Electrowetting-Controlled and Electrically-Tunable
Bio-Inspired Micro/Nanostructures and Optofluidic Devices
by
Supone Manakasettharn
A dissertation submitted in partial fulfillment of
the requirements for the degree of
Doctor of Philosophy
(Mechanical Engineering)
at the
UNIVERSITY OF WISCONSIN-MADISON
2013
Date of final oral examination: 01/16/2013
The dissertation is approved by the following members of the Final Oral Committee:
Tom N. Krupenkin, Associate Professor, Mechanical Engineering
Neil A. Duffie, Professor, Mechanical Engineering
Dan Negrut, Associate Professor, Mechanical Engineering
Hongrui Jiang, Professor, Electrical and Computer Engineering
Xudong Wang, Assistant Professor, Materials Science and Engineering
Justin Williams, Associate Professor, Biomedical Engineering
© Copyright by Supone Manakasettharn 2013
All Rights Reserved
i
Electrowetting-Controlled and Electrically-Tunable
Bio-Inspired Micro/Nanostructures and Optofluidic Devices
Supone Manakasettharn
Under the supervision of Professor Tom N. Krupenkin
At the University of Wisconsin-Madison
ABSTRACT
“The end and aim of all education is the development of character.”
- Francis W. Parker
Many man-made structures and devices have been inspired by the ingenious structures,
mechanisms, properties, and functions of plants and animals. This work has been inspired by a
number of unique properties, which biological organisms possess such as dynamic tunable
iridescence, self-cleaning properties, and brilliant structural color. The objective of this work is
to model, design, fabricate, and characterize novel bio-inspired micro-/nanostructures and
optofluidic devices.
To conceptually mimic the iridescence of cephalopods, microflowers have been modeled
and designed based on elasto-capillary bending, which is the interplay between the elastic energy
of petals and the capillary energy of a liquid droplet, which is used to actuate petal movement.
After microfabrication of the polycrystalline Si microflowers, two methods of petal actuation
have been demonstrated—one by volume change of the water droplet and the other by change of
ii
water contact angle on the petals using the electrowetting process. The experimental results are
in good agreement with a theoretical model.
By taking into account the self-cleaning properties of lotus leaves, transparent Ta2O5
nanostructured thin films have been fabricated using a multi-step anodization process of sputterdeposited Al-Ta bilayers on a quartz substrate.
The films then have been made
superhydrophobic by using a combination of nanostructures, called nanograss, along with the
deposition of hydrophobic coatings. The films also have been characterized by measuring water
contact angles and by obtaining optical transmittance spectra and SEM micrographs.
The
measured contact angles and transmittance spectra are in good agreement with theoretical
calculations.
Inspired by biological nanostructured surfaces possessing structural color and wettability
control, reflective Ta2O5 nanostructured thin films have been fabricated using the multi-step
anodization process of sputter-deposited Al-Ta bilayers on a Si substrate. The films have been
characterized by obtaining optical reflectance spectra, and SEM micrographs and by performing
Fourier analysis. When the nanograss surface has been wetted with methanol, it exhibits an
intense blue colored spot, which can be reversibly switched on and off by applying electrical
current to an indium-tin-oxide electrode positioned on top of the nanograss surface.
The results of this work form the basis for future development of a broad range of
optofluidic devices.
iii
To my parents, teachers, family, friends, and everyone
giving good things to me, others, and our society.
iv
ACKNOWLEDGEMENTS
“Try not to become a man of success, but rather try to become a man of value.”
“The value of a man should be seen in what he gives and not in what he is able to receive.”
- Albert Einstein
There are many people who have helped me both directly and indirectly to complete my
PhD degree. I would like to thank them all. I would like to express my sincerest appreciation to
my great research and academic advisor, Professor Tom N. Krupenkin, for his invaluable time,
guidance, advices, and financial support. I am very grateful to Professors Neil A. Duffie,
Hongrui Jiang, Dan Negrut, Xudong Wang, and Justin Williams who serve as a committee for
my PhD preliminary examination and dissertation defense.
I would also like to express my deep gratitude to Dr. J. Ashley Taylor for his invaluable
time, guidance, suggestions and help on my research and dissertation. I would like to thank
Tsung-Hsing (Kevin) Hsu for his help on the multi-step anodization processing of Ta2O5
nanostructured thin films and his helpful discussions on microfabrication.
I gratefully
acknowledge the assistance provided by Professor Stanley Pau and Graham Myhre at the
University of Arizona for their research collaboration on optical characterizations of Ta2O5
nanostructured thin films.
I would like to thank the Wisconsin Center for Applied
Microelectronics (WCAM) and Materials Science Center (MSC) at UW-Madison for processing
and characterization assistance. Additionally, I would like to thank Iliarys (Vicky) Matos for her
helpful discussions on my research. I also would like to thank my friends and wonderful lab
colleagues – Devin F. West, Rami Osman, Adnan Mortada, Fawzi Halimeh, and Karim Hannawi
for their supports.
v
Furthermore, I would like to express my deep gratitude to my former co-advisors (June
2008 – November 2009), Professors Kevin T. Turner and Ryan J. Kershner for their invaluable
time, guidance, and advice by providing the opportunities to work with them on lateral capillary
forces measured by PDMS microposts and interparticle capillary forces measured by optical
tweezers. I also would like to express my gratitude to Professor Gabe Spalding for his time,
guidance, and advice while collaborating with him. I am very grateful to Professor Douglas B.
Weibel, and Professor Justin Williams for allowing me to work in their laboratories. I would like
to thank Sanitta Thongpang and Dr. Shane Flickinger for assistance in using Professors
Williams’ and Weibel’s laboratories. In Professors Turner’s and Kershner’s Research Groups, I
would like to thank my friends and wonderful lab colleagues–Dr. David S. Grierson, Weixu
Chen, Ben Jasperson, Kevin V. Christ, Michael Wald, Bryce Smith, Hyun-Joon Kim, Jingjing
Liu, John Considine, Li Jiang, Stefan M. Oehrlein, Jose Roberto Sanchez Perez, Rouban Han,
Daniel A. Abras, Tao Wang, and Dan Hawk.
I am very grateful to Professors Roxann L. Engelstad, Alan Jeffrey Giacomin, Franz J.
Himpsel, Tom N. Krupenkin, Xiaochun Li, Heidi-Lynn Ploeg, David A. Rothamer, Christopher
J. Rutland, Izabela A. Szlufarska, Mario F. Trujillo, Justin Williams for teaching the courses
required for this degree and to my classmates for providing additional insight.
This work has been funded by the United States Air Force Office of Scientific Research
(AFOSR) Multi-University Research Initiative (MURI) Program under Award FA9550-09-10669-DOD35CAP. I also received partial financial support from the Thai Government Science
and Technology scholarship.
vi
TABLE OF CONTENTS
ABSTRACT ........................................................................................................................................ I
ACKNOWLEDGEMENTS.................................................................................................................... IV
TABLE OF CONTENTS ...................................................................................................................... VI
LIST OF FIGURES ............................................................................................................................. IX
NOMENCLATURE ........................................................................................................................... XIII
CHAPTER 1. INTRODUCTION.......................................................................................................... 1
1.1. Motivation ............................................................................................................................ 1
1.2. Objectives ............................................................................................................................ 1
1.3. Dissertation Outline ............................................................................................................. 3
CHAPTER 2.LITERATURE REVIEW ................................................................................................. 4
2.1. Capillary Origami ................................................................................................................ 4
2.1.1. Why capillary origami? ................................................................................................. 4
2.1.2. History........................................................................................................................... 4
2.1.3. Principles....................................................................................................................... 5
2.1.4. Applications .................................................................................................................. 9
2.2. Electrowetting .................................................................................................................... 11
2.2.1. Why electrowetting? ................................................................................................... 11
2.2.2. History......................................................................................................................... 11
2.2.3. Principles..................................................................................................................... 12
2.2.4. Dielectric materials ..................................................................................................... 14
2.2.5. Contact angle saturation .............................................................................................. 15
2.2.6. Switching speed .......................................................................................................... 15
2.2.7. Applications ................................................................................................................ 16
2.3. Optofluidics ....................................................................................................................... 18
2.3.1. Why optofluidics? ....................................................................................................... 18
2.3.2. History......................................................................................................................... 19
2.3.3. Principles..................................................................................................................... 19
2.3.4. Applications ................................................................................................................ 21
vii
CHAPTER 3. ELECTROWETTING-CONTROLLED CEPHALOPOD-INSPIRED ARTIFICIAL IRIDOPHORES
BASED ON CAPILLARY ORIGAMI .................................................................................................... 23
3.1. Abstract .............................................................................................................................. 23
3.2. Introduction ........................................................................................................................ 24
3.3. Theoretical Model .............................................................................................................. 25
3.4. Design ................................................................................................................................ 28
3.5. Fabrication ......................................................................................................................... 28
3.6. Results and Discussion ...................................................................................................... 29
3.7. Summary ............................................................................................................................ 34
CHAPTER 4. LOTUS LEAF-INSPIRED SUPERHYDROPHOBIC AND TRANSPARENT
TA2O5NANOSTRUCTURED THIN FILMS .......................................................................................... 35
4.1. Abstract .............................................................................................................................. 35
4.2. Introduction ........................................................................................................................ 35
4.3. Experimental Section ......................................................................................................... 37
4.4. Results and discussion ....................................................................................................... 39
4.5. Conclusion ......................................................................................................................... 45
CHAPTER 5. INTERPLAY BETWEEN IRIDESCENT AND OMNIDIRECTIONAL COLORATION IN BIOINSPIRED ELECTRICALLY-TUNABLE NANOSTRUCTURES ................................................................ 46
5.1. Abstract .............................................................................................................................. 46
5.2. Introduction ........................................................................................................................ 47
5.3. Experimental Section ......................................................................................................... 48
5.4. Results and Discussion ...................................................................................................... 49
5.5. Summary ............................................................................................................................ 56
CHAPTER 6. CONCLUSIONS AND PROPOSED FUTURE WORK ....................................................... 57
6.1. Conclusions ........................................................................................................................ 57
6.2. Proposed Future Work ....................................................................................................... 59
6.2.1. Microflowers ............................................................................................................... 59
6.2.2. Superhydrophobic and transparent Ta2O5 nanostructured thin films.......................... 60
6.2.3. Ta2O5 nanostructured thin films with electrically-tunable switching between
iridescent and non-iridescent coloration ............................................................................... 62
VITA .............................................................................................................................................. 65
REFERENCES .................................................................................................................................. 67
viii
APPENDIX ...................................................................................................................................... 81
A.1. Mathematica Code for Solving the Theoretical Model in Chapter 3 ................................ 81
A.2. Mathematica Code for Calculating the Theoretical Transmittance in Chapter 4 ............. 84
A.3. Mathematica Codes for Chapter 5 .................................................................................... 89
A.3.1. Analyzing the SEM Image with the 2D Discrete Fourier Transform ........................ 89
A.3.2. Deconvoluting the Measured Reflectance Spectra of the Nanostructured Thin Films
and the Planar Ta2O5 Thin Film to Predict the Scattering Spectra of the Nanostructured Thin
Films ..................................................................................................................................... 94
A.4. Process Parameters for Fabricating Microflowers in Chapter 3 ....................................... 99
A.5. Process Parameters for Fabricating Transparent Ta2O5 Nanostructured Thin Films in
Chapter 4 ................................................................................................................................. 102
A.6. Process Parameters for Fabricating Reflective Ta2O5 Nanostructured Thin Films in
Chapter 5 ................................................................................................................................. 104
ix
LIST OF FIGURES
Figure 2.1.
Folding criteria plotted from Eq. (2.1) assuming complete circular folding and
neglecting the effect of gravity.
Figure 2.2.
Capillary origami 3D structures of a pyramid, a cube, and a quasi-sphere obtained
by folding triangle-, cross-, and flower-shaped PDMS sheets, respectively,
actuated with a water droplet.
Figure 2.3.
Capillary origami 3D structures formed from triangle- and flower-shaped
templates using soap bubbles (scale bar: 2 cm).
Figure 2.4.
The folding of an artificial flower when submerged in water.
Figure 2.5.
(a) Schematics of initial templates. (b)-(d) SEM images of 3D microstructures
after folding (scale bar: 50 m).
Figure 2.6.
Three schematics from left to right showing steps to fabricate a spherical-shaped
silicon solar cell. The image at the far right shows the final spherical-shaped
silicon solar cell.
Figure 2.7.
Capillary origami controlled by an electric field. The schematic of the
experimental setup is shown at the far right and three images to the left show the
results of increasing voltage from 0 V to 700 V and decreasing voltage from 700
V to 200 V.
Figure 2.8.
Schematics of a classical electrowetting setup.
Figure 3.1.
(a) A microflower designed as an artificial iridophore based on capillary origami.
Flexible reflective petals (yellow) act as diffraction gratings reflecting incoming
light (shown schematically as red, green and blue beams).
(b) Schematics
x
representing the cross-section of a microflower (red) with a captured droplet
(blue). (c) Schematics representing the contact line
where total contact line
Figure 3.2.
∑
( )
( )
of a droplet on each petal,
.
(a)Petal curvature radius R b (in units of LEC) as a function of contact angle -0,
droplet radius R drop, and Cassie-Baxter fraction f0. Dashed lines correspond to f0 =
0.6 while solid lines correspond to f0 = 0.8. Droplet radius R
drop
and Cassie-
Baxter fraction f0 are determined at the state when the droplet forms an ideal
sphere with the petals completely wrapping the bottom half of the droplet. (b)
Droplet and petal shapes for the cases of contact angle -0 = 70° and -0 = 145°.
The results are shown for the droplet radius R drop = LEC.
Figure 3.3.
(a) Optical image of the top view of a microflower. (b) Part of a petal before
release. (c) SEM image of the cross-section of a petal on LTO layer next to an
array of Si nanograss before sacrificial etch. (d) The stem of a microflower
anchored to the substrate. (e) Optical images of microflowers on nanograss
substrate before oxidation and (f) after oxidation.
Figure 3.4.
(a) A microflower with a captured microdroplet. (b) Dependence of petal angle on
liquid volume (only half of the frame is shown to facilitate comparison). As the
amount of liquid captured by a microflower decreases the petal angle also
decreases. (c) Schematics of an electrowetting process. Blue represents a droplet;
the red line represents the dielectric layer, which surrounds the polycrystalline
silicon petal; and the yellow area shows the polycrystalline silicon microflower.
(d) Electrowetting actuation of a microflower. The Image shows the petal position
xi
without applied voltage, and the red dash lines indicate the petal positions with
the voltage applied.
Figure 4.1.
Schematics of the multi-step anodization process of an Al-Ta bilayer.
Figure 4.2.
Top view SEM images of (a) the Al surface after the first Al2O3 layer was
stripped, (b) the Al2O3 porous layer, (c) Ta2O5 nanoposts grown in the Al2O3
pores, and (d) the final nanostructured Ta2O5 thin film after the Al2O3 porous
layer was stripped. (e) The cross-section view of the final nanostructured Ta2O5
thin film.
Figure 4.3.
(a) A clean Ta2O5 nanograss surface which is highly hydrophilic showing a
contact angle of less than 3. (b) The same surface rendered superhydrophobic by
depositing a CFx coating showing a contact angle 155±2 with a hysteresis of 20.
(c) The transparent superhydrophobic nanograss film with two water droplets
deposited on the surface.
Figure 4.4.
(a) The predicted transmittance spectra of two films with different layer thickness:
the red curve is for a film consisting of a continuous Ta2O5 layer 300 nm thick
and a Ta2O5 nanograss layer of 90 nm thick and the blue curve is for a film with
a Ta2O5 continuous layer of 410 nm and a nanograss layer of 90 nm. (b)
Measured transmittance spectra of quartz (green) and transparent nanograss films
(blue, green) with the same dimensions.
Figure 5.1.
(a) Schematic illustrating light interference and scattering at Ta2O5 nanograss
film. (b) SEM cross-section image of the nanograss structure. (c) Top view
optical image of yellow nanograss structure. (d) Top view SEM image of the
xii
Ta2O5 nanograss. (e) Optical image of methanol spot on the Ta2O5 nanograss
substrate in ambient light. (f) Optical image of methanol spot taken with intense
backscattered illumination.
Figure 5.2.
Measured reflectance spectra of planar Ta2O5 film, dry Ta2O5 nanograss, and wet
blue spots of methanol, ethanol, IPA, acetone, toluene, and chloroform deposited
on the Ta2O5 nanograss substrate.
Figure 5.3.
(a) 2D DFT of the Ta2O5 nanograss (shown in Fig. 5.1(d)). (b) Radial average of
the power spectrum. (c) Scattered spectra predicted from the Fourier analysis of
the SEM image (Fig. 5.1(d)). (d) Scattering spectra obtained from the reflectance
measurements.
Figure 5.4.
(a) The predicted scattered spectrum (green) of the dry nanograss film and the
measured reflectance spectrum (black) of the planar Ta2O5 thin-film stack. (b)
The measured (orange) and predicted (green) reflectance spectra of the Ta2O5
nanograss. (c) The predicted scattered spectrum (intense blue) of the blue spot
and the measured reflectance spectrum (black) of the planar Ta2O5 thin-film stack.
(d) The measured (light blue) and predicted (dark blue) reflectance spectra of the
wet blue spot of methanol on Ta2O5 nanograss.
Figure 5.5.
(a) Electrically-induced switching of the methanol-induced coloration. (b)
Schematics illustrating how the methanol distributes inside the nanograss layer
while the electric current applied to an ITO electrode is off and is on.
xiii
NOMENCLATURE
Bending rigidity
Dielectric thickness.
Film thickness of layer
Phase of the light wave
Young’s modulus
()
Capillary energy of a droplet deposited on the petals
Bending energy
Capillary energy
Gravitational potential energy
Total energy
Ratio between the surface area in contact with a droplet and the projected area.
Volume fraction of medium
Gravitational acceleration
Thickness of the layer
Interference intensity
Incident light intensity
Scattered light intensity
Total length of contact line of a droplet on the petals
Length of the structure
Length of the interfacial surface of the fluid
xiv
Length of a structure
Capillary length
Critical length of a structure for folding to occur
Elasto-capillary length
Refractive index in medium
Effective refractive index
Ratio between the actual surface area of the rough surface and the projected area.
Amplitude of the reflectance at the interface
Amplitude of the light wave
Radius of curvature
Radius of curvature
Ratio of interference intensity to incident light intensity
Reflectance
Ratio of scattered light intensity to incident light intensity
Surface area
Transmittance
Applied voltage
Total volume of a droplet
Height of the center of mass
xv
Greek Letters
Effective absorption coefficient of the medium
Angle of a petal with respect to the substrate
Angle of a droplet with respect to a substrate,
Surface tension
Interfacial energy per unit area of liquid-vapor interface
Interfacial energy per unit area of solid-liquid interface
Interfacial energy per unit area of solid-vapor interface
Phase change of the light wave at the interface
Vacuum permittivity
Dielectric constant
Dielectric function of medium
Effective dielectric function of the medium
Observed contact angle
Incident/refracted angle in medium
Local contact angle that the liquid forms with the petal surface.
Contact angle with applied voltage
Contact angle
Wavelength of light
Poisson’s ratio
Density
1
CHAPTER 1. INTRODUCTION
“Look deep into nature, and then you will understand everything better.”
- Albert Einstein
1.1. Motivation
In nature, there are a wide variety of plants and animals, which possess unique useful
structures, mechanisms, properties, and functions which have evolved to ensure their survival.
When we look into nature, we may be inspired by plants and animals to create novel structures
and devices. For example, aircraft such as gliders and airplanes have been inspired by flying
animals such as birds and bats where their light and strong wings provide lift. In a similar spirit,
this work has been inspired by natural phenomena including dynamically tunable iridescence of
cephalopods, self-cleaning property of lotus leaves, and structural color of some biological
species.
1.2. Objectives
The objective of this work is to model, design, fabricate, and characterize novel bioinspired micro/nanostructures and optofluidic devices. The following structures will be studied
and make up the majority of work for this dissertation.
-Electrowetting-controlled cephalopod-inspired artificial iridophores based on capillary
origami
Iridophores of cephalopods can create dynamic tunable iridescence caused by thin-film
interference of the reflected light. This work will mimic the iridophore structure conceptually by
2
creating microflower like devices with flexible reflective petals based on capillary origami
microstructures. The theoretical model based on capillarity and elasticity will be developed to
design the microflowers and to characterize the dynamics of the actuation process.
The
microflowers will be micro-fabricated using a capillary origami approach where the capillary
forces of a liquid droplet will be used to actuate petal movement. Two methods of actuation will
be demonstrated and will be quantitatively characterized: (1) by changing the volume of the
liquid droplet and (2) by changing its contact angle on the petals using the electrowetting
process.
- Lotus leaf-inspired superhydrophobic and transparent Ta2O5 nanostructured thin films
The fabrication of nanostructures has been inspired by lotus leaves, which possess selfcleaning properties, where a rolling water droplet collects dusts as it falls from the leaf.
Transparent Ta2O5 nanostructured thin films will be fabricated using a multi-step anodization
process of sputter-deposited Ta thin films on a quartz substrate. The films will be made highly
water repellent or superhydrophobic by using a combination of a nanostructured surface and the
deposition of a hydrophobic CFx coating. The films will be characterized by measuring water
contact angles and by obtaining optical transmittance spectra and SEM micrographs.
The
measured contact angles and transmittance spectra will be compared to theoretical calculations.
-Interplay between iridescent and omnidirectional coloration in bio-inspired electricallytunable nanostructures
Brilliant structural color in many biological species often results from iridescent and noniridescent color generated by coherent scattering from nano-scale structural elements. Moreover,
nanostructured surfaces of some biological species are used to control wettability. Inspired by
3
this multifunctional approach, artificial Ta2O5 nanostructured thin films will be fabricated using
a multi-step anodization process of sputter-deposited Ta thin films on a Si substrate. The films
also will be made superhydrophilic by using a combination of a nanostructured surface and
exposure to oxygen plasma.
The iridescent and non-iridescent color of the reflective
nanostructured films will be investigated. The nanograss surface will be wetted by an organic
liquid, such as methanol, to produce additional coloration. The color then will be dynamically
switched on and off by applying electrical current to an indium-tin-oxide electrode positioned on
top of the nanograss surface. The nature of the methanol-induced coloration and the physics of
the electrically induced color switching mechanism will be studied.
1.3. Dissertation Outline
This work is organized in the following chapters. Chapter 2 provides a literature review
related to the objective of this work. Capillary origami, electrowetting, and optofluidics are
reviewed with the emphasis on why, when, and how they have been used. Chapter 3 explains
the theoretical model, design, fabrication, results and discussion of electrowetting-controlled
cephalopod-inspired artificial iridophores based on capillary origami. Chapter 4 describes the
fabrication process, experiments, results and discussion of lotus leaf-inspired superhydrophobic
transparent Ta2O5nanostructured thin films.
Chapter 5 reports experiments, results and
discussion of the interplay between iridescent and non-iridescent coloration in bio-inspired
electrically tunable nanostructures. Chapter 6 concludes the entire work and proposes future
work.
4
CHAPTER 2.LITERATURE REVIEW
“Education is the transmission of civilization.”
- Will Durant
2.1. Capillary Origami1
2.1.1. Why capillary origami?
The fabrication of 3D structures is one of the major challenges for micro- and nanofabrication. Folding of an elastic planar structure after patterning and release is one technique to
fabricate a 3D structure using self-assembly. The term origami is taken from the Japanese art of
paper folding; while the actuation of the folding is accomplished by using capillary forces of a
fluid droplet, hence the technique has been termed capillary origami. The combination of the
folding process with capillary forces has resulted in a new technique for micro- and nanofabrication.
2.1.2. History
The term capillary origami was first introduced in 2007 by Charlotte Py et al to describe
the folding of a polydimethylsiloxane (PDMS) sheet into a 3D structure by using capillary forces
created by a water droplet [1].
As early as 1993 Syms and Yeatman demonstrated that 3D
structures could be fabricated by folding surfaces using capillary forces produced by molten
solder [2]. Later Richard R. A. Syms introduced the term surface tension-powered self-assembly
to describe the technique [3, 4]. Both of these techniques are quite similar in that 3D structures
1
Copyright © 2012 Springer. Reprinted with permission from Manakasettharn, S., Taylor, J.A., and Krupenkin, T.,
2012, "Capillary Origami,"Encyclopedia of Nanotechnology, B. Bhushan, eds. Springer.
5
can be produced by folding elastic thin films. Both use capillary forces for self-assembly. In the
first example various liquids such as water are used, while for the second study molten metals
such as solder were used, which then solidified fixing the 3D microstructures.
2.1.3. Principles
At the macroscale level, the influence of capillary forces is negligible compared to other
forces such as gravity, electrostatic or magnetic. Because capillary forces scale linearly with the
characteristic size of the system, at sub-millimeter dimensions capillary forces begin to dominate
since the majority of other forces decrease much more rapidly than the first power of the length.
For example, a human cannot walk on water because capillary forces produced at the water
surface are much smaller than the gravitational force acting on a human, which scales as the cube
of the length. On the other hand, the much smaller water strider can easily walk on water
because capillary forces are large enough to balance the gravitational force produced by the
water strider. For capillary origami, capillary forces need to be large enough to counteract the
weight of the liquid droplet and the structural forces of the planar layer.
In terms of energy, for capillary origami one needs to consider the interplay of three
different energies: capillary energy, bending energy, and gravitational potential energy. For a
two-dimensional (2D) model, the capillary energy per unit length of the interface (2D analog of
the surface energy) is defined as
fluid and
is the length of the interfacial surface of the
is the surface tension [5]. The bending energy per unit length is approximately
, where
(
, where
is the length of the structure and
is the radius of curvature [6].
) is the bending rigidity of the structure, where
thickness of the layer, and
is Young’s modulus,
is
is Poisson’s ratio. If one only considers the mass of the fluid,
6
assuming that it is much larger than the mass of the structure, then the gravitational potential
energy per unit length is
constant of gravity, and
, where
is the density,
is the surface area,
is the
is the height of the center of mass. By neglecting the effect of gravity
and assuming complete circular folding, de Langre et al [7] derived simplified criteria for folding
considering the interplay between capillary and bending energies, which are expressed as
√
where
√
is the critical length of a structure for folding to occur,
length [5], and
√
(2.1)
√
is the capillary
is the elasto-capillary length [8].
Figure 2.1. Folding criteria plotted from Eq. (2.1) assuming complete circular folding and
neglecting the effect of gravity.
7
The simplified criteria for folding derived from Eq. (2.1) can be plotted as shown in Fig.
2.1. To fold a structure requires
or
or
√
indicating that the
capillary length must be larger than half of the elasto-capillary length so that the capillary effect
can overcome bending rigidity of the structure. The other requirement for folding is
√
or
confirming that the length of the structure should also be long
enough for a liquid droplet to wet the surface to produce sufficient capillary forces to fold the
structure. For the 3D structures the folding criteria become more complex. In particular in 3D the
critical length also depends on the shape of the initial template such that
squares and
for
for triangles [9]. Figure 2.2 shows examples of capillary origami
structures of a pyramid, a cube, and a quasi-sphere obtained from folding triangle-, cross-, and
flower-shaped PDMS sheets, respectively [10].
Figure 2.2. Capillary origami 3D structures of a pyramid, a cube, and a quasi-sphere obtained by
folding triangle-, cross-, and flower-shaped PDMS sheets, respectively, actuated with a water
droplet. Images reprinted from [10]with permission.
8
Besides using liquid droplets, capillary origami structures can be constructed by using
soap bubbles as shown in Fig. 2.3. The weight of a soap bubble is much less than that of a liquid
droplet especially for large droplets capable of covering centimeter size structures when
gravitational forces become significant. A soap bubble was shown to fold a centimeter size
elastic structure, which cannot be accomplished using a liquid droplet [11].
Figure 2.3. Capillary origami 3D structures formed from triangle- and flower-shaped templates
using soap bubbles (scale bar: 2 cm). Images reprinted from [11] with permission.
Petals of a flower also can be folded into a structure similar to capillary origami when
submerged in water as shown in Fig. 2.4. The folding of the flower in water is accomplished by
the interplay of elastic, capillary, and hydrostatic forces. During submersion hydrostatic pressure
pushes against the back of petals, and surface tension prevents water from penetrating through
the spacing between petals resulting in trapping an air bubble inside a flower. The inside of the
folded flower remains dry protected by the air bubble [12].
Figure 2.4. The folding of an artificial flower when submerged in water. Images reprinted from
[12] with permission.
9
2.1.4. Applications
Capillary origami has been used to fabricate a number of 3D microstructures. Figure 2.5
illustrates the self-assembly of structures with various geometries. The initial planar templates
are shown in Fig. 2.5(a), and the folded final 3D microstructures are shown in Fig. 2.5(b)-(d).
The initial planar templates with lengths ranging from 50-100 m and a thickness of 1 m were
fabricated from silicon nitride thin films deposited and patterned by using standard
micromachining processing typically used for integrated circuit and MEMS fabrication. Water
droplets then were deposited on the templates to fold 3D microstructures [13]. Figure 2.6 shows
another example of microfabrication of a quasi-spherical silicon solar cell based on capillary
origami. After fabrication by conventional micromachining processing, the initial flower-shaped
silicon template was folded into a sphere using a water droplet. Unlike, conventional flat solar
cells, this spherical solar cell enhanced light trapping and served as a passive tracking optical
device, absorbing light from a wide range of incident angles [14].
Figure 2.5. (a) Schematics of initial templates. (b)-(d) SEM images of 3D microstructures after
folding (scale bar: 50 m). Images reprinted from [13] with permission.
10
Figure 2.6. Three schematics from left to right showing steps to fabricate a spherical-shaped
silicon solar cell. The image at the far right shows the final spherical-shaped silicon solar cell.
Images reprinted from [14] with permission.
Structures formed by capillary origami also can be actuated by using electrostatic fields
to reversibly fold and unfold then. For this application we need to take into account the interplay
of capillary, elastic, and electrostatic forces. As shown in Fig. 2.7, an electric field was applied
between the droplet and the substrate. When the voltage was increased, the electrostatic force
increased eventually overcoming capillary forces resulting in unfolding of the PDMS sheet.
When the voltage was decreased below a certain threshold, the electrostatic force was no longer
strong enough to prevent capillary forces from again folding the elastic sheet [15].
Figure 2.7. Capillary origami controlled by an electric field. The schematic of the experimental
setup is shown at the far right and three images to the left show the results of increasing voltage
from 0 V to 700 V and decreasing voltage from 700 V to 200 V. Images reprinted from [15] with
permission.
Capillary origami is a simple and inexpensive method to fabricate 3D structures at the
sub-millimeter-scale. By using capillary forces, intricate and delicate 3D thin-film structures can
11
easily be fabricated, which would be difficult to obtain by other means. More applications
exploiting the advantages of capillary origami itself or in combination with electric fields can
readily be envisioned. Ultimately one expects to see more commercial products based on this
versatile technique.
2.2. Electrowetting
2.2.1. Why electrowetting?
Recently, manipulation of individual droplets has attracted increasing attention (e.g. labon-a-chip research) since it offers many potential advantages such as less volume of the fluid and
more multi-parallel activities by using many droplets in contrast to continuous flow of a fluid.
Electrowetting is one technique for manipulating the wetting of polar fluids on a surface with an
applied voltage. Much work of droplet manipulation has been done with electrowetting resulting
in a wide variety of applications ranging from tunable liquid lenses [16, 17] and mirrors [18] to
electronic displays [19, 20] and lab-on-a-chip systems [21-24].
2.2.2. History
In 1875, Gabriel Lippmann first found that variation of a voltage applied between
mercury and electrolyte could change the position of a mercury-electrolyte interface [25] (see the
appendix of [26], where Mugele and Baret translated Lippmann’s original paper from French
into English).
This electrocapillarity of mercury and electrolyte was the basis of modern
electrowetting. One problem of electrocapillarity was the electrolytic decomposition of water
when an applied voltage was over a few hundred millivolts. In 1993 Berge initiated the use of an
insulating film between a polar conducting liquid and an electrode in order to prevent electrolysis
12
[27]. This concept later has become known as electrowetting on dielectric (EWOD). There have
been a number of good reviews covering electrowetting and its applications published over the
last several years [23, 24, 26, 28, 29].
2.2.3. Principles
In electrowetting, a system generally consists of the partial wetting of a liquid droplet on
a planar dielectric film. One may consider horizontal forces balancing at a three-phase contact
line of a droplet. The force balance equation is then
,
, where
,
are interfacial energies (dimension of force per unit length or energy per unit area) of
solid-vapor, solid-liquid, liquid-vapor interfaces, respectively;
distance and
is a change in horizontal
is the contact angle. This equation may be rewritten as
(
)
, which is the well known Young’s equation [30, 31]. Combining effects from electrostatic
forces, one may obtain the basic equation for electrowetting [27]:
(2.2)
where
constant,
is the contact angle with applied voltage,
is an applied voltage, and
is vacuum permittivity,
is dielectric thickness.
Figure 2.8. Schematics of a classical electrowetting setup.
is dielectric
13
The classical electrowetting equation above was derived base on the assumption that a
droplet sits on a smooth flat surface, as shown in Fig. 2.8. In fact, surface roughness affects the
wetting of a droplet [32-35]. When a droplet fully wets a topographically rough surface, the
observed contact angle
in this state is given by the Wenzel equation [33]:
(2.3)
where
is the ratio between the actual surface area of the rough surface and the projected
(footprint) area. In the case of the surface being wet or hydrophilic (
), the Wenzel
equation predicts that the observed contact angle is less than the intrinsic or local contact angle
(
) on a smooth surface of the same material.
approaches
, the wetting is called complete or superhydrophilic [36-38]. In the case of the
surface being non-wet or hydrophobic (
If the observed contact angle
), the Wenzel equation predicts that the
observed contact angle is more than the intrinsic or local contact angle (
observed contact angle is larger than
). If the
, this is called superhydrophobic. However, the
hydrophobic surface is not achieved experimentally in the Wenzel state, where the liquid fully
penetrates and wets the topographically rough surface under the droplet, but it is in the CassieBaxter state [32, 39, 40]. In this state the droplet contacts only the very tips of the surface
topographic features by leaving non-wetted pockets in the structured layer. The observed contact
angle is described by the Cassie-Baxter equation [32]:
(
where
)
(2.4)
is the ratio between the surface area in contact with a droplet and the projected
(footprint) area.
14
Krupenkin et al.[41] firstly demonstrated the use of electrowetting to switch liquid
droplets on nanostructured surfaces from the Cassie-Baxter to the Wenzel state.
They
constructed nanoposts, each with an average diameter of 350 nm and an average height of 7 m
by etching a silicon wafer. Each post was covered with a thermally grown insulating oxide layer
and a hydrophobic top coating. In order to vary , the spacing between posts was changed from
1 to 4 m. For droplets of high-surface-tension liquids such as water (
molten salt (
= 62 mN/m) on nanostructured surfaces,
= 72 mN/m) and
increased linearly with , which
is in good agreement with the Cassie-Baxter equation. The highly mobile droplet with
on the nanostructured surface in the Cassie-Baxter state became immobile when
was lowered
with electrowetting. At an applied threshold voltage, the liquid penetrated the nanostructured
layer converting to the Wenzel state.
2.2.4. Dielectric materials
As indicated by the basic electrowetting equation, the insulating or dielectric layer should
be thin in order to use low voltage to tune the contact angle. The contact angle should be large at
zero voltage to obtain a large tuning range. Typical inorganic insulating or dielectric layers are
silicon dioxide [41-43] and silicon nitride [16, 42, 43]. Silicon dioxide (
) [42,
44]can be produced using either standard vacuum deposition or thermal oxidation techniques,
while silicon nitride (
) [16, 42, 44, 45]can be produced using chemical vapor
deposition. Since the surfaces of silicon dioxide and silicon nitride are hydrophilic, they need a
thin hydrophobic top coating in order to be used as electrowetting substrates. A hydrophobic top
coating can be self-assembled monolayers such as silanes [46] or amorphous fluoropolymer such
as Teflon AF [42] and Cytop [41].
Teflon AF (
) [47] and Cytop (
15
)[48]are also used as dielectric layers, which are produced by spin or dip coating techniques
[49-51]. Other polymer materials that have been used in electrowetting are parylene-N (
) [43, 52, 53], parylene-C (
) [42, 43, 54-56] and polydimethylsiloxane (PDMS,
) [57, 58].
2.2.5. Contact angle saturation
In previous experimental studies, the observed contact angle of a droplet sitting on a
dielectric layer shows a parabolic relationship with the applied voltage at low voltage, and
always becomes saturated at high voltage. Verheijen and Prins [52] explain that trapped charges
in a dielectric layer partially screen the applied electric field resulting in no change in the
observed contact angle beyond a threshold voltage. Vallet et al.[59] also observed that satellite
droplets of low conductive liquid (pure water) were emitted at the perimeter of the main droplet
since electrostatic repulsion beyond a threshold voltage was larger than surface tension. Adding
salt to pure water suppressed the droplet expulsion, probably by opposing gradients in surface
tension through gradients in concentration. In addition, Vallet et al.[60] found that luminescence
occurred at the contact line of a droplet of salted aqueous solution at high voltage, probably
because charges leaked from a droplet to the air and did not adsorb in the liquid at the liquidsolid interface. This air ionization then stopped the electrowetting. So far, various explanations
about the contact angle saturation have been proposed, but no consistent one has emerged.
2.2.6. Switching speed
In electrowetting, the observed contact angle of a droplet can be decreased with an
applied voltage and can be reversed to the original observed contact angle without an applied
voltage. The switching speed of the change in the observed contact angle with and without an
16
applied voltage has been observed to be faster than 1 kHz [61] and even the GHz range has been
achieved [62]. The switching speed depends on size, surface tension, density, viscosity of a
droplet, medium, and the applied voltage [61]. However, the size of a droplet seems to dominate
other factors and experimentally scales linearly with the switching speed at sub-millimeter size
[61]. Besides the fast switching speed, electrowetting also provides long term reliability where
more than 200,000 cycles were completed without obvious degradation [63].
This makes
electrowetting attractive for many new applications.
2.2.7. Applications
With its ability to manipulate individual droplets “digitally,” electrowetting has been used
in lab-on-a-chip microfluidic systems for chemical and biological applications [23, 24].
Electrowetting-based lab-on-a-chip devices generally consist of two parallel substrates with
integrated electrodes. Electrodes can be made of an indium-tin-oxide (ITO) transparent layer on
a glass substrate that allows operations of samples or droplets in the device to be observed from a
microscope. The operations combine dispensing, moving, merging, mixing, and splitting of
droplets [21, 22, 64-67]. The activation sequence of each operation can be freely programmable
on an array of individually addressable electrodes. Electrowetting can work with droplets of a
wide variety of solutions such as glucose [21], DNA, and protein solutions [68]. However, the
suppression of biomolecular adsorption on surfaces, such as using oil as the ambient medium
instead of air or controlling an applied voltage, needs to be done to maintain the hydrophobicity
of surfaces and the performance of droplet actuation [68]. Electrowetting-based lab-on-a-chip
devices have been commercialized by companies such as Cytonix and Advanced Liquid Logic
and many others have been developed by a large number of academic groups worldwide.
17
Electrowetting also has been used in a variety of optical applications. In 1981, Beni and
Hackwood [69] first reported the concept of electrowetting displays by moving a refractive index
matched liquid in and out of the porous solid to make a display either transparent or white
(diffusely reflecting). Later, Hayes and Freenstra at Philips Research demonstrated the color
concept of electrowetting displays.
They found that at a colored state a dyed oil film
continuously wetted a hydrophobic dielectric layer as a square pixel without an applied voltage,
and at a clear state the dyed oil contracted into one corner of the pixel with an applied
voltage[70].
In 2006, Philips Research spun out Liquavista which continued working on
commercial electrowetting displays; and in 2010 they were acquired by Samsung Electronics.
Furthermore, electrowetting has been used to control liquid lenses that are flexible and
tunable compared with classical lenses made from glass or other solid materials. In 2000, Berge
and Peseux [71] first described an electrowetting-controlled variable focal lens in a closed cell
that was filled with salt water and a non-polar droplet as a lens was deposited on a hydrophobic
surface at the center. After applying a voltage between the conducting liquid and the counterelectrode, these authors could switch the focal length of the lens more than 106 cycles without
noticing any degradation. Later, Krupenkin et al.[16] demonstrated a tunable liquid microlens
whose focal length could be adjusted and lateral position by varying the voltage applied to a set
of electrodes positioned underneath of the dielectric substrate. The same authors [17] also
demonstrated a tunable liquid microlens that later could be polymerized with UV light to lock its
position and shape. Kuiper and Hendriks [72] demonstrated one potential application of an
electrowetting-controlled variable-focus liquid lens for miniature cameras. Varioptic founded by
B. Berge has commercialized electrowetting-based liquid lenses for miniature cameras.
18
Another application includes electrowetting switches for optical fibers by moving a
mercury droplet, surrounded by an electrolyte, with an applied voltage inside a channel to either
reflect or transmit light in an optical multiplexer [73-75]. In addition, electrowetting has been
used to make a motor whose rotational motion of a non-circular plate floating on a fluid droplet
came from the deformation of a droplet on voltage-applied circular-track electrodes [76].
Moreover, electrowetting has been used to control the flow of an electrolyte through a
superlyophobic membrane to generate electrochemical energy which could be used to change a
battery [77].
Electrowetting is a simple and inexpensive technique to manipulate polar fluid droplets
on surfaces. When voltage is applied, the electric field created between charges in the droplet
and the dielectric layer, tends to pull the droplet to the surface. Consequently, the observable
contact angle of the droplet is lower and the droplet can be manipulated depending on the
individually addressable electrodes.
Many applications of electrowetting have been
commercialized as products for use in daily life.
In the future, one expects to see more
promising commercial products based on this versatile technique.
2.3. Optofluidics
2.3.1. Why optofluidics?
In general, the components in optical systems have been made from solid materials (e.g.
glasses), which obviously make them rigid. Fluidic materials (e.g. water), on the other hand, are
flexible and their interfaces are smooth, which are useful for optical systems. The integration of
optics and fluidics has created a new research field that has become known as “optofluidics.”
Optofluidic devices combine optical and fluidic advantages to improve their functionality.
19
Optofluidics has a broad variety of applications ranging from adjustable microlenses [78-80] and
lab-on-a-chip systems [81, 82] to data storage [83] and energy applications [84].
2.3.2. History
The concept of optical-fluidic devices, e.g. liquid mirror telescopes, can be traced back at
least in the 1850s. A liquid mirror telescope utilizes the principle of the free surface of a
reflective liquid (for example, mercury) uniformly rotating in a container to form a paraboloidal
mirror of a telescope. Compared to a conventional mirror, a liquid mirror is much cheaper and is
easier to scale up [85]. In 2003, the term optofluidics was introduced at California Institute of
Technology in the name of a University Research Center [86, 87]. After that, many optofluidic
devices have been developed and have utilized numerous optical and fluidic advantages. There
have been a number of good reviews published over the last several years [83, 84, 8792]covering optofluidics and its applications.
2.3.3. Principles
The principles of optofluidics are based on the principles of optics and fluidics.
Depending on underlying principles, some optofluidic devices may use light to manipulate fluids
while others may use fluids to manipulate light. Optofluidic devices can be designed in a variety
of ways to optimize optical and fluidic advantages.
Considering the properties of fluids, there are a number of ways that they can be utilized.
First, because of surface tension, an interface of two immiscible fluids, which cannot be mixed
homogeneously, is smooth, desirable for optical surfaces. For instance, an interface of air and
mercury rotating in a container can be used to form a smooth liquid mirror for the telescope
[85].Furthermore, miscible liquids allow diffusion across their interfaces, consequently creating
20
a concentration and refractive-index gradient. Since many optofluidic devices use microfluidic
channels dealing with low Reynolds number flow, the laminar flow of the miscible liquids help
control the concentration to form a continuous refractive-index gradient. For example, the
diffusion of CaCl2 from a core stream with a concentration of 3.5 M (refractive index, n=1.41) to
a cladding stream of de-ionized water (n=1.33) in co-injected microfluidic channels varies the
refractive index from 1.33-1.41 within the liquid medium for a tunable optofluidic microlens
[93].
Fluids can be transported in optofluidic devices by many ways. First, gravity-driven
flow[94] simply uses the weight of a liquid to drive the flow. Second, pressure-driven flow is
commonly used in optofluidic devices, in which a fluid is pumped by a positive displacement
pump, such as syringe pumps. Electrokinetic flow is another common technique which is the
coupling between an electric field and moving particles or fluids. When an ionized fluid is
moved under the effect of an electric field, it is called electro-osmosis [94, 95], which is one of
electrokinetic phenomena. When charged particles are moved in a resting fluid by an electric
field, this electrokinetic phenomenon is called electrophoresis [95]. When neutral particles are
moved by the application of an electric field gradient, it is called dielectrophoresis [95, 96].
On the side of optics, there are properties that can be utilized. First, when light, an
electromagnetic wave, is incident on an interface, light can be reflected, transmitted, absorbed or
diffused depending on an interface between the two media. When light is reflected on an
interface, the incident angle is equal to the reflected angle due to the law of reflection. When
light is transmitted through an interface, light will be refracted due to a change in its speed
between two media. Refraction can be described by Snell’s law:
, where
21
and
are refractive indices in two media, and
are incident/refracted angles in two media.
A lens with a gradual variation of the refractive index of a material can focus light and is called a
gradient refractive index (GRIN) lens as briefly explained below.
2.3.4. Applications
A few applications of optofluidics will now be reviewed. First, optofluidic lenses are an
example of an application using fluid to manipulate light. For example, as mentioned in 2.3.3.,
an optofluidic microlens, i.e. a liquid gradient refractive index (L-GRIN) lens, can change its
focal distance by variation in the concentration and refractive-index gradient of CaCl2 in coinjected microfluidic channels[93].Another way to adjust the focal distances of optofluidic lenses
is to tune their shapes (i.e. the radius of curvature of lenses). One technique to tune the shape of
a liquid lens is electrowetting [16, 71, 72] as mentioned in 2.2.7. The shape of optofluidic lenses
also can be tuned with stimuli responsive hydrogels [97, 98], electromagnetic [99], pneumatics
[100, 101], hydrodynamics [102], and dielectrophoresis[103, 104]. Varioptic is a company that
has commercialized electrowetting-based optofluidic lenses for miniature cameras. Optofluidic
lenses have recently been reviewed by Nguyen [91].
Another application is lab-on-a-chip (LOC) that integrates one or several laboratory
operations on a miniaturized chip. LOCs generally consist of a set of micro/nanofluidic unit
operations to handle (e.g., transport, mix, separate) sample fluids [92, 105-114]. Sample fluids
can be handled both as fluid streams in micro/nanofluidic channels and as fluid droplets
individually manipulated by electrowetting (see 2.2).
After sample fluids go through
micro/nanofluidic unit operations, in optofluidic LOCs, target samples are detected in optical
unit operations without direct contact.
The examples of optical detection methods are
22
fluorescence [115, 116], interferometry [117-119], and surface plasmon resonance imaging [120,
121]. The research and development of optofluidic LOCs may contribute to other fields such as
chemical engineering [112], biomedical and environmental monitoring [92, 111, 113, 114].
Other applications of optofluidics include optical waveguides [122, 123], liquid mirrors
[18], dye lasers [124], data storage [83, 125], and the field of energy [84]. Many applications
utilize the advantages of the integration of optics and fluidics. For these reasons, one expects to
see more applications based on optofluidics in the future.
23
CHAPTER 3. ELECTROWETTING-CONTROLLED
CEPHALOPOD-INSPIRED ARTIFICIAL IRIDOPHORES BASED ON
CAPILLARY ORIGAMI2
“Results? Why, man, I have gotten lot of results! If I find 10,000 ways something won’t work, I
haven’t failed. I am not discouraged, because every wrong attempt discarded is often a step
forward…”
- Thomas Alva Edison
3.1. Abstract
Cephalopods have evolved complex optical mechanisms of dynamic skin color control
based on mechanical actuation of micro-scale optical structures such as iridophores and
chromatophores. In this work, we describe the design, fabrication and characterization of bioinspired artificial iridophores, which resemble microflowers with flexible reflective petals, based
on capillary origami microstructures. Details of the microfabrication process are presented along
with the characterization of their mechanical and optical properties. Two methods of petal
actuation have been demonstrated—one based on the electrowetting process and the other by
volume change of the liquid droplet. These results were in good agreement with a model derived
to characterize the actuation dynamics. These initial results form the basis for future
development of a broad range of optofluidic devices.
2
Copyright © 2011 Applied Physics Letters. Reprinted with permission from Manakasettharn, S., Taylor, J. A., and
Krupenkin, T., 2011, "Bio-Inspired Artificial Iridophores Based on Capillary Origami: Fabrication and Device
Characterization," Applied Physics Letters, 99(14) pp. 144102.
This work has been supported by the United States Air Force Office of Scientific Research (AFOSR) MultiUniversity Research Initiative (MURI) Program Award # FA9550-09-1-0669-DOD35CAP.
24
3.2. Introduction
Many marine organisms have evolved complex optical mechanisms of skin color control
that allow them to change drastically their visual appearance to create a highly effective dynamic
camouflage[126-129].In particular, cephalopods have developed especially effective dynamic
color control structures based on the combination of absorptive (chromatophores) and reflective
(iridophores) layers. Recent studies of these structures in cuttlefish and squid [127, 128]have
revealed that the mechanism of the dynamic color change in these organisms is based on
mechanical actuation of micro-scale optical structures. Iridophores of squid consist of alternating
layers of reflective platelets of high and low index of refraction made out of reflectin proteins.
The squid can control the spacing and orientation of the platelets using muscle tissue thereby
creating dynamic tunable iridescence caused by thin-film interference of the reflected light [127,
128]. In a similar manner tunable iridescence can be achieved by changing the angle of a plate
with a diffraction grating etched on its surface.
In this work we have designed and fabricated an array of tunable microstructures which
mimic conceptually the adaptive coloration of the squid to create a class of artificial iridophores.
The microstructures consist of bendable elastic plates arranged as the petals of a flower designed
to dynamically change the direction of the reflected light as shown schematically in Fig. 3.1(a).
The bending of the plates is accomplished by using a process similar to capillary origami [1,
130].
25
Figure 3.1.(a) A microflower designed as an artificial iridophore based on capillary origami.
Flexible reflective petals (yellow) act as diffraction gratings reflecting incoming light (shown
schematically as red, green and blue beams). (b) Schematics representing the cross-section of a
microflower (red) with a captured droplet (blue). (c) Schematics representing the contact line
( )
of a droplet on each petal, where total contact line
∑
( )
.
3.3. Theoretical Model
Capillary origami or elasto-capillary folding is the folding of an elastic planar structure
into a three-dimensional (3D) structure using capillary forces of a fluid droplet [1, 130]. Unlike
the capillary origami process which requires complete folding of the petals around a liquid
droplet, we have designed the petals of our microstructures to bend and relax without completely
wrapping around the droplet. To accomplish this, the following model was derived to predict the
behavior of the microflower. At the submillimeter-scale, the folding process is the interplay
between capillary and structural forces. To fold a structure, the length of a structure needs to be
long enough so that capillary forces can overcome the bending rigidity of the structure. In terms
of energy, capillary origami is the interplay between the capillary energy of a fluid droplet and
the elastic bending energy of a structure. The total bending energy of petals can be expressed as
, where
(
) is the bending rigidity;
is Young’s modulus,
26
is the petal thickness,
Fig. 3.1(c)) of a droplet on the petals,
( )
∑
is Poisson’s ratio,
is the total length of contact line (see
is the length of a structure, and
is the radius of
curvature[6]. The capillary energy of a droplet can be considered as
surface tension,
where
is the angle of a droplet with respect to a substrate, and
is the
is the radius of
curvature[5]. We also consider the capillary energy of a droplet deposited on the petals as
()
(
). The total energy of the system can be written as
(
)
As shown in Fig. 3.1(b), we can define
(3.1)
and
angle of a petal with respect to the substrate and
, where
is the
is the angle of a droplet with respect to the
substrate. The geometric relations can be written as follows:
(3.2)
(3.3)
where
is the observed contact angle of a droplet. For this model it is assumed that the droplet
contacts the surface of the petals but does not wet the spaces between the petals. The observed
contact angle
of a fluid droplet then is given by the Cassie-Baxter equation [32]:
(
where
(3.4)
is the ratio between the surface line in contact with a droplet and the
projectedline,
line, and
)
is the horizontal radius of a droplet at the contact
is the local contact angle that the liquid forms with the petal surface. The total
volume of a droplet can be expressed as
27
where
(
(
)
) and
(
(
)
(3.5)
).
The energy of the system, expressed by Eq. (3.1), can be numerically minimized with
respect to the angle βb using equations (3.2)-(3.5) as constraints. The obtained results describe
petal behavior as a function of the contact angle, droplet size, and other system parameters. Fig.
3.2 shows the petal shape and radius of curvature R b as a function of contact angle  0 for various
droplet sizes and Cassie-Baxter f-ratios.
Figure 3.2. (a)Petal curvature radius R
b
(in units of LEC) as a function of contact angle -0,
droplet radius R drop, and Cassie-Baxter fraction f0. Dashed lines correspond to f0 = 0.6 while solid
lines correspond to f0 = 0.8. Droplet radius R drop and Cassie-Baxter fraction f0 are determined at
the state when the droplet forms an ideal sphere with the petals completely wrapping the bottom
half of the droplet. (b) Droplet and petal shapes for the cases of contact angle -0 = 70° and -0 =
145°. The results are shown for the droplet radius R drop = LEC.
28
3.4. Design
As one can see the flower petals tend to flatten (open up) as the local contact angle
decreases or the volume of the droplet decreases. The higher the value of the Cassie-Baxter
fraction , the higher the sensitivity of the flower curvature is to changes in droplet volume and
contact angle. The results of this model were then used to determine the desired dimensions of
the microflowers. Fig. 3.2(a) shows that with a elasto-capillary length [8] of
mm,
( )
mm, and
from  = 70 to  = 145.
√
, the angle of a petal increases as the contact angle increases
To achieve the maximum difference of the petal angle, an f-ratio of
0.8 was used. At  = 145b/2 = 97 and Rb = 1.35 mm; consequently Lb/2 = 1.35 x 97 x  / 180
= 2.286 mm. At least 30% of a petal was left free of liquid to reflect light, which defines the
length of petals as 2.972 mm and the width
petals was calculated from the equations
( )
as 1 mm for large flowers. The thickness of
(
)
and
√
. If
mm; water is used as the droplet ( = 72 mN/m); and the petals are made of polycrystalline
silicon (E = 169 GPa,  = 0.22)[131], the thickness would need to be 1.5 m. Small flowers with
petals 0.5 mm wide, 1.171 mm long, and 1.5 m thick were also fabricated.
3.5. Fabrication
The microflower structures were fabricated by releasing planar polycrystalline silicon
layers patterned to resemble the petals of a flower. The petals were anchored in the center
contacting the silicon substrate. First, an array of microposts (2 µm diameter with 10 µm pitch,
100 nm deep, often called nanograss [41]) was etched into the surface of the Si substrate using a
Si3N4 layer (270 nm thick deposited using LPCVD at 650C) as a hardmask. After photoresist
29
removal, a 2.40.2 µm sacrificial Si oxide layer (LTO) was deposited (LPCVD 450C) on top of
the Si3N4 hardmask. Contact holes were then etched through the LTO and Si3N4 layers into the
Si substrate to create the stems of the microflowers. To form the petals of the flowers a
polycrystalline silicon layer 1.30.3 µm thick was deposited on top of the LTO. Deep Si etching
(the Bosch process) was used to pattern the array of flowers stopping on the LTO layer. The
exposed LTO was then etched (without removing the photoresist used to pattern the flowers)
with 6:1 buffered HF exposing the underlying Si3N4 hardmask. At this point the polycrystalline
silicon petals are mostly unreleased. The nanograss structure was then created by continuing the
deep Si etch with the photoresist protecting the flowers. To control excessive curling and
deformation of the petals, the unreleased structures were annealed at 750C in forming gas for 6
h. The residual stress in the polycrystalline silicon was large enough to significantly deform the
petals after release for the unannealed structures. After release (in 49% HF) the wafer was rinsed
in DI water, which was then replaced with isopropyl alcohol (IPA). The IPA was removed by
rapid evaporation at 150C. For electrowetting, a SiO2 dielectric layer with a thickness of 102
nm was thermally grown (O2, 950C) to completely incase the polycrystalline silicon flowers. To
control curling of petals due to thermal stress produced by the oxide growth, a quartz wafer was
placed on top of microflowers using Si spacers to form a 1 mm gap. Finally, a CFx hydrophobic
coating was deposited using plasma deposition from C4F8.
3.6. Results and Discussion
Microflowers resulting from this fabrication process are shown in Fig. 3.3. An array of
20 um x 20 um holes, spaced 0.25 mm apart were etching into the petals to decrease the HF
release time (Fig 3.3(b)). The arrays of micron-size holes on petals were designed to allow the 1
30
mm and 0.5 mm petals to be released in about the same time. The cross-section of a petal on the
LTO layer before release is shown in Fig. 3.3(c) next to an array of a Si nanograss patterned into
the Si substrate. The center stem of a microflower can be seen in Fig. 3.3(d). This design
allowed the center portion of the flowers to remain connected mechanically and electrically to
the Si substrate, even after the petals were released.
The array of nanograss (100 nm post height) etched into the Si substrate before sacrificial
oxide and polycrystalline silicon deposition greatly helped to reduce the problem of stiction,
which occurred after petal release and drying. The nanograss textured Si substrate not only helps
to prevent stiction, but also creates a superhydrophobic surface surrounding the microflowers,
which helps to capture the deposited droplet in the center of the microflower as shown in Fig.
3.4(a). As shown in Fig. 3.3(c) the posts are etched much more deeply into the Si surface around
the petals than under them. The taller posts, after depositing a thin CFx layer, create a
superhydrophobic surface by causing a water droplet to remain in a stable Cassie-Baxter state
(contact angle 157±3), where the water droplet only contacts the tips of the posts without
wetting the rest of surface [32, 35, 41]. The superhydrophobic surface is much more water
repellant than the center of the flower causing the droplet to be pinned without wetting the
surrounding substrate surface.
31
Figure 3.3. (a) Optical image of the top view of a microflower. (b) Part of a petal before release.
(c) SEM image of the cross-section of a petal on LTO layer next to an array of Si nanograss
before sacrificial etch. (d) The stem of a microflower anchored to the substrate. (e) Optical
images of microflowers on nanograss substrate before oxidation and (f) after oxidation.
Two methods of petal actuation were demonstrated as indicated from our model: one by
changing the volume of the liquid droplet and the other by using the electro-wetting process.
As shown in Fig 3.4(b) (the entire bending process was video recorded), when a droplet (up to
about 3 L) was deposited on the microflower by using an automated syringe pump, the petals
curled up away from the substrate and cured down towards the substrate as the liquid was
withdrawn. The petal angle could be reversibly changed about 10±1 in agreement with our
model. In principle, the volume of a liquid droplet can be changed by placing it in a humidity
controlled chamber. As the humidity changes, the volume of the droplet will change because of
evaporation or condensation [132].
32
For the case of electrowetting the capillary force exerted by the water droplet on the petal
can be changed by changing its contact angle. In classical electrowetting, the contact angle can
be changed by applying a voltage to a conducting liquid placed on a dielectric film coated
conductive substrate. The basic equation for electrowetting [27] can be expressed as
, where
angle without applied voltage,
an applied voltage, and
is the contact angle with an applied voltage,
is vacuum permittivity,
is the contact
is the film dielectric constant,
is
is film thickness. The electrowetting equation predicts that
changes with the square of the
and that for relatively thin dielectric layers (about 100 nm)
significant changes in contact angle can be observed by applying
on the order of 101 - 102 V.
For classical electrowetting, the dielectric layer and the electrode are rigid and flat, but for the
microflowers, as shown schematically in Fig. 3.4(c), the SiO2 dielectric layer, the polycrystalline
silicon petal electrodes, and the liquid droplet are flexible and movable. As shown Fig. 3.4(d)),
the angle of the petals was controlled by applying a voltage of 140 V between the captured liquid
droplet and a large microflower and the Si substrate. The advancing contact angle of the liquid
droplet on the petal surface changed from about 120 to about 100 when the voltage was
applied. This change in the contact angle of about 20 resulted in about 10 change in the angle
of the petals in agreement with the model.
33
Figure 3.4. (a)A microflower with a captured microdroplet. (b) Dependence of petal angle on
liquid volume (only half of the frame is shown to facilitate comparison). As the amount of liquid
captured by a microflower decreases the petal angle also decreases. (c) Schematics of an
electrowetting process. Blue represents a droplet; the red line represents the dielectric layer,
which surrounds the polycrystalline silicon petal; and the yellow area shows the polycrystalline
silicon microflower. (d) Electrowetting actuation of a microflower. The Image shows the petal
position without applied voltage, and the red dash lines indicate the petal positions with the
voltage applied.
34
3.7. Summary
In summary, we have demonstrated the fabrication of microflowers, which can be
dynamically tuned to change their reflective color and iridescence. The structures have been
fabricated using capillary origami approach where the capillary forces of a liquid droplet are
used to actuate petal movement. Two methods of actuation were demonstrated and quantitatively
characterized: (1) by changing the volume of the liquid droplet and (2) by changing its contact
angle on the petals using the electrowetting process. A model was derived to characterize the
dynamics of the actuation process. In general good agreement with the results from petal
movement was obtained.
35
CHAPTER 4. LOTUS LEAF-INSPIRED SUPERHYDROPHOBIC AND
TRANSPARENT TA2O5NANOSTRUCTURED THIN FILMS3
“Many of life’s failures are experienced by people who did not realize how close they were to
success when they gave up.”
- Thomas Alva Edison
4.1. Abstract
Transparent Ta2O5 nanostructured thin films have been fabricated using a multi-step
anodization process. Obtained by a combination of the nanostructured surface and the deposition
of the hydrophobic CFx coating, the transparent films can be made highly water repellent or
superhydrophobic useful for self-cleaning and anti-fogging optical coatings. Contact angle
measurements and optical transmittance curves of the nanostructured films are in good
agreement with theoretical calculations.
4.2. Introduction
In nature there are many examples of highly textured or superhydrophobic surfaces that
easily shed water. These surfaces typically consist of micron and submicron structures with low
surface energy coatings. When water contacts the surface, a highly mobile rolling ball is formed
with a contact angle greater than 150. Plants such as the lotus leaf (Nelumbonucifera) have
evolved these surfaces for self-cleaning, as the rolling water droplet collects particulates as it
falls from the leaf [133]. By removing water in a similar manner, the surface can have anti3
Copyright © 2012 Optical Materials Express. Reprinted with permission from Manakasettharn, S., Hsu, T.-H.,
Myhre, G., Pau, S., Taylor, J. A., and Krupenkin, T., 2012, "Transparent and Superhydrophobic Ta 2O5
Nanostructured Films," Optical Materials Express 2(2), pp.214-221.
This work has been supported by the United States Air Force Office of Scientific Research (AFOSR) MultiUniversity Research Initiative (MURI) Program Award # FA9550-09-1-0669-DOD35CAP.
36
fogging properties. Recently it was shown that water droplets impinging on superhydrophobic
surfaces at below freezing temperatures are able to recoil from the surface before freezing, thus
preventing ice to accumulate [134]. Transparent superhydrophobic surfaces with self-cleaning
and anti-fogging properties are especially useful for glass windows and optical coatings for
lenses. Various methods for preparing transparent superhydrophobic surfaces have already been
reported [135-155]. In addition to being transparent, superhydrophobic optical coatings should
possess feature sizes of less than 200 nm in order to avoid scattering loss of visible light. It is
also desirable that the film can be conformally coated on curved or complex topographical
surfaces. The superhydrophobic Ta2O5 optical thin film, which is the subject of this work,
possesses both of these properties.
Tantalum pentoxide (Ta2O5) is a transparent, low absorption, high refractive index
material, which can be utilized as optical coatings from near ultraviolet (350 nm) to infrared (> 8
µm). The refractive index of Ta2O5 is in the range of 1.65-2.3 [156-159] and depends on
preparation techniques.
It has been extensively studied because of its promising optical,
physical, chemical and electrical properties. The material is robust, has melting temperature of
greater than 1785 °C [160, 161], and is useful for applications requiring high temperature. It is
insoluble in water, ethanol and most acids, excluding hydrofluoric acid [160]. Its dielectric
constant of greater than 22 [162, 163] is useful for electronic applications such as capacitors.
Many different preparation techniques of Ta2O5 have been reported [164], none of which
describe the fabrication of superhydrophobic surfaces.
37
4.3. Experimental Section
The transparent Ta2O5 nanostructured thin films were fabricated using a multi-step
anodization process of tantalum (Ta) thin films as shown in Fig. 4.1. The Ta layer (75 nm thick)
was sputter-deposited (DC power 1 kW, 5 mT Ar pressure) on a cleaned quartz wafer. An
aluminum (Al) layer 600 nm thick then was sputter-deposited (DC power 1 kW, 8 mT Ar
pressure) on top of the Ta layer without breaking vacuum. The anodization process to form the
porous Al layer was similar to recipes which have previously been published [165-167]. The Al
layer was anodized in 0.2 M oxalic acid with a constant current density of 10 mA/cm 2 while
ramping the voltage up to 53 V. As soon as 53 V was reached, the anodization process was
stopped preventing the thin Al film from being completely anodized. The thin top layer of Al 2O3
was stripped in a chromium trioxide (CrO3) / phosphoric acid (H3PO4) solution (2 g CrO3 / 100
ml H2O, 4.1 ml conc. H3PO4) at 70 C for 15 min. The surface of the remaining Al layer is
shown in Fig. 4.2(a). The remaining Al layer was reanodized by sweeping the voltage from 0 V
to 45 V (1 V/s) in 0.0005 M oxalic acid, and then the Al layer was reanodized in 0.2 M oxalic
acid at 53 V with constant current density of 10 mA/cm2 to completely oxidize the Al as shown
in Fig. 4.2(b). When the applied voltage was stable at 53 V and the current density dropped to
0.5 mA/cm2 or lower, the voltage was then increased to 200 V for 12 min to grow Ta2O5 posts in
the Al2O3 pores as shown in Fig. 4.2(c). After that, the Al2O3 layer was stripped in the
CrO3/H3PO4 solution at 70 C for 120 min. The remaining metallic Ta layer was reanodized in
0.2 M oxalic acid ramping the voltage to 200 V with a constant current density of 0.2 mA/cm2.
Complete oxidation of the film occurred at a voltage below 200 V, at which point the current
would decrease.
This process formed a transparent textured Ta2O5 thin film on a quartz
38
substrate. To make the structure superhydrophobic a thin CFx film was deposited using plasma
excitation of C4F8.
Figure 4.1. Schematics of the multi-step anodization process of an Al-Ta bilayer.
Contact angles of water droplets were measured using an automated drop shape analyzer
(FTA1000 C Frame, First Ten Angstroms, Portsmouth, Virginia, US). Contact angles were
measured at five different points. Advancing and receding contact angles were measured by
video recording the change in drop shape as the drop, attached to the injection needle, was
dragged across the moving surface. The nanostructure was observed by SEM (Zeiss 1500XB).
39
The transmittance spectrum was measured with UV/VIS/NIR spectrometer (Lambda 900, Perkin
Elmer, Waltham, Massachusetts, US).
4.4. Results and discussion
The final structure, as shown in SEM micrographs in Fig. 4.2(d) and 4.2(e), consists of a
layer of Ta2O5 nanoposts (often called nanograss [41]) on top of a continuous Ta2O5 layer
supported by a quartz substrate. The Ta2O5 nanoposts are about 40 nm in diameter and 200 nm
in height. The thickness of the continuous Ta2O5 layer is 200 nm. The height of the nanoposts is
controlled by the anodization voltage. The thickness of the continuous Ta2O5 layer is controlled
by the initial thickness of the deposited Ta layer. Post diameter and density are somewhat
dependent on anodization conditions used to form the porous Al2O3 mask.
Figure 4.2. Top view SEM images of (a) the Al surface after the first Al 2O3 layer was stripped,
(b) the Al2O3 porous layer, (c) Ta2O5 nanoposts grown in the Al2O3 pores, and (d) the final
nanostructured Ta2O5 thin film after the Al2O3 porous layer was stripped. (e) The cross-section
view of the final nanostructured Ta2O5 thin film.
40
Figure 4.3. (a) A clean Ta2O5 nanograss surface which is highly hydrophilic showing a contact
angle of less than 3. (b) The same surface rendered superhydrophobic by depositing a CFx
coating showing a contact angle 155±2 with a hysteresis of 20.
(c) The transparent
superhydrophobic nanograss film with two water droplets deposited on the surface.
The Ta2O5 nanograss surface, which was cleaned using O2 plasma, is highly hydrophilic.
When a 10 L water droplet was deposited on the surface, the diameter of the spreading droplet
was about 10 mm. The water contact angle was estimated to be about 2.9 when assuming the
spreading droplet possessed a hemispherical shape (see Fig. 4.3(a)). Surfaces with contact
angles less than 5 are often defined as superhydrophilic. However, the contact angle of a water
41
droplet on a cleaned non-textured Ta2O5 surface was also less than 5. The Ta2O5 nanostructured
surface was easily made superhydrophobic by depositing a low surface energy CF x coating using
plasma deposition from C4F8. As shown in Fig. 4.3(b), the observed contact angle of water on
the Ta2O5 nanostructured surface with the hydrophobic coating was 155±2 with a hysteresis of
20±2, as compared with the water contact angle of 107±2 for a non-textured surface with the
same hydrophobic coating. The contact angle hysteresis [168] is the difference between the
advancing angle and the receding angle which is observed when the front contact line of a
droplet advances, and its back contact line recedes. If the hysteresis is high, a droplet will tend
to be pinned on a surface. In our case, the water droplet on the textured surface was highly
mobile typical of superhydrophobic surfaces. The observed contact angle
of 155 is in good
agreement with the contact angle predicted by the Cassie Baxter equation [32]:
(
where
and
)
(4.1)
is the ratio between the surface area in contact with a droplet and the total projected area
is the local contact angle or the contact angle formed on the posts. For our case, is
approximately equal to 0.126, and
is estimated to be ~107o or the contact angle observed on a
similar flat surface. The observed contact angle can be increased by decreasing the diameter of
the nanoposts or by increasing the average distance between them.
The transmittance ( ) of the transparent multilayered Ta2O5 film (as shown in Fig.
4.3(c)) can be estimated by calculating the reflectance (
) and subtracting it from one, assuming
the absorption is small. For light at normal incidence, the observed reflectance is given by [169]
(
)
(4.2)
42
where
is the amplitude and
is the phase of the light wave and (
) is the complex
conjugate. In this case of three interfaces (i.e. by a double layer), the reflectance is given by
[169]
(
)
(
where
(4.3)
)
is the amplitude of the reflectance at the interface ,
phase change of the light wave at the interface ,
medium , and
= 0, 1, 2, 3,
is the
is the refractive index in
is film thickness of layer . In our case, we assumed the first, second, third,
and fourth layers are air, Ta2O5 nanograss, Ta2O5 continuous, and air layers, respectively. The
500 m thick quartz substrate was not included in the calculation since its thickness is much
larger than the coherence length of the visible light source (here a tungsten halogen lamp), which
is just a few micrometers and within which thin-film interference is still strong. The refractive
indices of air and Ta2O5 continuous layer are well known. The effective refractive index of
Ta2O5 nanograss layer was calculated from [170]
√
where
,
is the wavelength of light,
(4.4)
is the permittivity of free space,
is the
effective absorption coefficient of the medium. The effective dielectric function of the medium
can be approximated by [170]
(4.5)
where
and
are volume fraction and dielectric functions of air medium and Ta2O5 medium.
43
Figure 4.4. (a) The predicted transmittance spectra of two films with different layer thickness:
the red curve is for a film consisting of a continuous Ta2O5 layer 300 nm thick and a Ta2O5
nanograss layer of 90 nm thick and the blue curve is for a film with a Ta2O5 continuous layer of
410 nm and a nanograss layer of 90 nm. (b) Measured transmittance spectra of quartz (green)
and transparent nanograss films (blue, green) with the same dimensions.
The calculated transmittance of the Ta2O5 bi-layer film is shown in Fig. 4.4(a). Two
cases are shown for films with 90 nm thick nanograss layer on top of a continuous Ta2O5 layer
44
with thickness of 300 nm and 410 nm. The measured transmittance of quartz and the two Ta2O5
films are shown in Fig. 4.4(b).
Good qualitative agreement between the calculated and
measured transmittance curves was obtained. Similar calculation shows that the thickness of the
continuous Ta2O5 layer has the major effect on the reflectance as compared to the nanograss
thickness. About 10% of the incident light is reflected at the quartz Ta2O5 interface which is not
taken into account in the calculated curves. For much of the visible range the index of refraction
for Ta2O5 remains relatively constant. For example, if the index of refraction of Ta2O5 at 550 nm
is taken as 1.807 and the volume fraction of the nanograss as 0.15, the calculated index of
refraction of the nanograss layer is 1.103.
The calculated index of refraction of the nanograss layer from Eq. (4.4) is almost the
same as the index of refraction of air since most of the volume of the nanograss layer consists of
air. Consequently, the nanograss layer does not have a major influence on the optical
transmittance spectrum based on the calculation from Eq. (4.2) and Eq. (4.3). One may vary
anodization conditions to increase the volume fraction of the solid nanograss resulting in the
increase of the index of refraction of the nanograss layer. If its index of refraction is high
enough, the nanograss layer may affect the optical properties of Ta2O5 nanostructured thin films.
However, an increase of the volume fraction of the nanograss also would affect the observed
contact angle of water on the Ta2O5 nanostructured surface as indicated by Eq. (4.1). To
maintain superhydrophobic surfaces (
at
),
limits the maximum index of refraction of the nanograss layer.
should be less than 0.189 which
45
4.5. Conclusion
In conclusion, we have demonstrated the fabrication of transparent Ta2O5 nanostructured
thin films by using a multi-step anodization process of sputter-deposited Ta thin films. The
transparent films can easily be made highly water repellent or superhydrophobic by depositing a
low surface energy coating. The films have been characterized by measuring water contact
angles and by obtaining optical transmittance spectra and SEM micrographs. The measured
contact angles and transmittance curves are in good agreement with calculations. Our simple
low-cost process can potentially be used on an industrial scale to fabricate superhydrophobic
transparent Ta2O5 nanostructured thin films to create durable optical coatings with self-cleaning
and anti-fogging properties.
46
CHAPTER 5. INTERPLAY BETWEEN IRIDESCENT AND
OMNIDIRECTIONAL COLORATION IN BIO-INSPIRED
ELECTRICALLY-TUNABLE NANOSTRUCTURES4
“Just because something doesn’t do what you planned it to do in the first place doesn’t mean it’s
useless….”
- Thomas Alva Edison
5.1. Abstract
We have investigated a novel nanostructured film consisting of tantalum pentoxide
(Ta2O5) nanograss formed on top of a continuous Ta2O5 layer on a reflective Ta thin-film. The
nanograss layer exhibits short-range order and produces non-iridescent coloration, which is
confirmed by the predicted two-dimensional discrete Fourier transform (2D DFT) power spectra
and our developed model. The underlying continuous Ta2O5 layer provides long-range order and
produces iridescent coloration, which is confirmed by the measured reflectance spectra. When
the nanograss surface is wetted with organic liquids, non-iridescent blue coloration obscures the
thin-film interference color. Moreover, this blue color can be dynamically and reversibly
switched on and off by applying electrical current to an indium-tin-oxide (ITO) electrode
positioned on top of the nanograss surface. This work forms the basis for future development of
optofluidic devices, especially in the field of display technology.
4
Copyright © 2013 Manakasettharn, S., Hsu, T.-H., Myhre, G., Pau, S., Taylor, J. A., and Krupenkin, T., "Interplay
between Iridescent and Omnidirectional Coloration in Bio-Inspired Electrically-Tunable Nanostructures,”in
preparation.
This work has been supported by the United States Air Force Office of Scientific Research (AFOSR) MultiUniversity Research Initiative (MURI) Program Award # FA9550-09-1-0669-DOD35CAP.
47
5.2. Introduction
The bright coloration, found in a broad range of different biological species from frogs, to
birds, to insects, to cephalopods, often is created by the interplay between coherent (phase
dependent) and incoherent (phase independent) light scattering from nano-scale structural
elements. In particular it has been demonstrated that both iridescent and non-iridescent color can
be generated by coherent scattering [171]. For iridescent structures the color of the light changes
when viewed at different angles, while for non-iridescent structures color does not change with
viewing angle. Structures that exhibit only short-range order tend to produce non-iridescent
color. To the contrary, well ordered structures, such as laminar reflective arrays in avian feathers
and butterfly wings generate strongly iridescent coloration [171].
Not surprisingly, many
brightly-colored biological tissues exhibit nanostructures with elaborate combinations of both
short and long-range order. Such a combined approach provides great flexibility in selecting the
most evolutionary and advantageous coloration strategy.
Besides color production, micro and nanostructured surfaces are commonly used by
insects for a totally different purpose that is to control wettability. In particular, butterfly and
cicada wings [172], Namib Desert beetle elytra [173], and water strider legs [174, 175] are
reported to have superhydrophobic properties.
Inspired by this multifunctional approach,
encountered in nature, we have investigated the interplay between the short-range and the longrange order in the structural color generation exhibited by the artificial nanostructured surfaces.
Moreover, the interplay between optical properties of the nanostructured surfaces, and their
wetting behavior, has been investigated.
48
5.3. Experimental Section
To this end we developed a novel nanostructured film consisting of tantalum pentoxide
(Ta2O5) nanograss formed on top of a continuous Ta2O5 layer [176]. A process to fabricate the
nanostructured film deposited on a Si substrate was developed and has already been described in
Chapter 4. The nanograss structure was made by anodic oxidation of Al-Ta bilayers. The
height and spacing of the posts were controlled by the anodization conditions. The dimensions
of the nanostructure were obtained from SEM (Zeiss 1500XB) micrographs.
The nanograss film was fabricated and was cleaned with O2 plasma. After that, droplets
of various organic liquid, such as methanol, ethanol, isopropyl alcohol (IPA), acetone, toluene,
and chloroform, were deposited on the nanograss surface. As the liquid drop evaporated, a thin
uniform liquid film was formed within the nanostructure, creating a blue colored spot. When the
color of the liquid film was uniform, a quartz wafer was positioned on top of the liquid film and
the nanograss surface to reduce the evaporation rate of the thin liquid film. The transmittance
spectra of dry nanograss and of the blue spot formed by various kinds of droplets were measured
with UV/VIS/NIR spectrometer (Lambda 900, Perkin Elmer, Waltham, Massachusetts, US).
Furthermore, dynamic color control of the blue spot was investigated by using the
following procedure. When the color of the organic liquid film, such as methanol, was uniform,
an indium-tin-oxide (ITO) electrode was positioned on top of the methanol film and the
nanograss surface. Electrical current was then applied to the 1 mm wide ITO electrode to control
coloration of the methanol film on the nanograss surface by controlling the thickness of the
liquid film. To create the optimal lighting conditions for the blue spot observation, the fiber
49
optic light (Schott KL 2500 LCD, Germany with Osram 64653 250W/24V GX5.3 lamp) was
used.
Figure 5.1. (a) Schematic illustrating light interference and scattering at Ta2O5 nanograss film.
(b) SEM cross-section image of the nanograss structure. (c) Top view optical image of yellow
nanograss structure.(d)Top view SEM image of the Ta2O5 nanograss. (e) Optical image of
methanol spot on the Ta2O5 nanograss substrate in ambient light. (f) Optical image of methanol
spot taken with intense backscattered illumination.
5.4. Results and Discussion
The nanostructured film integrates two major types of structural elements, an array of 50100 nm diameter Ta2O5 posts, which exhibits only short-range order and underlying laminar
thin-film stack, which provides long-range order, as shown in Fig. 5.1(b),(d). The incident light
interacts with both types of structures simultaneously generating a combination of iridescent and
omnidirectional coloration (Fig. 5.1(a)). The iridescent color of the dry nanograss sample is
yellow, as shown in Fig. 5.1(c).
This yellow appearance is predominantly determined by
interference of light with the underlying film stack, which consisted of 120 nm thick Ta2O5 layer
50
deposited on a reflective Ta thin-film, while the top layer of nanograss contributed little to the
observed color.
The reflective Ta layer tends to enhance any color produced on the
nanostructured film surface. The structure of the entire stack is further confirmed by comparing
the nanostructured film spectrum shown in Fig. 5.2 with the yellow appearance and reflectance
spectrum of a planar 120 nm thick Ta2O5 film (refractive index: n =1.78-1.87 in visible light)
anodically grown on top of a reflective Ta layer.
Figure 5.2. Measured reflectance spectra of planar Ta2O5 film, dry Ta2O5 nanograss, and wet
blue spots of methanol, ethanol, IPA, acetone, toluene, and chloroform deposited on the Ta2O5
nanograss substrate.
One of the most interesting properties of this film is a profound dependence of its optical
properties on the nanograss wetting state. In particular, when wetted by an organic liquid, such
as methanol (n = 1.31-1.37), the nanograss surface exhibits light blue coloration in ambient light
and intense blue coloration when directly illuminated with a fiber optic light, as shown in Fig.
5.1(e),(f). The nanograss surface was wetted with a number of other organic liquids besides
methanol: ethanol (n = 1.36-1.37), IPA (n = 1.37), acetone (n = 1.35-1.36), toluene (n = 1.5), and
51
chloroform (n = 1.45-1.5), all of which exhibited the same blue coloration similar to methanol, as
demonstrated by comparing their reflectance spectra shown in Fig. 5.2.
Figure 5.3. (a) 2D DFT of the Ta2O5 nanograss (shown in Fig. 5.1(d)). (b) Radial average of the
power spectrum. (c) Scattered spectra predicted from the Fourier analysis of the SEM image
(Fig. 5.1(d)). (d) Scattering spectra obtained from the reflectance measurements.
What is the physical nature of the observed blue spot? We observe that the blue color of
the wet spot does not change with viewing angle and that the dry structures show only a yellow
interference color. This indicates that the blue spot may be caused by the back scattered light
from nanograss filled with the organic liquid. This model was tested by theoretically analyzing
coherent light scattering by the Ta2O5 nanograss array by implementing the nano-optic Fourier
approach, developed by Prum and Torrest [171]. Our calculations were carried out using the
52
computational software program Mathematica [177] as follows. First, the top view SEM image
of the Ta2O5 nanograss or nanopost array (Fig. 5.1(d)) was analyzed with the two-dimensional
discrete Fourier transform (2D DFT) to convert the original spatial domain of the nanopost array
to the frequency domain. 2D DFT represents the image as a function which contains coefficients
of a finite combination of sinusoids. The result from the 2D DFT analysis of the SEM image are
presented as the 2D DFT power spectrum (Fig. 5.3(a)) reporting the magnitude of the periodicity
of nanopost distances and diameters of a specific spatial frequency in all directions within the
original image. The power spectrum is the squared magnitudes of the complex coefficients. The
ring-shaped distribution in the 2D DFT power spectrum demonstrates the equivalency of spatial
frequencies of the nanopost periodicity and confirms the absence of the long-range order. The
distribution of power or density among different spatial frequencies in Fig. 5.3(a) can be plotted
as the radial average of the power spectrum shown in Fig. 5.3(b). To express the results in terms
of wavelength, the spatial frequency is inverted and then multiplied by twice the weighted
average refractive index. A weighted average refractive index for the dry nanograss is calculated
by assigning the refractive index of Ta2O5 to the nanoposts and the refractive index of air to the
surroundings. Analysis of the 2D DFT power spectrum (green dot and dash line) shown in Fig.
5.3(c) indicates that for the dry nanograss film the maximum scattering occurs in the UV region
(100 nm – 200 nm wavelength). Thus, the iridescent color of the dry samples is predominantly
determined by interference of light with the underlying laminar Ta2O5thin-film stack. The
situation is very different for the cases when the nanopost array is wetted by an organic liquid,
such as methanol. The presence of the liquid changes the weighted average refractive index of
the nanograss film (since there is a large difference in refractive index between methanol and air)
53
and shifts the scattering peak of the 2D DFT power spectrum (blue dot and dash line) shown in
Fig. 5.3(c) towards the visible wavelength region. This causes non-iridescent blue coloration
that obscures the interference color.
We have confirmed that the nanopost array exhibits short-range order from the predicted
scattering 2D DFT power spectrum and that the underlying Ta2O5 thin-film stack exhibits the
long-range order from the measured reflectance spectrum. We can also predict the scattering
spectrum of the dry nanopost array and the blue spot by deconvoluting the measured reflectance
spectra of the nanostructured film and the planar Ta2O5 thin-film stack. As shown in Fig. 5.1(a)
we assume that there is no absorption in the nanostructured film, and we assume that the
scattering loss occurs twice, when the light enters and exits the nanostructured film. We then
model the reflectance of light from the nanostructured film by taking into account that light is
lost by scattering both while entering and while exiting the film
(
where
)
(5.1)
is the ratio of the interference light intensity ( ) to incident light intensity ( ) and
is the ratio of the scattered light intensity ( ) to the incident light intensity ( ). For thin-film
interference, we used the measured reflectance spectrum (black line) of the 120 nm planar
Ta2O5layer on a reflective Ta thin-film as shown in Fig. 5.4(a),(c). To match the predicted
reflectance spectra (green line in Fig. 5.4(b) and intense blue line in Fig. 5.4(d)) with the
measured reflectance spectra (orange line in Fig. 5.4(b) and light blue line in Fig. 5.4(d)) of the
nanostructured film, we select the fitting function that is similar to scattered spectra from the
Fourier analysis of the SEM image as
( ⁄ ) [
((
)⁄ )
]
(5.2)
54
where
is wavelength, for dry nanograss scattering
and for wet nanograss scattering
. The results
of predicted scattered spectra by our model are shown in Fig. 5.4(a),(b) and Fig. 5.3(d), which
are in good agreement with the predicted scattered 2D DFT power spectra in Fig. 5.3(c). One
can use this model to predict the scattered spectrum of the nanostructured film by deconvoluting
the measured reflectance spectra of the nanostructured film and the planar Ta2O5 thin-film stack.
Figure 5.4. (a) The predicted scattered spectrum (green) of the dry nanograss film and the
measured reflectance spectrum (black) of the planar Ta2O5 thin-film stack. (b) The measured
(orange) and predicted (green) reflectance spectra of the Ta2O5 nanograss. (c) The predicted
scattered spectrum (intense blue) of the blue spot and the measured reflectance spectrum (black)
of the planar Ta2O5 thin-film stack. (d) The measured (light blue) and predicted (dark blue)
reflectance spectra of the wet blue spot of methanol on Ta2O5 nanograss.
55
The interplay between non-iridescent coloration produced by the nanograss layer and
iridescent coloration produced by the underlying laminar Ta2O5 thin-film stack potentially opens
a possibility of a dynamic color control. Indeed, the blue color of methanol deposited on the
nanostructured film can be dynamically and reversibly switched on and off by applying electrical
current to an ITO electrode positioned on top of the nanograss surface as shown in Fig. 5.5. This
allows reversible switching of the sample color from non-iridescent blue coloration to the
iridescent color of the dry film originating from thin film interference.
How exactly does the wetting control operate? As shown in Fig. 5.5(a), when the
electrical current is not applied to the ITO electrode placed on top of the nanograss surface, the
methanol inside the nanograss layer is uniformly distributed as indicated by a uniform blue
coloration which is only observed for thin methanol layers within the nanograss. When the
electrical current is applied to the ITO electrode, it seems that the methanol flows underneath the
ITO electrode. The heated ITO electrode may create a thermal gradient which causes a variation
in the surface tension of the methanol, and this may induce the methanol to flow into the hot
nanograss directly underneath the ITO electrode as shown in Fig. 5.5(b). This thermal-gradientinduced flow may be related to the phenomenon of thermocapillarity [178-185]. To further test
whether the origin of the electrically induced color switching mechanism is indeed the
thermocapillary flow, the beam of the fiber optic light was used to heat the nanograss and the
methanol film, also resulting in the reversible switching of the blue color. This simple test also
indicates that the electrically induced color switching mechanism is due to the thermocapillary
flow.
Further research in this area will be focused on obtaining a better quantitative
understanding of the physics of the electrically-controlled film wetting.
56
Figure 5.5. (a) Electrically-induced switching of the methanol-induced coloration. (b)
Schematics illustrating how the methanol distributes inside the nanograss layer while the electric
current applied to an ITO electrode is off and is on.
5.5. Summary
We have investigated iridescent and non-iridescent color of a nanostructured Ta2O5 film
when it is dry and when it is wetted with organic liquids. We have demonstrated that noniridescent coloration is produced by the nanograss layer. This is confirmed by analysis of the
predicted scattered 2D DFT power spectra and by the theoretical model which we developed,
while iridescent coloration is produced by the underlying laminar Ta2O5 thin-film stack as shown
by the measured reflectance spectra. We also have demonstrated the reversible switching of the
sample color from non-iridescent blue coloration to iridescent color, produced by thin-film
interference, by applying electrical current to an ITO electrode positioned on top of the
nanograss surface to induce thermocapillary flow of methanol inside the nanograss layer.
57
CHAPTER 6. CONCLUSIONS AND PROPOSED FUTURE WORK
“I shall never be content until the beneficent influence of the University
reaches every home in the state.”
- Charles Van Hise
6.1. Conclusions
Novel micro/nanostructures and optofluidic devices inspired by dynamic tunable
iridescence of cephalopods in Chapter 3, self-cleaning property of lotus leaves in Chapter 4,
and structural color of some biological species in Chapter 5 have been successfully fabricated
and characterized.
In Chapter 3, polycrystalline Si microflowers have been modeled, designed and microfabricated. Actuation of the petal movement is based on the interplay between the elastic forces
of petals and the capillary forces of a liquid droplet. Dynamic control of the petal movement can
be achieved by changing the volume of the droplet or by changing the droplet contact angle
using the electrowetting process.
In Chapter 4, transparent Ta2O5 nanostructured thin films have been fabricated using a
multi-step anodization process.
The films have been made highly water repellent by a
combination of their nanostructured surface and the deposition of the hydrophobic coating to
create the self-cleaning property, similar to lotus leaves. Theoretical calculations, in agreements
with the experimental measurements, indicate that the thickness of the continuousTa2O5layer,
located between the nanostructured Ta2O5 layer and the quartz substrate, dominates the
transmittance of the entire film stack, while the size and density of the Ta2O5nanostructure
determines the water contact angle and wetting properties of the surface. The thickness of the
58
film and the height, size and density of the nanoposts can be controlled by the anodization
conditions. The superhydrophobic transparent Ta2O5 nanostructured thin films can be used to
create durable optical coatings with the self-cleaning properties.
In Chapter 5, reflective Ta2O5 nanostructured thin films have been fabricated using the
same multi-step anodization process as described in Chapter 4, except that the sputter-deposited
Ta layer deposited on a Si substrate has not been completely anodized leaving a thin Ta metallic
layer. This reflective Ta layer tends to enhance any color produced on the nanostructured film
surface.
The films have been made superhydrophilic by exposing their surface to oxygen
plasma. The nanostructured surface with a yellow color appearance, resulting from the thin-film
interference, has been wetted by an organic liquid, such as methanol, creating a uniform intense
blue colored spot, resulting from light scattering. Due to the phenomenon of thermocapillarity,
the blue spot can be reversibly switched on and off by applying electrical current to heat an ITO
electrode positioned on top of the nanostructure surface.
The results of this work establish three new approaches to manipulate light. Chapter 3
uses capillary forces of a fluid droplet to actuate microstructures that are capable of creating
tunable iridescence. Chapter 4 describes the fabrication of transparent nanostructured thin films
which are made highly-water repellent for application as self-cleaning optical coatings. In
Chapter 5 the interplay between iridescent and non-iridescent color produced by nanostructured
thin films was investigated. Methods to control wettability were developed allowing reversible
control of dynamic color change. This work forms the basis for future development of a broad
range of novel optofluidic devices.
59
6.2. Proposed Future Work
6.2.1. Microflowers
6.2.1.1. Textured diffraction grating on petals
The structure of the microflowers can be modified in several ways to improve their
capabilities to manipulate light and to change color. The top surface of the petals can be textured
to create a diffraction grating, which can be dynamically tuned to change reflective color and
iridescence, imitating dynamically tunable iridescence produced by cephalopod iridophores.
Two different types of patterns of diffraction gratings may be used. First, a concentric circular
pattern of lines may be textured on each petal surface. The processing of the diffraction grating
would occur after the polycrystalline silicon (poly-Si) has been deposited. A patterned SiO2
layer would act as a mask. After the petals are etched, and the photoresist is stripped, the
diffraction pattern can be etched into the poly-Si petals. In this manner, the apparent color of the
microflowers will change as the angle of the petals is changed due to the diffraction
phenomenon. Second, a square grid pattern may be textured in the same way on each petal to
create periodic distribution of the color spots, which would change their position with the petal
angle.
6.2.1.2. Optimized petal movement
Careful modeling of the system to determine the precise dimensions of the petals is
required since the system is quite sensitive to the correct balance between capillary forces of a
droplet and mechanical bending forces, mainly determined by the dimensions of the petals. If
the petals are too long and thin, capillary forces of a droplet will be too high, resulting in the
60
complete wrapping of petals around the droplet. On the other hand, capillary forces will not be
large enough to actuate the petal movement if the petals are too short and thick.
The precise
geometry of the petals can be calculated to optimize the petal movement by using a numerical
technique such finite element analysis.
6.2.1.3. Controlled self-assembly of different geometrical structures
In this work, the petal movement of microflowers is actuated by capillary forces of a
droplet, whose contact angle can be controlled by electrowetting. As described in Chapter 2.1,
many sophisticated sub-millimeter 3D structures can be self-assembled by folding elastic planar
structures using capillary forces of fluid droplets.
Electrowetting then can be explored to
precisely and dynamically assemble or disassemble the sub-millimeter 3D structures with
different shapes. This can be used in applications such as photovoltaics to control the angle of
solar cell surfaces to follow the change in angle of incident sunlight on the surfaces.
6.2.2. Superhydrophobic and transparent Ta2O5 nanostructured thin films
6.2.2.1. Anti-icing properties
Many superhydrophobic surfaces have anti-icing properties which prevent ice formation
on such surfaces by repelling the water droplets before freezing on cold surfaces can occur. The
typical size of structures, which create superhydrophobic surfaces, can be on the micro or
nanoscale.
As predicted by the Cassie-Baxter equation, the change in contact angle is
determined by the area ratio f or the ratio of area of the structures are in contact with the water
droplet to the total area of the surface rather than the actual size of the structures. However, for
optical coatings, the size of structures on surfaces should be less than ~102 nm in order to avoid
61
scattering loss of visible light. Our Ta2O5 nanostructured thin films satisfy these requirements by
being both transparent and superhydrophobic with nanoscale structures. However, the size and
shape of the nanostructures affect the dynamic repulsion of water droplets from the surface
before freezing can occur, or whether or not transition from the Cassie to the Wenzel state
occurs. Before being used for optical coatings in freezing conditions, a detailed investigation
needs to be conducted to determine the dynamic interaction of water with these specific surfaces.
The goal would be to create nanostructures whose size and shape maximize anti-icing properties
without decreasing self-cleaning properties and transmittance of the thin films.
6.2.2.2. Mechanical wear resistance
The transparent Ta2O5 nanostructured thin films will remain superhydrophobic as long as
an array of nanoposts and hydrophobic coating on the surfaces are undamaged. To determine the
life of the superhydrophobic surfaces, one may examine mechanical wear resistance of the
nanostructured surfaces by performing erosion experiments by using water jets and air-born
particles. An atomic force microscope (AFM) and scanning electron microscope (SEM) can then
be used to characterize the damage to the surfaces.
6.2.2.3. Improved theoretical prediction of the film transmittance
As shown in Fig. 4.4, the measured transmittance of the thin films is in good qualitative
agreement with the theoretical calculation, which is estimated by calculating the reflectance and
subtracting it from one. By assuming the absorption is small, the difference in amplitude
between the measured transmittance and the theoretical calculation is about 20%. To be in good
quantitative agreement with the measured transmittance, the theoretical calculation should
include the absorption of the Ta2O5 nanostructured thin films and the quartz substrate. By
62
refining the calculations better agreement may be obtained and further insight may be gained
leading to improvement in the transmittance of the film.
6.2.2.4. Optimized transmittance of thin films
The transmittance of our superhydrophobic Ta2O5 nanostructured thin films may be
maximized by finding proper film thickness and effective refractive index, which will reduce the
reflection caused by thin-film interference. By controlling proper film thickness and effective
refractive index, the thin films may be used as either antireflection coatings, which reduce
reflection from surfaces or optical filters, which selectively transmit light of different
wavelengths.
6.2.3. Ta2O5 nanostructured thin films with electrically-tunable switching between
iridescent and non-iridescent coloration
6.2.3.1. Better quantitative understanding of the electrically-controlled film wetting
As discussed in Chapter 5, the heating induced color switching mechanism is understood
qualitatively to be due to the thermocapillary flow of methanol towards the hot nanograss
directly underneath an ITO electrode heated by an applied electrical current.
One may
quantitatively measure the diameter and the speed of the moving boundary of the wet spot from
top view images. However, it is difficult to measure the thickness of methanol inside the
nanograss layer. The thickness of methanol layer may be predicted by analyzing the thin-film
interference of the wet film having 5 layers: (1) an air layer, (2) an air-Ta2O5 nanograss layer, (3)
a methanol-Ta2O5 nanograss layer, (4) a Ta2O5 continuous layer, and (5) a reflective Ta
continuous layer. The calculated spectrum can then be compared to the measured reflectance
63
spectrum. To analyze this thin-film interference, refractive indices and thickness of the layers
will be needed, most of which are well-know or can be measured. The thickness of the methanol
layer inside the nanograss will be the key to a better quantitative understanding of the physics of
the thermocapillary flow of methanol inside the nanograss layer. After the thickness of the
methanol layer is obtained by analyzing the thin-film interference, the model of thermalgradient-induced flow of methanol can be refined to predict the dynamic heating induced color
switching mechanism.
6.2.3.2. Precise coloration
With the electrically induced color switching mechanism, one can dynamically and
reversibly switch between iridescent color caused by interference from the underlying laminar
Ta2O5 continuous layer and non-iridescent color caused by scattered light from the nanograss
layer filled with methanol. To further apply this mechanism in the field of displays, one needs to
be able to precisely control the iridescent and non-iridescent coloration of the nanostructured thin
films. It has been experimentally demonstrated that the iridescent color can be changed by
controlling the thickness of the Ta2O5 continuous layer. However, the non-iridescent color has
only been experimentally demonstrated to possess a blue color. To produce other colors, one
needs to create a larger shift of the scattering peak towards the visible wavelength region, which
can be accomplished by making the array of nanoposts larger and by increasing the weighted
average refractive index of the nanograss layer.
6.2.3.3. Variety of color switching
In Chapter 5, the electrically induced color switching between reflective interference and
scattering has been experimentally demonstrated. In a similar manner, one may switch from
64
transparent to scattering by using the transparent Ta2O5 nanostructured thin film, whose
fabrication is described in Chapter 4, but instead by making it highly hydrophilic to allow the
methanol to evenly spread inside the nanograss layer. Furthermore, one may superimpose highly
hydrophilic and transparent Ta2O5 nanostructured thin films with different non-iridescent colors
on top of one another to create a wide variety of color switching devices.
65
VITA
Supone Manakasettharn
Place of birth: Chiang Mai, Thailand
Education
M.S.
University of California, Berkeley, 2008
Major: Mechanical Engineering
Focus: Micro-Electro-Mechanical Systems (MEMS)/ Nanoengineering
B.Eng. Chiang Mai University, Thailand, 2004
Major: Mechanical Engineering
Publications
Book Chapter/Encyclopedia
1.Manakasettharn, S., Taylor, J.A., and Krupenkin, T., 2012, "Capillary Origami,"Encyclopedia
of Nanotechnology, B. Bhushan, eds. Springer.
2.Manakasettharn, S., Taylor, J.A., and Krupenkin, T., 2011, "Superhydrophobicity at Micron
and Submicron Scale,"Comprehensive Nanoscience and Technology, D. Andrews, G. Scholes,
and G. Wiederrecht, eds. Academic Press, Amsterdam, 4, pp. 383-411, Chap. 4.13.
Refereed Journal Articles
3.Manakasettharn, S., Hsu, T.-H., Myhre, G., Pau, S., Taylor, J. A., and Krupenkin, T., 2012,
"Transparent and Superhydrophobic Ta2O5 Nanostructured Films," Optical Materials Express
2(2), pp.214-221.
66
4.Manakasettharn, S., Taylor, J. A., and Krupenkin, T., 2011, "Bio-Inspired Artificial
Iridophores Based on Capillary Origami: Fabrication and Device Characterization," Applied
Physics Letters, 99(14) pp. 144102.
Archival Conference Paper
5.Manakasettharn, S., Taylor, J. A., and Krupenkin, T., 2011, "Electrowetting-controlled Bioinspired Artificial Iridophores," Paper No. 8097-23, Optical Trapping and Optical
Micromanipulation VIII, K. Dholakia and G. C. Spalding, eds. Proceeding of SPIE, 8097.
Contributed Conference Presentations
6. Krupenkin, T., Taylor, J. A., and Manakasettharn, S., 2012, “Reverse electrowetting – A new
approach to high-power harvesting of mechanical energy,” (Oral) APS March Meeting, Boston,
Massachusetts.
7. Krupenkin, T., Taylor, J. A., and Manakasettharn, S., 2011, “Reverse electrowetting – A new
approach to high-power harvesting of mechanical energy,” (Oral) MRS Fall Meeting, Boston,
Massachusetts.
8. Manakasettharn, S., Taylor, J. A., and Krupenkin, T., 2011, "Electrowetting-controlled Bioinspired Artificial Iridophores," (Oral) Optical Trapping and Optical Micromanipulation VIII,
SPIE Optics+Photonics, San Diego, California.
9.Manakasettharn, S., Taylor, J. A., and Krupenkin, T., 2011, "Bio-inspired artificial iriodphores
based on capillary origami," (Oral) APS March Meeting, Dallas, Texas.
10. Bryce B. Smith, Benjamin A. Jasperson, Supone Manakasettharn, and Frank Pfefferkorn,
2009, “Effect of Mold Surface Texture on Adhesion of Cast PDMS to Glass,” (Poster) ASME
International Conference on Manufacturing Science and Engineering, West Lafayette, Indiana.
67
REFERENCES
[1] Py, C., Reverdy, P., Doppler, L., 2007, "Capillary Origami: Spontaneous Wrapping of a
Droplet with an Elastic Sheet," Physical Review Letters, 98(15) pp. 156103.
[2] Syms, R. R. A., and Yeatman, E. M., 1993, "Self-Assembly of Three-Dimensional
Microstructures using Rotation by Surface Tension Forces," Electronics Letters, 29(8) pp. 662664.
[3] Syms, R. R. A., 1999, "Surface Tension Powered Self-Assembly of 3-D MicroOptomechanical Structures," Microelectromechanical Systems, Journal of, 8(4) pp. 448-455.
[4] Syms, R. R. A., Yeatman, E. M., Bright, V. M., 2003, "Surface Tension-Powered SelfAssembly of Microstructures-the State-of-the-Art," Microelectromechanical Systems, Journal of,
12(4) pp. 387-417.
[5] Berthier, J., 2008, "Microdrops and digital microfluidics," William Andrew Publishing, New
York.
[6] Timoshenko, S., and Woinowsky-Krieger, S., 1959, "Theory of plates and shells," McGrawHill New York, New York.
[7] de Langre, E., Baroud, C. N., and Reverdy, P., 2010, "Energy Criteria for Elasto-Capillary
Wrapping," Journal of Fluids and Structures, 26(2) pp. 205-217.
[8] Bico, J., Roman, B., Moulin, L., 2004, "Adhesion: Elastocapillary Coalescence in Wet Hair,"
Nature, 432(7018) pp. 690-690.
[9] Py, C., Reverdy, P., Doppler, L., 2009, "Capillarity Induced Folding of Elastic Sheets," The
European Physical Journal-Special Topics, 166(1) pp. 67-71.
[10] Py, C., Reverdy, P., Doppler, L., 2007, "Capillary Origami," Physics of Fluids, 19pp.
091104.
[11] Roman, B., and Bico, J., 2010, "Elasto-Capillarity: Deforming an Elastic Structure with a
Liquid Droplet," Journal of Physics: Condensed Matter, 22pp. 493101.
[12] Jung, S., Reis, P. M., James, J., 2009, "Capillary Origami in Nature," Physics of Fluids,
21pp. 091110.
[13] van Honschoten, J. W., Berenschot, J. W., Ondarçuhu, T., 2010, "Elastocapillary
Fabrication of Three-Dimensional Microstructures," Applied Physics Letters, 97pp. 014103.
68
[14] Guo, X., Li, H., Yeop Ahn, B., 2009, "Two-and Three-Dimensional Folding of Thin Film
Single-Crystalline Silicon for Photovoltaic Power Applications," Proceedings of the National
Academy of Sciences, 106(48) pp. 20149.
[15] Piñeirua, M., Bico, J., and Roman, B., 2010, "Capillary Origami Controlled by an Electric
Field," Soft Matter, 6pp. 4491–4496.
[16] Krupenkin, T., Yang, S., and Mach, P., 2003, "Tunable Liquid Microlens," Applied Physics
Letters, 82pp. 316.
[17] Yang, S., Krupenkin, T. N., Mach, P., 2003, "Tunable and Latchable Liquid Microlens with
Photopolymerizable Components," Advanced Materials, 15(11) pp. 940-943.
[18] Bucaro, M. A., Kolodner, P. R., Taylor, J. A., 2008, "Tunable Liquid Optics:
Electrowetting-Controlled Liquid Mirrors Based on Self-Assembled Janus Tiles," Langmuir,
25(6) pp. 3876-3879.
[19] Zhou, K., Heikenfeld, J., Dean, K. A., 2009, "A Full Description of a Simple and Scalable
Fabrication Process for Electrowetting Displays," Journal of Micromechanics and
Microengineering, 19pp. 065029.
[20] Heikenfeld, J., Drzaic, P., Yeo, J. S., 2011, "Review Paper: A Critical Review of the Present
and Future Prospects for Electronic Paper," Journal of the Society for Information Display, 19pp.
129.
[21] Srinivasan, V., Pamula, V. K., and Fair, R. B., 2004, "An Integrated Digital Microfluidic
Lab-on-a-Chip for Clinical Diagnostics on Human Physiological Fluids," Lab on a Chip, 4(4) pp.
310-315.
[22] Pollack, M. G., Shenderov, A. D., and Fair, R. B., 2002, "Electrowetting-Based Actuation of
Droplets for Integrated Microfluidics," Lab on a Chip, 2(2) pp. 96-101.
[23] Fair, R. B., 2007, "Digital Microfluidics: Is a True Lab-on-a-Chip Possible?" Microfluidics
and Nanofluidics, 3(3) pp. 245-281.
[24] Fair, R. B., Khlystov, A., Tailor, T. D., 2007, "Chemical and Biological Applications of
Digital-Microfluidic Devices," Design & Test of Computers, IEEE, 24(1) pp. 10-24.
[25] Lippmann, G., 1875, "Relations Entre Les Phénomenes Électriques Et Capillaires," Annales
De Chimie Et De Physique, 5pp. 494.
[26] Mugele, F., and Baret, J. C., 2005, "Electrowetting: From Basics to Applications," Journal
of Physics: Condensed Matter, 17pp. R705.
69
[27] Berge, B., 1993, "Electrocapillarity and Wetting of Insulator Films by Water," Comptes
Rendus De l’Academie Des Sciences Serie II, 317pp. 157–163.
[28] Quilliet, C., and Berge, B., 2001, "Electrowetting: A Recent Outbreak," Current Opinion in
Colloid & Interface Science, 6(1) pp. 34-39.
[29] Heikenfeld, J., Smith, N., Dhindsa, M., 2009, "Recent Progress in Arrayed Electrowetting
Optics," Optics and Photonics News, 20(1) pp. 20-26.
[30] Young, T., 1805, "An Essay on the Cohesion of Fluids," Philosophical Transactions of the
Royal Society of London, 95pp. 65-87.
[31] Wenzel, R. N., 1936, "Resistance of Solid Surfaces to Wetting by Water," Industrial &
Engineering Chemistry, 28(8) pp. 988-994.
[32] Cassie, A. B. D., and Baxter, S., 1944, "Wettability of Porous Surfaces," Trans.Faraday
Soc., 40pp. 546-551.
[33] Wenzel, R. N., 1949, "Surface Roughness and Contact Angle." The Journal of Physical
Chemistry, 53(9) pp. 1466-1467.
[34] Quéré, D., 2005, "Non-Sticking Drops," Reports on Progress in Physics, 68pp. 2495.
[35] Manakasettharn, S., Taylor, J.A., and Krupenkin, T., 2011, "Comprehensive Nanoscience
and Technology,"Academic Press, Amsterdam, pp. 383-411.
[36] Shi, F., Chen, X., Wang, L., 2005, "Roselike Microstructures Formed by Direct in Situ
Hydrothermal Synthesis: From Superhydrophilicity to Superhydrophobicity," Chemistry of
Materials, 17(24) pp. 6177-6180.
[37] Martines, E., Seunarine, K., Morgan, H., 2005, "Superhydrophobicity
Superhydrophilicity of Regular Nanopatterns," Nano Letters, 5(10) pp. 2097-2103.
and
[38] de Gennes, P. G., 1985, "Wetting: Statics and Dynamics," Reviews of Modern Physics,
57(3) pp. 827.
[39] Callies, M., and Quéré, D., 2005, "On Water Repellency," Soft Matter, 1(1) pp. 55-61.
[40] Ahuja, A., Taylor, J. A., Lifton, V., 2008, "Nanonails: A Simple Geometrical Approach to
Electrically Tunable Superlyophobic Surfaces," Langmuir, 24(1) pp. 9-14.
[41] Krupenkin, T. N., Taylor, J. A., Schneider, T. M., 2004, "From Rolling Ball to Complete
Wetting: The Dynamic Tuning of Liquids on Nanostructured Surfaces," Langmuir, 20(10) pp.
3824-3827.
70
[42] Moon, H., Cho, S. K., and Garrell, R. L., 2002, "Low Voltage Electrowetting-onDielectric," Journal of Applied Physics, 92pp. 4080.
[43] Wan, Z., Zeng, H., and Feinerman, A., 2007, "Reversible Electrowetting of Liquid-Metal
Droplet," Journal of Fluids Engineering, 129pp. 388.
[44] Pierson, H.O., 1999, "Handbook of chemical vapor depostion: principles, technology, and
applications," William Andrew.
[45] Piccirillo, A., and Gobbi, A. L., 1990, "Physical‐Electrical Properties of Silicon Nitride
Deposited by PECVD on III–V Semiconductors," Journal of the Electrochemical Society,
137(12) pp. 3910-3917.
[46] Mugele, F., and Herminghaus, S., 2002, "Electrostatic Stabilization of Fluid
Microstructures," Applied Physics Letters, 81pp. 2303.
[47] DuPont, 2011, "Teflon® AF Properties," 2011(July 21) .
[48] Bellex International Corporation, 2009, "Cytop," 2011(July 21) .
[49] Seyrat, E., and Hayes, R. A., 2001, "Amorphous Fluoropolymers as Insulators for
Reversible Low-Voltage Electrowetting," Journal of Applied Physics, 90pp. 1383.
[50] Klingner, A., and Mugele, F., 2004, "Electrowetting-Induced Morphological Transitions of
Fluid Microstructures," Journal of Applied Physics, 95pp. 2918.
[51] Quinn, A., Sedev, R., and Ralston, J., 2003, "Influence of the Electrical Double Layer in
Electrowetting," The Journal of Physical Chemistry B, 107(5) pp. 1163-1169.
[52] Verheijen, H. J. J., and Prins, M. W. J., 1999, "Reversible Electrowetting and Trapping of
Charge: Model and Experiments," Langmuir, 15(20) pp. 6616-6620.
[53] Quinn, A., Sedev, R., and Ralston, J., 2005, "Contact Angle Saturation in Electrowetting,"
The Journal of Physical Chemistry B, 109(13) pp. 6268-6275.
[54] Jones, T. B., Wang, K. L., and Yao, D. J., 2004, "Frequency-Dependent Electromechanics
of Aqueous Liquids: Electrowetting and Dielectrophoresis," Langmuir, 20(7) pp. 2813-2818.
[55] Pollack, M. G., Fair, R. B., and Shenderov, A. D., 2000, "Electrowetting-Based Actuation of
Liquid Droplets for Microfluidic Applications," Applied Physics Letters, 77pp. 1725.
[56] Rosslee, C., and Abbott, N. L., 2000, "Active Control of Interfacial Properties," Current
Opinion in Colloid & Interface Science, 5(1-2) pp. 81-87.
71
[57] Kuo, J. S., Spicar-Mihalic, P., Rodriguez, I., 2003, "Electrowetting-Induced Droplet
Movement in an Immiscible Medium," Langmuir, 19(2) pp. 250-255.
[58] Satoh, W., Loughran, M., and Suzuki, H., 2004, "Microfluidic Transport Based on Direct
Electrowetting," Journal of Applied Physics, 96pp. 835.
[59] Vallet, M., Berge, B., and Vovelle, L., 1996, "Electrowetting of Water and Aqueous
Solutions on Poly (Ethylene Terephthalate) Insulating Films," Polymer, 37(12) pp. 2465-2470.
[60] Vallet, M., Vallade, M., and Berge, B., 1999, "Limiting Phenomena for the Spreading of
Water on Polymer Films by Electrowetting," The European Physical Journal B-Condensed
Matter and Complex Systems, 11(4) pp. 583-591.
[61] Smith, N. R., Hou, L., Zhang, J., 2008, "Experimental Validation of >1 kHz Electrowetting
Modulation," University/Government/Industry Micro/Nano Symposium, 2008. UGIM 2008.
17th Biennial, Anonymous IEEE, pp. 11-14.
[62] Chen, J. Y., Kutana, A., Collier, C. P., 2005, "Electrowetting in Carbon Nanotubes,"
Science, 310(5753) pp. 1480.
[63] Prins, M. W. J., Welters, W. J. J., and Weekamp, J. W., 2001, "Fluid Control in
Multichannel Structures by Electrocapillary Pressure," Science, 291(5502) pp. 277.
[64] Cho, S. K., Fan, S. K., Moon, H., 2002, "Towards digital microfluidic circuits: creating,
transporting, cutting and merging liquid droplets by electrowetting-based actuation," Micro
Electro Mechanical Systems, 2002. The Fifteenth IEEE International Conference on,
Anonymous IEEE, pp. 32-35.
[65] Cho, S. K., Moon, H., and Kim, C. J., 2003, "Creating, Transporting, Cutting, and Merging
Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits,"
Microelectromechanical Systems, Journal of, 12(1) pp. 70-80.
[66] Aizenberg, J., Krupenkin, T., and Kolodner, P., 2006, "Accelerated Chemical Reactions for
Lab-on-a-Chip Applications Using Electrowetting-Induced Droplet Self-Oscillations," Materials
Research Society Symposium Proceedings, Anonymous Warrendale, Pa.; Materials Research
Society; 1999, 915, pp. 103.
[67] Wang, E. N., Bucaro, M. A., Taylor, J. A., 2009, "Droplet Mixing using Electrically
Tunable Superhydrophobic Nanostructured Surfaces," Microfluidics and Nanofluidics, 7(1) pp.
137-140.
[68] Yoon, J. Y., and Garrell, R. L., 2003, "Preventing Biomolecular Adsorption in
Electrowetting-Based Biofluidic Chips," Analytical Chemistry, 75(19) pp. 5097-5102.
72
[69] Beni, G., and Hackwood, S., 1981, "Electro‐wetting Displays," Applied Physics Letters,
38(4) pp. 207-209.
[70] Hayes, R. A., and Feenstra, B., 2003, "Video-Speed Electronic Paper Based on
Electrowetting," Nature, 425(6956) pp. 383-385.
[71] Berge, B., and Peseux, J., 2000, "Variable Focal Lens Controlled by an External Voltage:
An Application of Electrowetting," The European Physical Journal E: Soft Matter and Biological
Physics, 3(2) pp. 159-163.
[72] Kuiper, S., and Hendriks, B., 2004, "Variable-Focus Liquid Lens for Miniature Cameras,"
Applied Physics Letters, 85pp. 1128.
[73] Beni, G., Hackwood, S., and Jackel, J., 1982, "Continuous Electrowetting Effect," Applied
Physics Letters, 40(10) pp. 912-914.
[74] Jackel, J., Hackwood, S., and Beni, G., 1982, "Electrowetting Optical Switch," Applied
Physics Letters, 40(1) pp. 4-5.
[75] Jackel, J., Hackwood, S., Veselka, J., 1983, "Electrowetting Switch for Multimode Optical
Fibers," Applied Optics, 22(11) pp. 1765-1770.
[76] Takei, A., Matsumoto, K., and Shomoyama, I., 2010, "Capillary Motor Driven by
Electrowetting," Lab Chip, 10(14) pp. 1781-1786.
[77] Lifton, V., Taylor, J., Vyas, B., 2008, "Superhydrophobic Membranes with Electrically
Controllable Permeability and their Application to “smart” Microbatteries," Applied Physics
Letters, 93(4) pp. 043112-043112-3.
[78] Rosenauer, M., and Vellekoop, M. J., 2009, "3D Fluidic Lens Shaping-A Multiconvex
Hydrodynamically Adjustable Optofluidic Microlens," Lab on a Chip, 9(8) pp. 1040-1042.
[79] Song, C., Nguyen, N. T., Tan, S. H., 2010, "A Tuneable Micro-Optofluidic Biconvex Lens
with Mathematically Predictable Focal Length," Microfluidics and Nanofluidics, 9(4) pp. 889896.
[80] Li, H., Wong, T. N., and Nguyen, N. T., 2011, "A Tunable Optofluidic Lens Based on
Combined Effect of Hydrodynamics and Electroosmosis," Microfluidics and Nanofluidics, pp. 111.
[81] Schaap, A., Bellouard, Y., and Rohrlack, T., 2011, "Optofluidic Lab-on-a-Chip for Rapid
Algae Population Screening," Biomedical Optics Express, 2(3) pp. 658-664.
73
[82] Isikman, S. O., Bishara, W., Zhu, H., 2011, "Optofluidic Tomography on a Chip," Applied
Physics Letters, 98pp. 161109.
[83] Erickson, D., Mandal, S., Yang, A., 2008, "Optofluidics: Fluidics Enabling Optics and
Optics Enabling Fluidics," Anonymous ASME, .
[84] Erickson, D., Sinton, D., and Psaltis, D., 2011, "Optofluidics for Energy Applications,"
Nature Photonics, .
[85] Gibson, B. K., 1991, "Liquid Mirror Telescopes: History," Journal of the Royal
Astronomical Society of Canada, 85(4) pp. 158-171.
[86] Fainman, Y., Lee, L.P., Psaltis, D., 2010, "Optofluidics: fundamentals, devices, and
applications," McGraw-Hill, New York, .
[87] Erickson, D., Yang, C., and Psaltis, D., 2008, "Optofluidics Emerges from the Laboratory,"
Photonics Spectra, 42(2) pp. 74–79.
[88] Psaltis, D., Quake, S. R., and Yang, C., 2006, "Developing Optofluidic Technology through
the Fusion of Microfluidics and Optics," Nature, 442(7101) pp. 381-386.
[89] Monat, C., Domachuk, P., and Eggleton, B., 2007, "Integrated Optofluidics: A New River
of Light," Nature Photonics, 1(2) pp. 106-114.
[90] Horowitz, V., Awschalom, D., and Pennathur, S., 2008, "Optofluidics: Field Or
Technique?" Lab on a Chip, 8(11) pp. 1856.
[91] Nguyen, N. T., 2010, "Micro-Optofluidic Lenses: A Review," Biomicrofluidics, 4pp.
031501.
[92] Brennan, D., Justice, J., Corbett, B., 2009, "Emerging Optofluidic Technologies for Pointof-Care Genetic Analysis Systems: A Review," Analytical and Bioanalytical Chemistry, 395(3)
pp. 621-636.
[93] Mao, X., Lin, S. C. S., Lapsley, M. I., 2009, "Tunable Liquid Gradient Refractive Index (LGRIN) Lens with Two Degrees of Freedom," Lab Chip, 9(14) pp. 2050-2058.
[94] Cui, X., Lee, L. M., Heng, X., 2008, "Lensless High-Resolution on-Chip Optofluidic
Microscopes for Caenorhabditis Elegans and Cell Imaging," Proceedings of the National
Academy of Sciences, 105(31) pp. 10670.
[95] Cordovez, B., Tung, S., and Erickson, D., 2006, "Optofluidic particle manipulation and
characterization in nanowell sensors," LEOS Summer Topical Meetings, 2006 Digest of the,
Anonymous IEEE, pp. 25-26.
74
[96] Chien, C. C., and Fan, S. K., 2009, "Liquid-core liquid-cladding optical waveguide by liquid
dielectrophoresis," Solid-State Sensors, Actuators and Microsystems Conference, 2009.
TRANSDUCERS 2009. International, Anonymous IEEE, pp. 2210-2213.
[97] Dong, L., Agarwal, A. K., Beebe, D. J., 2006, "Adaptive Liquid Microlenses Activated by
Stimuli-Responsive Hydrogels," Nature, 442(7102) pp. 551-554.
[98] Zeng, X., and Jiang, H., 2008, "Tunable Liquid Microlens Actuated by Infrared LightResponsive Hydrogel," Applied Physics Letters, 93pp. 151101.
[99] Lee, S. W., and Lee, S. S., 2007, "Focal Tunable Liquid Lens Integrated with an
Electromagnetic Actuator," Applied Physics Letters, 90pp. 121129.
[100] Pang, L., Levy, U., Campbell, K., 2005, "Set of Two Orthogonal Adaptive Cylindrical
Lenses in a Monolith Elastomer Device," Optics Express, 13(22) pp. 9003-9013.
[101] Zhang, D. Y., Justis, N., and Lo, Y. H., 2004, "Fluidic Adaptive Lens of Transformable
Lens Type," Applied Physics Letters, 84(21) pp. 4194-4196.
[102] Tang, S. K. Y., Stan, C. A., and Whitesides, G. M., 2008, "Dynamically Reconfigurable
Liquid-Core Liquid-Cladding Lens in a Microfluidic Channel," Lab Chip, 8(3) pp. 395-401.
[103] Ren, H., and Wu, S. T., 2008, "Tunable-Focus Liquid Microlens Array using
Dielectrophoretic Effect," Optics Express, 16(4) pp. 2646-2652.
[104] Ren, H., Xianyu, H., Xu, S., 2008, "Adaptive Dielectric Liquid Lens," Optics Express,
16(19) pp. 14954-14960.
[105] Abgrall, P., and Gue, A., 2007, "Lab-on-Chip Technologies: Making a Microfluidic
Network and Coupling it into a Complete microsystem—a Review," Journal of Micromechanics
and Microengineering, 17pp. R15.
[106] Haeberle, S., and Zengerle, R., 2007, "Microfluidic Platforms for Lab-on-a-Chip
Applications," Lab Chip, 7(9) pp. 1094-1110.
[107] Kovarik, M. L., and Jacobson, S. C., 2009, "Nanofluidics in Lab-on-a-Chip Devices,"
Analytical Chemistry, 81(17) pp. 7133-7140.
[108] Mark, D., Haeberle, S., Roth, G., 2010, "Microfluidic Lab-on-a-Chip Platforms:
Requirements, Characteristics and Applications," Chem.Soc.Rev., 39(3) pp. 1153-1182.
[109] Dittrich, P. S., and Manz, A., 2006, "Lab-on-a-Chip: Microfluidics in Drug Discovery,"
Nature Reviews Drug Discovery, 5(3) pp. 210-218.
75
[110] Craighead, H., 2006, "Future Lab-on-a-Chip Technologies for Interrogating Individual
Molecules," Nature, 442(7101) pp. 387-393.
[111] Chin, C. D., Linder, V., and Sia, S. K., 2006, "Lab-on-a-Chip Devices for Global Health:
Past Studies and Future Opportunities," Lab Chip, 7(1) pp. 41-57.
[112] Chow, A. W., 2002, "Lab‐on‐a‐chip: Opportunities for Chemical Engineering," AIChE
Journal, 48(8) pp. 1590-1595.
[113] Toner, M., and Irimia, D., 2005, "Blood-on-a-Chip," Annu.Rev.Biomed.Eng., 7pp. 77-103.
[114] Gardeniers, J., and Van den Berg, A., 2004, "Lab-on-a-Chip Systems for Biomedical and
Environmental Monitoring," Analytical and Bioanalytical Chemistry, 378(7) pp. 1700-1703.
[115] Brennan, D., Lambkin, P., Moore, E. J., 2008, "An Integrated Optofluidic Platform for
DNA Hybridization and Detection," Sensors Journal, IEEE, 8(5) pp. 536-542.
[116] Seo, J., and Lee, L. P., 2004, "Disposable Integrated Microfluidics with Self-Aligned
Planar Microlenses," Sensors and Actuators B: Chemical, 99(2–3) pp. 615-622.
[117] Ymeti, A., Greve, J., Lambeck, P. V., 2007, "Fast, Ultrasensitive Virus Detection using a
Young Interferometer Sensor," Nano Letters, 7(2) pp. 394-397.
[118] Berney, H., and Oliver, K., 2005, "Dual Polarization Interferometry Size and Density
Characterisation of DNA Immobilisation and Hybridisation," Biosensors and Bioelectronics,
21(4) pp. 618-626.
[119] Lillis, B., Manning, M., Berney, H., 2006, "Dual Polarisation Interferometry
Characterisation of DNA Immobilisation and Hybridisation Detection on a Silanised Support,"
Biosensors and Bioelectronics, 21(8) pp. 1459-1467.
[120] Chinowsky, T. M., Grow, M. S., Johnston, K. S., 2007, "Compact, High Performance
Surface Plasmon Resonance Imaging System," Biosensors and Bioelectronics, 22(9–10) pp.
2208-2215.
[121] Lindquist, N. C., Lesuffleur, A., Im, H., 2008, "Sub-Micron Resolution Surface Plasmon
Resonance Imaging Enabled by Nanohole Arrays with Surrounding Bragg Mirrors for Enhanced
Sensitivity and Isolation," Lab Chip, 9(3) pp. 382-387.
[122] Acharya, B. R., Krupenkin, T., Ramachandran, S., 2003, "Tunable Optical Fiber Devices
Based on Broadband Long-Period Gratings and Pumped Microfluidics," Applied Physics Letters,
83pp. 4912.
76
[123] Mach, P., Krupenkin, T., Yang, S., 2002, "Dynamic Tuning of Optical Waveguides with
Electrowetting Pumps and Recirculating Fluid Channels," Applied Physics Letters, 81pp. 202204.
[124] Li, Z., Zhang, Z., Emery, T., 2006, "Single Mode Optofluidic Distributed Feedback Dye
Laser," Optics Express, 14(2) pp. 696-701.
[125] Erickson, D., Li, B., Adleman, J. R., 2005, "Spectrographic Microfluidic Memory,"
Proc.ICMM, .
[126] Vukusic, P., and
," Nature, 424pp. 852.
Sambles,
J.
R.,
2003,
"Photonic
Structures
in
biology
[127] Crookes, W. J., Ding, L. L., Huang, Q. L., 2004, "Reflectins: The Unusual Proteins of
Squid Reflective Tissues," Science, 303(5655) pp. 235.
[128] Kramer, R. M., Crookes-Goodson, W. J., and Naik, R. R., 2007, "The Self-Organizing
Properties of Squid Reflectin Protein," Nature Materials, 6(7) pp. 533-538.
[129] Herring, P. J., 1977, "Bioluminescence of Marine Organisms," Nature, 267(5614) pp. 788793.
[130] Manakasettharn, S., Taylor, J.A., and Krupenkin, T.N., 2012, "Capillary Origami,"
Encyclopedia of Nanotechnology, B. Bhushan, eds. Springer.
[131] Sharpe, W. N.,Jr, Bin Yuan, Vaidyanathan, R., 1997, "Measurements of Young's modulus,
Poisson's ratio, and tensile strength of polysilicon," Micro Electro Mechanical Systems, 1997.
MEMS '97, Proceedings, IEEE., Tenth Annual International Workshop on, Anonymous pp. 424429.
[132] Berthier, E., Warrick, J., Yu, H., 2008, "Managing Evaporation for More Robust
Microscale assaysPart 1. Volume Loss in High Throughput Assays," Lab on a Chip, 8(6) pp.
852-859.
[133] Barthlott, W., and Neinhuis, C., 1997, "Purity of the Sacred Lotus, Or Escape from
Contamination in Biological Surfaces," Planta, 202(1) pp. 1-8.
[134] Mishchenko, L., Hatton, B., Bahadur, V., 2010, "Design of Ice-Free Nanostructured
Surfaces Based on Repulsion of Impacting Water Droplets," ACS Nano, 4(12) pp. 7699–7707.
[135] Tadanaga, K., Katata, N., and Minami, T., 1997, "Formation Process of Super-WaterRepellent Al2O3 Coating Films with High Transparency by the Sol-Gel Method," Journal of the
American Ceramic Society, 80(12) pp. 3213-3216.
77
[136] Nakajima, A., Fujishima, A., Hashimoto, K., 1999, "Preparation of Transparent
Superhydrophobic Boehmite and Silica Films by Sublimation of Aluminum Acetylacetonate,"
Advanced Materials, 11(16) pp. 1365-1368.
[137] Miwa, M., Nakajima, A., Fujishima, A., 2000, "Effects of the Surface Roughness on
Sliding Angles of Water Droplets on Superhydrophobic Surfaces," Langmuir, 16(13) pp. 57545760.
[138] Nakajima, A., Hashimoto, K., Watanabe, T., 2000, "Transparent Superhydrophobic Thin
Films with Self-Cleaning Properties," Langmuir, 16(17) pp. 7044-7047.
[139] Tadanaga, K., Morinaga, J., Matsuda, A., 2000, "Superhydrophobic-Superhydrophilic
Micropatterning on Flowerlike Alumina Coating Film by the Sol-Gel Method," Chemistry of
Materials, 12(3) pp. 590-592.
[140] Shang, H. M., Wang, Y., Limmer, S. J., 2005, "Optically Transparent Superhydrophobic
Silica-Based Films," Thin Solid Films, 472(1-2) pp. 37-43.
[141] Yabu, H., and Shimomura, M., 2005, "Single-Step Fabrication of Transparent
Superhydrophobic Porous Polymer Films," Chemistry of Materials, 17(21) pp. 5231-5234.
[142] Fresnais, J., Chapel, J. P., and Poncin-Epaillard, F., 2006, "Synthesis of Transparent
Superhydrophobic Polyethylene Surfaces," Surface and Coatings Technology, 200(18-19) pp.
5296-5305.
[143] Kemell, M., Färm, E., Leskelä, M., 2006, "Transparent Superhydrophobic Surfaces by
self‐assembly of Hydrophobic Monolayers on Nanostructured Surfaces," Physica Status Solidi
(a), 203(6) pp. 1453-1458.
[144] Su, C., Li, J., Geng, H., 2006, "Fabrication of an Optically Transparent Super-Hydrophobic
Surface Via Embedding Nano-Silica," Applied Surface Science, 253(5) pp. 2633-2636.
[145] Bravo, J., Zhai, L., Wu, Z., 2007, "Transparent Superhydrophobic Films Based on Silica
Nanoparticles," Langmuir, 23(13) pp. 7293-7298.
[146] Vourdas, N., Tserepi, A., and Gogolides, E., 2007, "Nanotextured Super-Hydrophobic
Transparent Poly (Methyl Methacrylate) Surfaces using High-Density Plasma Processing,"
Nanotechnology, 18pp. 125304.
[147] Zhang, X., Kono, H., Liu, Z., 2007, "A Transparent and Photo-Patternable
Superhydrophobic Film," Chemical Communications, (46) pp. 4949-4951.
78
[148] Chang, K. C., Chen, Y. K., and Chen, H., 2008, "Fabrication of Highly Transparent and
Superhydrophobic Silica-Based Surface by TEOS/PPG Hybrid with Adjustment of the pH
Value," Surface and Coatings Technology, 202(16) pp. 3822-3831.
[149] Han, J. T., Kim, S. Y., Woo, J. S., 2008, "Transparent, Conductive, and Superhydrophobic
Films from Stabilized Carbon nanotube/silane Sol Mixture Solution," Advanced Materials,
20(19) pp. 3724-3727.
[150] Xu, Q. F., Wang, J. N., Smith, I. H., 2008, "Superhydrophobic and Transparent Coatings
Based on Removable Polymeric Spheres," J.Mater.Chem., 19(5) pp. 655-660.
[151] Li, Y., Liu, F., and Sun, J., 2009, "A Facile Layer-by-Layer Deposition Process for the
Fabrication of Highly Transparent Superhydrophobic Coatings," Chemical Communications,
(19) pp. 2730-2732.
[152] Ling, X. Y., Phang, I. Y., Vancso, G. J., 2009, "Stable and Transparent Superhydrophobic
Nanoparticle Films," Langmuir, 25(5) pp. 3260-3263.
[153] Venkateswara Rao, A., Latthe, S. S., Nadargi, D. Y., 2009, "Preparation of MTMS Based
Transparent Superhydrophobic Silica Films by sol–gel Method," Journal of Colloid and Interface
Science, 332(2) pp. 484-490.
[154] Yang, J., Zhang, Z., Men, X., 2009, "Fabrication of Stable, Transparent and
Superhydrophobic Nanocomposite Films with Polystyrene Functionalized Carbon Nanotubes,"
Applied Surface Science, 255(22) pp. 9244-9247.
[155] Im, M., Im, H., Lee, J. H., 2010, "A Robust Superhydrophobic and Superoleophobic
Surface with Inverse-Trapezoidal Microstructures on a Large Transparent Flexible Substrate,"
Soft Matter, 6(7) pp. 1401-1404.
[156] Ingrey, S., Westwood, W. D., Cheng, Y., 1975, "Variable Refractive Index and
Birefringent Waveguides by Sputtering Tantalum in O2—N2 Mixtures," Applied Optics, 14(9)
pp. 2194-2198.
[157] Z. Tepehan, F., Ghodsi, F. E., Ozer, N., 1999, "Optical Properties of sol–gel Dip-Coated
Ta2O5 Films for Electrochromic Applications," Solar Energy Materials and Solar Cells, 59(3)
pp. 265-275.
[158] Cheng, W., Chi, S., and Chu, A., 1999, "Effect of Thermal Stresses on Temperature
Dependence of Refractive Index for Ta2O5 Dielectric Films," Thin Solid Films, 347(1-2) pp.
233-237.
[159] Schmitt, K., Oehse, K., Sulz, G., 2008, "Evanescent Field Sensors Based on Tantalum
Pentoxide waveguides–a Review," Sensors, 8(2) pp. 711-738.
79
[160] Patnaik, P., 2003, "Handbook of Inorganic Chemicals," McGraw-Hill, New York, pp. 911912.
[161] Martienssen, W., and Warlimont, H., 2005, "Springer handbook of condensed matter and
materials data," Springer Verlag, Berlin, Germany, pp. 440.
[162] Husted, D., Gruss, L., and Mackus, T., 1971, "Electrical Properties of Anodic Oxide Films
of Ta, Nb, Zr, Ti, W, and V Formed by the Ion‐Cathode Method," Journal of the Electrochemical
Society, 118pp. 1989.
[163] Young, L., 1958, "The Determination of the Thickness, Dielectric Constant, and Other
Properties of Anodic Oxide Films on Tantalum from the Interference Colours," Proceedings of
the Royal Society of London.Series A.Mathematical and Physical Sciences, 244(1236) pp. 41.
[164] Chaneliere, C., Autran, J. L., Devine, R. A. B., 1998, "Tantalum Pentoxide (Ta2O5) Thin
Films for Advanced Dielectric Applications," Materials Science and Engineering: R: Reports,
22(6) pp. 269-322.
[165] Mozalev, A., Sakairi, M., Saeki, I., 2003, "Nucleation and Growth of the Nanostructured
Anodic Oxides on Tantalum and Niobium Under the Porous Alumina Film," Electrochimica
Acta, 48(20-22) pp. 3155-3170.
[166] Wu, C., Ko, F., and Hwang, H., 2006, "Self-Aligned Tantalum Oxide Nanodot Arrays
through Anodic Alumina Template," Microelectronic Engineering, 83(4-9) pp. 1567-1570.
[167] Mozalev, A., Smith, A. J., Borodin, S., 2009, "Growth of Multioxide Planar Film with the
Nanoscale Inner Structure Via Anodizing Al/Ta Layers on Si," Electrochimica Acta, 54(3) pp.
935-945.
[168] McHale, G., Shirtcliffe, N., and Newton, M., 2004, "Contact-Angle Hysteresis on SuperHydrophobic Surfaces," Langmuir, 20(23) pp. 10146-10149.
[169] Anders, H., 1967, "Thin films in optics," Focal Press, London and New York, .
[170] Bruggeman, D. A. G., 1935, "Berechnung Verschiedener Physikalischer Konstanten Von
Heterogenen Substanzen. I. Dielektrizitätskonstanten Und Leitfähigkeiten Der Mischkörper Aus
Isotropen Substanzen," Annalen Der Physik, 416(7) pp. 636-664.
[171] Prum, R. O., and Torres, R. H., 2003, "A Fourier Tool for the Analysis of Coherent Light
Scattering by Bio-Optical Nanostructures," Integrative and Comparative Biology, 43(4) pp. 591602.
[172] Zheng, Y., Gao, X., and Jiang, L., 2007, "Directional Adhesion of Superhydrophobic
Butterfly Wings," Soft Matter, 3(2) pp. 178-182.
80
[173] Parker, A. R., and Lawrence, C. R., 2001, "Water Capture by a Desert Beetle," Nature,
414(6859) pp. 33-34.
[174] Feng, X. Q., Gao, X., Wu, Z., 2007, "Superior Water Repellency of Water Strider Legs
with Hierarchical Structures: Experiments and Analysis," Langmuir, 23(9) pp. 4892-4896.
[175] Gao, X., and Jiang, L., 2004, "Biophysics: Water-Repellent Legs of Water Striders,"
Nature, 432(7013) pp. 36-36.
[176] Manakasettharn, S., Hsu, T. H., Myhre, G., 2012, "Transparent and Superhydrophobic
Ta2O5 Nanostructured Thin Films," Optical Materials Express, 2(2) pp. 214-221.
[177] Wolfram, S., 1999, "The MATHEMATICA® Book, Version 4," Cambridge university
press, .
[178] Pimputkar, S. M., and Ostrach, S., 1980, "Transient Thermocapillary Flow in Thin Liquid
Layers," Physics of Fluids, 23(7) pp. 1281.
[179] Costa, G. D., 1993, "Optical Visualization of the Velocity Distribution in a Laser-Induced
Thermocapillary Liquid Flow," Applied Optics, 32(12) pp. 2143-2151.
[180] Marchuk, I., and Kabov, O., 1998, "Numerical Modeling of Thermocapillary Reverse Flow
in Thin Liquid Films Under Local Heating," Russ.J.Eng.Thermophys, 8(1-4) pp. 17-46.
[181] Chen, J. Z., Troian, S. M., Darhuber, A. A., 2005, "Effect of Contact Angle Hysteresis on
Thermocapillary Droplet Actuation," Journal of Applied Physics, 97pp. 014906.
[182] Jiao, Z., Huang, X., Nguyen, N. T., 2008, "Thermocapillary Actuation of Droplet in a
Planar Microchannel," Microfluidics and Nanofluidics, 5(2) pp. 205-214.
[183] Darhuber, A. A., Valentino, J. P., and Troian, S. M., 2010, "Planar Digital Nanoliter
Dispensing System Based on Thermocapillary Actuation," Lab Chip, 10(8) pp. 1061-1071.
[184] Chraïbi, H., and Delville, J. P., 2012, "Thermocapillary Flows and Interface Deformations
Produced by Localized Laser Heating in Confined Environment," Physics of Fluids, 24pp.
032102.
[185] Scheid, B., van Nierop, E. A., and Stone, H. A., 2012, "Thermocapillary-Assisted Pulling
of Contact-Free Liquid Films," Physics of Fluids, 24pp. 032107.
81
APPENDIX
A.1. Mathematica Code for Solving the Theoretical Model in Chapter 3
(* spherical cap droplet shape calculations *)
(* Supone\MURI\math\muri_3D_3_rb_shape.nb *)
Symbolize[ 1, 2, r1, r2, ls,0]
(*
1 = c = Angle of a droplet with respect to a substrate,
2 = b = Angle of a petal with respect to the substrate,
r1 = Rc / LEC,
r2 = Rb / LEC,
( )
ls = ∑
/ LEC,
0 = Local contact angle
*)
r2=(ls*(Cos[0]+1))/(2**Sin[2/2]*(1 + Cos[(1+2)/2]))
(* derived from the Cassie-Baxter equation (3.4) *)
r1=r2*Sin[2/2]/Sin[1/2]
(* equation (3.2) *)
constr = 3/ V==Subscript[r, 1]3*(1-Cos[Subscript[, 1]/2])2*(2+Cos[1/2]) + Subscript[r,
2]3*(1-Cos[Subscript[, 2]/2])2*(2+Cos[2/2])
(* derived from equation (3.5) *)
funcb1[ bbt2_,VVt_,llt_,tht0_ ]:= 1/.FindRoot[constr/.{2bbt2,
VVVt,lsllt,0tht0*/180}, {1,-1.99, 1.99} ]
=2*1*Subscript[r, 1]2 + 1/4*ls/r2*2 + (r2/2)*(4*r2-ls)*2
(* The total energy equation (3.1) *)
funcE[bbt2_,VVt_,llt_,tht0_]:=/.{2bbt2, VVVt,lsllt,0tht0*/180, 1funcb1[
bbt2,VVt,llt,tht0]}
b2func[VVt_,llt_,tht0_]:=x/.FindMinimum[Evaluate[Interpolation[Table[{beta2,
funcE[beta2,VVt, llt,tht0] }, {beta2,0.1, 2, 0.01}]]][x], {x,0.1, 2}] [[2]]
b1func[VVt_,llt_,tht0_]:=funcb1[ b2func[VVt,llt,tht0],VVt,llt,tht0]
b2funcM[RR_, frac_,th0_]:=180/*b2func[4/3 *RR3,frac*2**RR,th0]
b1funcM[RR_, frac_,th0_]:=180/*b1func[4/3 *RR3,frac*2**RR,th0]
82
r2funcM[RR_, frac_,th0_]:= r2/.{2b2func[4/3 *RR3,frac*2**RR,th0], V4/3
*RR3,lsfrac*2**RR,0th0*/180, 1b1func[4/3 *RR3,frac*2**RR,th0]
}
r1funcM[RR_, frac_,th0_]:= r1/.{2b2func[4/3 *RR3,frac*2**RR,th0], V4/3
*RR3,lsfrac*2**RR,0th0*/180, 1b1func[4/3 *RR3,frac*2**RR,th0]
}
(* plate & droplet shapes *)
RR=1.0;frac=0.8;th0=70;
(* RR = radius of a droplet, frac = Cassie-Baxter fraction, th0 = contact angle *)
rr=r1funcM[RR,frac,th0];
=(1/2)*(/180)*b1funcM[RR,frac,th0];
sh=-r1funcM[RR,frac,th0]+(r1funcM[RR,frac,th0]*(1Cos[(1/2)*(/180)*b1funcM[RR,frac,th0]])+r2funcM[RR,frac,th0]*(1Cos[(1/2)*(/180)*b2funcM[RR,frac,th0]]));
p4=ParametricPlot[{-rr*Sin[],sh+rr*Cos[]},{,-,},AxesTrue,PlotStyleRed];
RR=1.0;frac=0.8;th0=150;
rr=r1funcM[RR,frac,th0];

=(1/2)*(/180)*b1funcM[RR,frac,th0];
sh=-r1funcM[RR,frac,th0]+(r1funcM[RR,frac,th0]*(1Cos[(1/2)*(/180)*b1funcM[RR,frac,th0]])+r2funcM[RR,frac,th0]*(1Cos[(1/2)*(/180)*b2funcM[RR,frac,th0]]));
p5=ParametricPlot[{-rr*Sin[],sh+rr*Cos[]},{,-,},AxesTrue,PlotStyleRed];
RR=1.0;frac=0.8;th0=150;
rr=r2funcM[RR,frac,th0];
delta=/6;
v=delta+(1/2)*(/180)*b2funcM[RR,frac,th0];
p6=ParametricPlot[{rr*Sin[],rr*(1+Cos[])},{,-v,2-(-v)},AxesTrue,PlotStyleRed];
83
RR=1.0;frac=0.8;th0=70;
rr=r2funcM[RR,frac,th0];
delta=/6;
v=delta+(1/2)*(/180)*b2funcM[RR,frac,th0];
p7=ParametricPlot[{rr*Sin[],rr*(1+Cos[])},{,-v,2-(-v)},AxesTrue,PlotStyleRed];
(* r = 1.0 frac = 0.8 contact ang=70 and 150 *)
(* The predicted shape of a microflower with change in contact angle between 70 and 150
degrees *)
comb1=Show[ p4,p5,p6,p7, PlotRange{{2,2},{0,2}},AspectRatio1/2,AxesOrigin{0,0},Ticks->None]
84
A.2. Mathematica Code for Calculating the Theoretical Transmittance in Chapter 4
(* Reflection of Light by 3 Layers and
Effective Refractive Index *)
(* MURI\Publications\OSA\Color_R3layers_Oblique_MURI_01.nb *)
Symbolize[n0,n1,n2,n3,n4,d1,d2,d3,wl,A1,A2,A3,A4]
(* ni = refractive index, di = thickness, wli = wavelength, Ai = angle *)
(* The Reflection of Light at Four Interfaces (i.e. by Three Layers) *)
(* Ref: H. Anders (1965),Thin Films in Optics, tranlated by J.N. Davidson (1967),Focal Press,
London, New York, Ch .1.4. *)
(* Incident/refractive angles in the medium 0, 1, 2, 3, 4 *)
A0=0Degree;
A1=ArcSin[n0*Sin[A0]/n1];
A2=ArcSin[n1*Sin[A1]/n2];
A3=ArcSin[n2*Sin[A2]/n3];
A4=ArcSin[n3*Sin[A3]/n4];
(* Phase relative to the reflection *)
del1=4*Pi*n1*d1*Cos[A1]/(wl*n1);
del2=4*Pi*n2*d2*Cos[A2]/(wl*n2);
del3=4*Pi*n3*d3*Cos[A3]/(wl*n3);
(* Reflection of the perpendicular components *)
rpp1=(n0*Cos[A0]-n1*Cos[A1])/(n0*Cos[A0]+n1*Cos[A1]);
rpp2=(n1*Cos[A1]-n2*Cos[A2])/(n1*Cos[A1]+n2*Cos[A2]);
rpp3=(n2*Cos[A2]-n3*Cos[A3])/(n2*Cos[A2]+n3*Cos[A3]);
rpp4=(n3*Cos[A3]-n4*Cos[A4])/(n3*Cos[A3]+n4*Cos[A4]);
Rpp=((rpp1+rpp2*Exp[-I*del1]+rpp3*Exp[-I*(del1+del2)]+rpp4*Exp[I*(del1+del2+del3)]+rpp1*rpp2*rpp3*Exp[-I*del2]+rpp1*rpp3*rpp4*Exp[I*del3]+rpp1*rpp2*rpp4*Exp[-I*(del2+del3)]+rpp2*rpp3*rpp4*Exp[I*(del1+del3)])/(1+rpp1*rpp2*Exp[-I*del1]+rpp1*rpp3*Exp[-I*(del1+del2)]+rpp1*rpp4*Exp[I*(del1+del2+del3)]+rpp2*rpp3*Exp[-I*del2]+rpp3*rpp4*Exp[-I*del3]+rpp2*rpp4*Exp[I*(del2+del3)]+rpp1*rpp2*rpp3*rpp4*Exp[-I*(del1+del3)]))*Conjugate[((rpp1+rpp2*Exp[I*del1]+rpp3*Exp[-I*(del1+del2)]+rpp4*Exp[-I*(del1+del2+del3)]+rpp1*rpp2*rpp3*Exp[I*del2]+rpp1*rpp3*rpp4*Exp[-I*del3]+rpp1*rpp2*rpp4*Exp[I*(del2+del3)]+rpp2*rpp3*rpp4*Exp[-I*(del1+del3)])/(1+rpp1*rpp2*Exp[I*del1]+rpp1*rpp3*Exp[-I*(del1+del2)]+rpp1*rpp4*Exp[I*(del1+del2+del3)]+rpp2*rpp3*Exp[-I*del2]+rpp3*rpp4*Exp[-I*del3]+rpp2*rpp4*Exp[I*(del2+del3)]+rpp1*rpp2*rpp3*rpp4*Exp[-I*(del1+del3)]))];
85
(* Reflection of the parallel components *)
rpa1=(n0*Cos[A1]-n1*Cos[A0])/(n0*Cos[A1]+n1*Cos[A0]);
rpa2=(n1*Cos[A2]-n2*Cos[A1])/(n1*Cos[A2]+n2*Cos[A1]);
rpa3=(n2*Cos[A3]-n3*Cos[A2])/(n2*Cos[A3]+n3*Cos[A2]);
rpa4=(n3*Cos[A4]-n4*Cos[A3])/(n3*Cos[A4]+n4*Cos[A3]);
Rpa=((rpa1+rpa2*Exp[-I*del1]+rpa3*Exp[-I*(del1+del2)]+rpa4*Exp[I*(del1+del2+del3)]+rpa1*rpa2*rpa3*Exp[-I*del2]+rpa1*rpa3*rpa4*Exp[I*del3]+rpa1*rpa2*rpa4*Exp[-I*(del2+del3)]+rpa2*rpa3*rpa4*Exp[I*(del1+del3)])/(1+rpa1*rpa2*Exp[-I*del1]+rpa1*rpa3*Exp[-I*(del1+del2)]+rpa1*rpa4*Exp[I*(del1+del2+del3)]+rpa2*rpa3*Exp[-I*del2]+rpa3*rpa4*Exp[-I*del3]+rpa2*rpa4*Exp[I*(del2+del3)]+rpa1*rpa2*rpa3*rpa4*Exp[-I*(del1+del3)]))*Conjugate[((rpa1+rpa2*Exp[I*del1]+rpa3*Exp[-I*(del1+del2)]+rpa4*Exp[-I*(del1+del2+del3)]+rpa1*rpa2*rpa3*Exp[I*del2]+rpa1*rpa3*rpa4*Exp[-I*del3]+rpa1*rpa2*rpa4*Exp[I*(del2+del3)]+rpa2*rpa3*rpa4*Exp[-I*(del1+del3)])/(1+rpa1*rpa2*Exp[I*del1]+rpa1*rpa3*Exp[-I*(del1+del2)]+rpa1*rpa4*Exp[I*(del1+del2+del3)]+rpa2*rpa3*Exp[-I*del2]+rpa3*rpa4*Exp[-I*del3]+rpa2*rpa4*Exp[I*(del2+del3)]+rpa1*rpa2*rpa3*rpa4*Exp[-I*(del1+del3)]))];
(* Reflectance *)
R=(Rpp+Rpa)/2;
R;
(* Plot[{R/.{n01,n11.39,n22.12, n31.63, n41.52,d1550/4, d2550/2,
d3550/4,fff0.15}},
{wl,350,850},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Example p.62",
FrameLabel{"wavelength(nm)","Reflectance"},PlotStyle{Red,Blue,Green }] *)
(* Plot[{R/.{n01,n11,n21.38, n32.15, n41.52,d10, d297.8, d3238,fff0.15}},
{wl,400,800},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Example p.55",
FrameLabel{"wavelength(nm)","Reflectance"},PlotStyle{Red,Blue,Green }] *)
(* Interface description *)
(* Air *)
nn0 = 1.000
(* Methanol *)
nnn1=1.33722 (* @ 546.1 nm *)
nn1 = 2.887*10^-12*wl^4-7.890*10^-9*wl^3+8.198*10^-6*wl^2-3.923*10^-3*wl+2.063
(* index depending on wavelength (wl,300,800) *)
(* Ref: refractiveindex.info *)
(* Ta2O5 nanograss *)
(* n2 = neff *)
86
(* Ta2O5 layer *)
nnn3=1.8079 (* @ 546.1 nm *)
nn3=7.163*10^-12*wl^4-1.898*10^-8*wl^3+1.888*10^-5*wl^2-8.459*10^-3*wl+3.249
nn31=7.165*10^-12*wl^4-1.894*10^-8*wl^3+1.891*10^-5*wl^2-8.554*10^-3*wl+3.303
(* index depending on wavelength (wl,300,800) *)
(* Ref: refractiveindex.info *)
(* Ta layer *)
nnn4=3.43-3.66*I (* @ 546.1 nm for tetragonal (beta) Ta *)
(* Ref: D.G. Muth (1969), J. Vac. Sci. Technol. 6, 749. *)
nn4=(2.396*10^-10*wl^4-4.242*10^-7*wl^3+2.412*10^-4*wl^2-4.715*10^-2*wl+4.049)+(3.908*10^-10*wl^4+9.184*10^-7*wl^3-7.683*10^-4*wl^2+2.716*10^-1*wl-32.19)*I
(* index depending on wavelength (wl,350,750) *)
(* Ref: refractiveindex.info *)
(* SiO2 layer *)
nSiO2 = 1.55
nnSiO2=1.391*10^-12*wl^4-3.602*10^-9*wl^3+3.525*10^-6*wl^2-1.58*10^-3*wl+1.82
(* Si layer *)
nSi = 4
Plot[{nn1},{wl,380,800},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Refractiv
e Index of Methanol",
FrameLabel{"wavelength(nm)","n"},PlotStyle{Red}]
Plot[{nn3},{wl,380,800},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Refractiv
e Index of Ta2O5",
FrameLabel{"wavelength(nm)","n"},PlotStyle{Red}]
Plot[{2.396*10^-10*wl^4-4.242*10^-7*wl^3+2.412*10^-4*wl^2-4.715*10^2*wl+4.049},{wl,380,750},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Refract
ive Index (Real Part) of Ta",
FrameLabel{"wavelength(nm)","n"},PlotStyle{Red}]
Plot[{-3.908*10^-10*wl^4+9.184*10^-7*wl^3-7.683*10^-4*wl^2+2.716*10^-1*wl32.19},{wl,380,750},FrameTrue,GridLinesAutomatic,AxesTrue,PlotLabel"Refractive
Index (Imaginary Part) of Ta",
FrameLabel{"wavelength(nm)","n"},PlotStyle{Red}]
87
Refractive Index of Methanol
1.38
1.37
n
1.36
1.35
1.34
1.33
1.32
400
500
600
700
800
700
800
wavelength nm
Refractive Index of Ta2O5
1.86
n
1.84
1.82
1.80
1.78
400
500
600
wavelength nm
Refractive Index Real Part of Ta
n
2.5
2.0
1.5
400
450
500
550
600
wavelength nm
650
700
750
88
Refractive Index Imaginary Part of Ta
3.0
2.8
n
2.6
2.4
2.2
2.0
1.8
400
450
500
550
600
650
700
750
wavelength nm
(* Effective dielectric function of the medium *)
(* D.A.G. Bruggeman, Ann. Phys. (Leizig) 24, 636 (1935) *)
fft=.
neff=Sqrt[eeff/.(Solve[((1-fft)*(nn0^2-eeff)*(nn3^2+2*eeff)+fft*(nn3^2eeff)*(nn0^2+2*eeff)0)/.{wlwwlt, fftffft},eeff][[2]])]
neff/.{wwlt550, ffft0.15}
1.10335
Rsub=R/.{n2(neff/.{ffftfff, wwltwl})} ;
R4=Rsub/.{n0nn0,n1nn1, n3nn3, n4nn0};
Plot[{1-R4/.{d10, d290, d3300,fff0.15},
1-R4/.{d10, d290, d3410,fff0.15}},
{wl,350,750},FrameTrue,AxesTrue,AxesOriginAutomatic,PlotLabel"",
FrameLabel{"Wavelength(nm)","Transmittance"},PlotStyle{Red,Blue}]
Transmittance
1.00
0.95
0.90
0.85
0.80
400
500
600
Wavelength nm
700
89
A.3. Mathematica Codes for Chapter 5
A.3.1. Analyzing the SEM Image with the 2D Discrete Fourier Transform
(*set input file*)
inputfile="070111-09-top_M2.tif";
(*set working directory*)
SetDirectory[$UserDocumentsDirectory]
(*
workingdirectory="C:/Users/tnk/Funding/squid/images2012";
SetDirectory[workingdirectory]
*)
(*read image *)
Ai =Import[inputfile];
(* length of image in nm *)
L0=2000
2000
Ag =ColorConvert[Ai,"Grayscale"]
ImageDimensions[Ag]
{1015,670}
(* length per one pixel in nm *)
Lp=L0/ImageDimensions[Ag][[1]]//N
1.97044
Agnc=ImageCrop[Ag,{650,650}]
90
(* length of croped image in nm *)
L=Lp*ImageDimensions[Agnc][[1]]//N
1280.79
data = ImageData[Agnc];
ArrayPlot[data]
ps=RotateRight[(Abs[Fourier[data]])2, {325,325}];
trsh=2.0;
psn=Table[If[ps[[i,j]]>trsh,trsh, ps[[i,j]]], {i, 1, 650}, {j, 1, 650}];
roi=75*Lp
drange={{-((2)/L)*roi,(2)/L*roi}, {-((2)/L)*roi,(2)/L*roi}}
147.783
{{-0.724983,0.724983},{-0.724983,0.724983}}
ps1=Take[psn, {250,400}, {250,400}];
(* Plot the 2D DFT power spectrum, the unit of axis is nm^-1 *)
ArrayPlot[ps1, FrameTrue, FrameTicksAutomatic,ColorFunctionScalingTrue,
PlotRangeClippingTrue, DataRangedrange]
91
0.
0.
0.
0.
Rdist1=Table[0, {i, 325}];Do[Do[If[Floor[1+
]<326,Rdist1[[Floor[1+
i
325
2
j
325
i
325
2
j
i
325
325
2
2
2
j
325
]]]=Rdist1[[Floor[1+
]]] +ps[[i,j]]], {i, 1, 650}], {j, 1, 650}];
Rdist2=Table[If[i<4,0,Rdist1[[i]]], {i, 325}];
ListPlot[Rdist2, PlotRange{{0,50},{0, 1000}}]
1000
800
600
400
200
0
0
10
20
30
40
50
Rtot2=0;
Do[Rtot2=Rtot2+Rdist2[[i]], {i,1, Length[Rdist1]}];
Rdist3=Table[Rdist2[[i]]/Rtot2, {i, 325}];
ListPlot[Rdist3, PlotRange{{0,50},{0, 0.15}}]
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
10
20
30
40
50
2
92
Rdist4=Table[{(2)/L*i,Rdist3[[i]]*100}, {i, 325}];
ListPlot[Rdist4, PlotRange{{0,0.25},{0,
12}},FrameTrue,AspectRatio0.75,GridLinesAutomatic,JoinedFalse,PlotStyle{PointSize
[0.02`],RGBColor[0,0,1]},PlotLabel" ",FrameLabel{"k [1/nm]","Total Fourier Power
[%]",},BaseStyle{FontSize14}]
12
Total Fourier Power
10
8
6
4
2
0
0.00
0.05
0.10
0.15
0.20
0.25
k 1 nm
Rdist5=Table[{4*((2)/L*i)-1,Rdist3[[i]]*100}, {i, 325}];
Rgr2 =
ListPlot[Rdist5,FrameTrue,PlotRange{{0,800},{0,12}},GridLinesAutomatic,JoinedFalse,
PlotStyle{PointSize[0.02`],RGBColor[0,0.8,0]},PlotLabel"NG",FrameLabel{"
[nm]","Backscattered Power [Arb. units]",},BaseStyle{FontSize14}]
NG
12
BackscatteredPower Arb. units
10
8
6
4
2
0
0
200
400
600
800
nm
Rdist6=Table[{4*1.36*((2)/L*i)-1,Rdist3[[i]]*100}, {i, 325}];
Rgr3=ListPlot[Rdist6,FrameTrue,PlotRange{{0,800},{0,12}},GridLinesAutomatic,Joined
False,PlotStyle{PointSize[0.02`],RGBColor[0,0,1]},PlotLabel"NG",FrameLabel{"
[nm]","Backscattered Power [Arb. units]",},BaseStyle{FontSize14}]
93
NG
12
BackscatteredPower Arb. units
10
8
6
4
2
0
0
200
400
600
800
nm
Show[ Rgr2, Rgr3, AspectRatio0.75, PlotLabel" " ]
(* Green represents the dry nanograss and blue represents the wet spot *)
BackscatteredPower Arb. units
12
10
8
6
4
2
0
0
200
400
nm
600
800
94
A.3.2. Deconvoluting the Measured Reflectance Spectra of the Nanostructured Thin Films
and the Planar Ta2O5 Thin Film to Predict the Scattering Spectra of the Nanostructured
Thin Films
(* comparison of a blue spot on yellow nanograss with a planar yellow Ta2O5 sample *)
(* MURI\Publications\ACS\spectrum_4_sm-acs.nb *)
(* set input file *)
inputfile1= "Ta_NG_Blue_sm.txt";
(* set working directory *)
SetDirectory[$UserDocumentsDirectory]
(* workingdirectory = "C:/Users/tnk/Funding/squid/trip_2012/spectra";
SetDirectory[workingdirectory] *)
(* read data *)
stream = OpenRead[inputfile1];
Skip[stream,Record, 1];
rawdata1= ReadList[stream, {Number, Number, Number, Number, Number}];
Close[stream];
(* generate NG and Ta2O5 reflectance *)
NGBSdata = Table[ {rawdata1[[i,1]], rawdata1[[i,5]]/100}, {i, 1, Length[rawdata1]}];NGdata =
Table[ {rawdata1[[i,1]], rawdata1[[i,3]]/100}, {i, 1, Length[rawdata1]}];TOdata = Table[
{rawdata1[[i,1]], rawdata1[[i,2]]/100}, {i, 1, Length[rawdata1]}];
(* plot NG and Ta2O5 reflectance *)
(* Measured reflectance spectrum of the blue spot *)
NGBSgr1=ListPlot[NGBSdata,PlotRange{0,
1},FrameTrue,AspectRatio0.75,GridLinesAutomatic,JoinedTrue,PlotStyle{Thickness[0
.007`],RGBColor[0,1,1]},PlotLabel" ",FrameLabel{" [nm]",
"Reflectance"},BaseStyle{FontSize14}]
95
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
nm
(* Measured reflectance spectrum of the dry nanograss sample with yellow appearance *)
NGgr1=ListPlot[NGdata,PlotRange{0,
1},AspectRatio0.75,FrameTrue,GridLinesAutomatic,JoinedTrue,PlotStyle{Thickness[0
.007`],RGBColor[1,0.5,0]},PlotLabel" ",FrameLabel{" [nm]",
"Reflectance"},BaseStyle{FontSize14}]
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200
300
400
500
nm
600
700
800
(* Measured reflectance spectrum of the planar thin film with yellow appearance *)
TOgr1=ListPlot[TOdata,PlotRange{0, 1},AspectRatio0.75,
FrameTrue,GridLinesAutomatic,JoinedTrue,PlotStyle{Thickness[0.007`],RGBColor[0,0,
0]},FrameLabel{" [nm]", "Reflectance"},BaseStyle{FontSize14}]
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
nm
Show[NGgr1, NGBSgr1,TOgr1, PlotLabel" ", FrameLabel{" [nm]", "Reflectance"}]
96
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
nm
SCBSfunc1=.;
SCBSfunc1[a_,b_,c_,s_,d_,_]:=Tanh[(/d)2]*(a+ b*Exp[-((-s)/c)2]);
(* Equation (5.2) *)
(* Tanh[(/d)^2] (or other functions) makes the predicted scattering function go to zero; (a+
b*Exp[-((-s)/c)^2]): Gaussian function *)
at=0.23; bt=0.50; ct=90; st=260;dt=150;
SCBSgr2=Plot[SCBSfunc1[at,bt,ct,st,dt,x], {x,0, 800}, PlotRange{0,1}, FrameTrue,
FrameLabel{" [nm]", "I /I0"},GridLinesAutomatic,
PlotStyle{Thickness[0.007`],RGBColor[0,0,1]},BaseStyle{FontSize14}]
(* Predicted scattering spectrum from the blue spot *)
1.0
I I0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
nm
NGBSdata1= Table[{NGdata[[i,1]], TOdata[[i,2]]*(1-SCBSfunc1[at,bt,ct,st,dt,NGdata[[i,1]]])2},
{i, 1, Length[rawdata1]}];
(* Equation (5.1): R = R_interference * ( 1 - R_scattering )^2 *)
NGBSgr2=ListPlot[NGBSdata1,FrameTrue,GridLinesAutomatic,JoinedTrue,PlotStyle{T
hickness[0.007`],RGBColor[0,0,1]},PlotLabel"NG",FrameLabel{"wavelength",
"Reflectance"},BaseStyle{FontSize14}];
Show[ NGBSgr1, NGBSgr2,AspectRatio0.75]
97
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200 300
400 500 600 700 800
nm
Show[ TOgr1, SCBSgr2,AspectRatio0.75,FrameLabel{" [nm]", "I /I0"}]
1.0
I I0
0.8
0.6
0.4
0.2
0.0
200
300
400 500
600
700 800
nm
SCfunc1=.;
SCfunc1[a_,b_,c_,s_,d_,_]:=Tanh[(/d)2]*(a+ b*Exp[-((-s)/c)2]);
at=0.08; bt=0.79; ct=80; st=200; dt=80;
SCgr2=Plot[SCfunc1[at,bt,ct,st,dt, x], {x,0, 800}, PlotRange{0,1}, FrameTrue,
FrameLabel{" [nm]", "I /I0"},GridLinesAutomatic,
PlotStyle{Thickness[0.007`],RGBColor[0,1,0]}, BaseStyle{FontSize14}]
(* Predicted scattering spectrum from the dry nanograss *)
1.0
I I0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
nm
NGdata1= Table[{NGdata[[i,1]], TOdata[[i,2]]*(1-SCfunc1[at,bt,ct,st,dt,NGdata[[i,1]]])2}, {i, 1,
Length[rawdata1]}];
NGgr2=ListPlot[NGdata1,FrameTrue,GridLinesAutomatic,JoinedTrue,PlotStyle{Thickne
ss[0.007`],RGBColor[0,1,0]},PlotLabel"NG",FrameLabel{"wavelength",
"reflectance"},BaseStyle{FontSize14}];
98
Show[ NGgr1, NGgr2,AspectRatio0.75]
1.0
Reflectance
0.8
0.6
0.4
0.2
0.0
200 300 400 500 600 700 800
nm
Show[ TOgr1, SCgr2,AspectRatio0.75,FrameLabel{" [nm]", "I /I0"}]
1.0
I I0
0.8
0.6
0.4
0.2
0.0
200 300 400 500 600 700 800
nm
(* Scattering spectrum obtained from the reflectance measurements *)
(* Green represents dry nanograss scattering and blue represents wet spot scattering *)
Show[SCgr2, SCBSgr2, AspectRatio0.75,FrameLabel{" [nm]", "I /I0"}]
1.0
I I0
0.8
0.6
0.4
0.2
0.0
0
200
400
nm
600
800
99
A.4. Process Parameters for Fabricating Microflowers in Chapter 3
To fabricate microflowers, there are 5 major steps as follows.
I. The deposition and patterning of a Si3N4 mask layer
1. A Si wafer was pre-furnace-cleaned using a typical RCA wet cleaning procedure.
1.1. Piranha, H2SO4 : H2O2 = 60 : 1, temp = 100 C for 15 min
1.2. Rinse 10 cycles
1.3. NH4OH : H2O2 : H2O = 1 : 1 : 5, megasonic, temp = 75 C for 15 min
1.4. Rinse 10 cycles
1.5. HCl : H2O2 : H2O = 1 : 1 : 5, megasonic, temp = 75 C for 15 min
1.6. Rinse 10 cycles
1.7. H2O : HF = 50 : 1, temp = ambient for 2 min
1.8. Rinse 10 cycles
1.9. Rinse and dry a wafer in the spin-rinse dryer
2. A Si3N4 layer 270 nm thick was then deposited using low pressure chemical vapor
deposition (LPCVD) at 650 C in the silicon nitride LPCVD furnace (Tystar nitride tube),
recipe: lowznit for 36 min.
3. Next, photoresist was patterned as a mask for the Si3N4 layer. Typical steps for photoresist
patterning at all levels were as follows.
3.1. Hexamethyldisilazane (HMDS) was applied on the substrate by the vapor prime
method at 105 C.
3.2. S1813 photoresist was applied by spin-coating at 3000 rpm for 30 s.
100
3.3. The photoresist was soft-baked at 115 C for 1 min 30 s producing a final thickness
of 1.5 m.
3.4. The photoresist was exposed using contact printing (Karl-Suss MA-6) for 6 s to
create the pattern for the nanograss array (2 m diameter posts with 10 m pitch).
3.5. The photoresist was then developed for 60 s with MF-321 developer.
4. The Si3N4 hardmask pattern for the nanograss array was etched using reactive ion etching
(RIE) dry plasma etch (PlasmaTherm).
5. The photoresist was then stripped with acetone followed by wet cleaning.
II. The deposition and patterning of a sacrificial oxide layer
6. After pre-furnace RCA wet cleaning, the low temperature silicon dioxide (LTO) layer
(2.4±0.2 m thick) was deposited on top of the Si3N4 hardmask using LPCVD in the LTO
furnace (Tystar LTO tube), recipe: stdlto, for 30 min at 450 C.
7. Photoresist (STR1045) then was spin-coated at 4000 rpm for 30 s, soft-baked at 110 C for
1 min 30 s, patterned using contact printing for 30 s, developed, rinsed, and hard-baked at
125 C for 20 min producing a final thickness of 4.4 um.
8. The LTO layer was etched using RIE stopping on the Si substrate to create the contact
holes.
9. The photoresist was then stripped.
III. The deposition and patterning of the poly-Si microflowers
10. After the pre-furnace wet clean, a 1.3±0.3 m poly-Si layer was deposited on top of the
patterned LTO using LPCVD in the poly-Si LPCVD furnace (Tystar Poly tube) at 625 C.
11. The poly-Si layer was patterned using contact printing with STR104 photoresist.
101
12. The poly-Si layer was etched with deep Si plasma etch (DRIE) using the Bosch process
stopping on the LTO layer.
IV. The patterning of an array of nanoposts (often called nanograss) on the Si substrate
13. The LTO was etched with 6:1 buffered hydrofluoric acid for 27 min, without removing
the photoresist used to pattern the microflowers. After the wet-etch, the wafer was rinsed in
DI water for 15 min and then dried.
14. The Si substrate was again plasma etched (DRIE) using the Si3N4 hardmask to form the
nanograss structures and the photoresist mask layer to protect the poly-Si microflowers.
15. The photoresist was stripped and the wafer was pre-furnace wet cleaned.
V. The annealing, final release and oxidation of microflowers
16. The unreleased structures were annealed at 750C in forming gas for 6 hr.
17. After release (in 49% HF) the wafer was rinsed in DI water for 15 min, which was then
replaced with isopropyl alcohol (IPA). The IPA was removed by rapid evaporation at 150C
to finally release the microflower petals.
18. For electrowetting purposes, a SiO2 dielectric layer was thermally grown on poly-Si
flowers. The SiO2 layer was grown (102 nm thick) using wet oxidation at 950 C.
19. The microflowers and nanograss were coated with a hydrophobic CFx layer using plasma
deposition from C4F8 gas.
102
A.5. Process Parameters for Fabricating Transparent Ta2O5 Nanostructured Thin
Films in Chapter 4
The transparent Ta2O5 nanostructured thin films were fabricated using a multi-step anodization
process of tantalum (Ta) thin films as follows.
1. The Ta layer (75 nm thick) was sputter-deposited using the DC sputterer (CVC 601, power 1
kW, 5 mT Ar pressure, deposition time 3.75 min) on a cleaned quartz wafer.
2. An aluminum (Al) layer 600 nm thick then was sputter-deposited (DC power 1 kW, 8 mT Ar
pressure, deposition time 17.5 min) on top of the Ta layer without breaking vacuum.
3. The Al layer was anodized in 0.2 M oxalic acid with a constant current density of 10 mA/cm 2
while ramping the voltage up to 53 V. As soon as 53 V was reached, the anodization process
was stopped preventing the thin Al film from being completely anodized.
4. The thin top layer of Al2O3 was stripped in a chromium trioxide (CrO3) / phosphoric acid
(H3PO4) solution (2 g CrO3 / 100 ml H2O, 4.1 ml conc. H3PO4) at 70 C for 15 min.
5. The remaining Al layer was reanodized by sweeping the voltage from 0 V to 45 V (1 V/s) in
0.0005 M oxalic acid.
6. The Al layer was reanodized in 0.2 M oxalic acid at 53 V with constant current density of 10
mA/cm2 to completely oxidize the Al.
7. When the applied voltage was stable at 53 V and the current density dropped to 0.5 mA/cm2 or
lower, the voltage was then increased to 200 V for 12 min to grow Ta2O5 posts in the Al2O3
pores.
8. The Al2O3 layer was stripped in the CrO3/H3PO4 solution at 70 C for 120 min.
103
9. The remaining metallic Ta layer was reanodized in 0.2 M oxalic acid ramping the voltage to
200 V with a constant current density of 0.2 mA/cm2. Complete oxidation of the film occurred
at a voltage below 200 V, at which point the current would decrease.
10. After being cleaned using O2 plasma, the Ta2O5 nanograss surface became highly
hydrophilic.
11. To make the Ta2O5 nanograss surface superhydrophobic, a thin CFx film was deposited using
plasma excitation of C4F8.
104
A.6. Process Parameters for Fabricating Reflective Ta2O5 Nanostructured Thin
Films in Chapter 5
The reflective Ta2O5 nanostructured thin films were fabricated using a multi-step anodization
process of tantalum (Ta) thin films as follows.
1. The Ta layer (150 nm thick) was sputter-deposited (DC power 1 kW, 5 mT Ar pressure,
deposition time of 7.5 min) on a cleaned Si wafer.
2. An aluminum (Al) layer 600 nm thick then was sputter-deposited (DC power 1 kW, 8 mT Ar
pressure, deposition time of 15 min) on top of the Ta layer without breaking vacuum.
3. The Al layer was anodized in 0.2 M oxalic acid with a constant current density of 10 mA/cm 2
while ramping the voltage up to 53 V. As soon as 53 V was reached, the anodization process
was stopped preventing the thin Al film from being completely anodized.
4. The thin top layer of Al2O3 was stripped in a chromium trioxide (CrO3) / phosphoric acid
(H3PO4) solution (2 g CrO3 / 100 ml H2O, 4.1 ml conc. H3PO4) at 70 C for 15 min.
5. The remaining Al layer was reanodized by sweeping the voltage from 0 V to 45 V (1 V/s) in
0.0005 M oxalic acid.
6. The Al layer was reanodized in 0.2 M oxalic acid at 53 V with constant current density of 10
mA/cm2 to completely oxidize the Al.
7. When the applied voltage was stable at 53 V and the current density dropped to 0.5 mA/cm2 or
lower, the voltage was then increased to 200 V for 12 min to grow Ta2O5 posts in the Al2O3
pores.
8. The Al2O3 layer was stripped in the CrO3/H3PO4 solution at 70 C for 120 min.
9. The Ta2O5 nanograss surface was cleaned using O2 plasma.