communications
Microlithography
‘‘Lock-and-Key’’ Geometry Effect of Patterned Surfaces:
Wettability and Switching of Adhesive Force**
Xing-Jiu Huang,* Dong-Haan Kim, Maesoon Im, Joo-Hyung Lee, Jun-Bo Yoon, and
Yang-Kyu Choi*
A rough surface can be a regular (engineered surface), a
random (irregular rough surface), or an intermediate case
(hierarchical rough surface).[1] Whichever case is used for
wettability, a truly superhydrophobic surface exhibits not only
a high contact angle (>150 8) but also a low-contact-angle
hysteresis (sliding angle).[2] Quéré et al. theoretically
described how contact-angle hysteresis generates an adhesive
force and the contact angle and hysteresis can be controlled by
tailoring the surface topography of the solid substrate.[3]
Researchers have since attempted to capture these properties
in synthetic materials with nanoscale surface features[4] or
changes of surface topography.[5,6]
For flexibility in adapting to rough surfaces, poly
(dimethylsiloxane) (PDMS) elastomer, an ideal elastic
material in terms of its stress–strain response, has attracted
great attention. Particular interest has focused on introducing
nanoscale structures onto microscale surfaces using surface
treatments to reach a hydrophobic state, such as mechanically
assembled monolayers,[7] CO2 pulsed-laser etching,[8–10] UV/
ozone surface treatments,[11] SF6 plasmas,[12] oxygen plasma
and chemical surface treatments,[13,14] and laser etching.[15]
Besides these, Hang et al. created an artificial lotus leaf by
[] Dr. X.-J. Huang, Prof. Y.-K. Choi, M. Im
Nano-Oriented Bio-Electronic Lab
School of Electrical Engineering and Computer Science
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (South Korea)
E-mail: [email protected]; [email protected]
D.-H. Kim, J.-H. Lee, Prof. J.-B. Yoon
3D Micro-Nano Structures Lab
School of Electrical Engineering and Computer Science
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (South Korea)
[] X.-J.H. would like to express appreciation for the financial support
of the Brain Korea 21 project, the school of Information Technology, and the Korea Advanced Institute of Science and Technology
in 2007. This work was also partially supported by the NRL
program of the Korea Science and Engineering Foundation grant
funded by the Korea government (MOST) (No.R0A-2007-00020028-0).
: Supporting Information is available on the WWW under http://
www.small-journal.com or from the author.
DOI: 10.1002/smll.200800649
90
nanocasting[16] and Lee et al. fabricated PDMS micropillar
structures.[17]
Patterned surfaces thus exhibit their unique advantages in
hydrophobicity due to their large-scale surface uniformity.
However, the adhesive force still exists at some interfaces.
Inspired by the ‘‘lock-and-key’’ model, a new type of
patterned surface consisting of dense arrays of microfabricated
PDMS lenses (lock) and bowls (key) for the wettability and
switching of the adhesive force is presented here. The
dimension of the arrayed PDMS microlens is studied to
optimize the hydrophobicity and adhesive force. The
imprinted microbowl-arrayed surface exhibits a superhydrophobicity with a high contact angle (approximately 164.6 8)
and a low adhesive force (the work of adhesion decreased
nearly one-tenth in comparison with microlens arrays). This
approach provides a simple method to investigate the
transformation of an adhesive force into an anti-adhesive
force. Significantly, the anti-adhesive properties of materials
with microbowl arrays should be very useful in a wide range of
biomedical applications, including blood pumps, cardiac
pacemaker leads, and other PDMS-based medical devices,
due to their good biocompatibility and flexibility.[18]
A superhydrophobic surface is created by close-packing
trifluoromethyl groups to decrease the surface energy. The
strategy in this study employed microlens and microbowl
arrays without a thin fluorinated carbon film coating
(Figure 1). First, an AZ9260 positive photoresist (PR, Clariant
Co. Ltd.) layer with a thickness of 80 mm was spin coated onto
a Si substrate at 1500 rpm for 0.5 s. Using the threedimensional (3D) diffuser lithography that has been reported
previously,[19] a highly ordered array of PR microbowls was
formed on the substrate. For the microlens array, the primary
design criteria included the diameters of the heights of the
lens. The microlens array is supported on a continuous PDMS
(Sylgard 184, Dow Corning) or h-PDMS (KE-1606, Shin-Etsu,
Japan) film (2–3 mm in thickness). A microlens array of
10-mm diameter and 6-mm height (10 6), a 10 2 microlens
array, and a flat PDMS were also tested for adhesive force.
To comparably check the wettability and switching of adhesive
force, 10 6 PDMS microbowl arrays, with imprinted inverse
microlens structure following the lock-and-key domains) were
fabricated. A second replication of PDMS was performed on
the h-PDMS microlens-arrayed template in the same manner
as the first process. Before casting, an antistick monolayer
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Scanning electron microscopy (SEM,
Philips XL 30 AFEG Eindhoven, The
Netherlands) images are shown in
Figure 2 to demonstrate the features of
the fabricated microlens arrays. A uniform
feature is clearly observed at low magnification (Figure 2a and b). The surface
exhibits a high ‘‘flatness’’ that was inherited
from the supporting PR microbowl-arrayed
substrate. No distortion was observed on
the array surface, suggesting that this
approach eliminates significant drawbacks
(e.g., small areas, non-homogeneity, nonuniform distribution) encountered with
conventional bottom-up techniques. Looking at the surface properties, it can be
deduced that the highly uniform surface
with very few defects contributes significantly to the smallest sliding angle, an
important characteristic of the rough
surfaces. In contrast, it was observed that
the structure exhibits hydrophobic properFigure 1. Rational design and fabrication of lens and bowl microarrays based on a lock-and- ties with a high-contact-angle hysteresis, as
key principle. A three-dimensional diffuser lithograph was used to create an array of
will be considered in more detail in the
microbowls in a photoresist (PR) thin film supported on Si (PR/Si master). PDMS or h-PDMS
following section. Figure 2c and d shows
casting onto the master was followed by curing, and the lift-off resulted in the microlens
high-magnification images of the PDMS
array containing the lock aspect. Finally, an anti-stick monolayer was coated onto the
microlens, and the convex shape and
fabricated h-PDMS microlens array (used as a template). A second replication of PDMS
was then performed on the microlens array and peeled off, resulting in the matching key
smooth exterior surface of the lens can
PDMS microbowl array.
be clearly seen.
The first attempt at measuring the
contact angle (u) of a droplet was undertaken to find the geometrical effect and
dimension-dependent hydrophobicity of
the perfectly ordered microlens surface (u
was measured on a Dataphysics OCA20
CA system at ambient temperature; the
droplet volume used in the experiments
was 6 mL). Arrays of PDMS microlens 5,
10, and 20 mm in diameter and 2 mm in
height, as well as 10 mm in diameter and 6,
7, 8, and 12 mm in height, were successfully
fabricated using a 3D diffuser lithograph.
Figure 3a clearly shows the comparison
results of a flat surface and the microlensarrayed surface. The water-contact angle of
the flat surface of PDMS was approximately 108.3 8 (Figure 3b). A heightdependent hydrophobicity was also found
(Figure 3a, violet square and fitted line);
Figure 2. A 40 8 view of the fabricated PDMS microlens arrays with different magnifications. A
uniform feature is clearly observed at low magnification (a, b). The microlens-arrayed surface the apparent contact angle noticeably
increased with an increasing height and
morphology is apparent at high magnification (c, d).
the surface changed from moderately
hydrophobic (which is always the case with
(vapor-phase tridecafluoro-1,1,2,2-tetrahydrooctyltricholrosi- water where the contact angles never exceed 120 8)[20] close to
lane (CF3-(CF2)5(CH2)2 -SiCl3, Fluka) was evaporated on the a superhydrophobic state (150 8). This is a very interesting
PDMS microlens array surface as a release agent for 30 s, and result: it is entirely different from previous reports on
the PDMS solidification was performed at 85 8C for 60 min. microfabricated pillars or post arrays that show a highly
The PDMS film was then peeled off, which imprinted the hydrophobic state if given the smallest possible values of a/H
(a: pillar size; H: height; the contact angle drastically increased
inverse microlens array structures.
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Figure 3. The diameter- and height-dependent hydrophobic behavior
of a microlens- arrayed surface. a) The comparison between the
wettability of a flat surface and the microlens array with different
dimensions. The inset is a simple model of a water droplet pinned on a
microlens-arrayed surface; b–d) Micrographic pictures of a droplet on a
flat surface and microlens array with different diameters and heights.
with increasing H).[21] Therefore, the fact can be explained by
considering the unique structure of the microlens and the
adhesive force between the PDMS microlens and water
droplet, which results in a heterogeneous contact.
As shown in Figure 3a, the variation of the contact angles
measured at different sites further implies that the PDMS
microlens-arrayed surface has hydrophobicity with a high
adhesive force. Again, the wettability of the microlens-arrayed
surfaces with different diameters was investigated. As highlighted by the black circle in Figure 3a (black square), a good
diameter-dependent wettability is evident; that is, the contact
angle decreased as the diameter of the microlens increased
(Figure 3c). In particular, the contact angle approaches 110.5 8
for the 20 2 microlens-arrayed surface, which is very close to
that of the flat surface (108.3 8). It is not difficult to understand
this situation. If the structure is too low and too wide, the
microlens-arrayed surface becomes almost flat and the droplet
is able to wet the surface and enters the Wenzel regime[22] that
is based on the hypothesis of a saturated surface. Therefore, it
was concluded that, in these situations, a perfectly ordered
microlens array geometrically and crucially influences the
hydrophobic behavior of the surfaces. Furthermore, the
diameter and height of the microlens are crucial parameters
that govern the hydrophobicity. Besides, this type of patterned
surface is very useful for quantitative studies of the
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Figure 4. The adhesive force of a microlens array. a,b) Behavior of a water
droplet on a 10 6 microlens-arrayed surface with tilt angles of 90 8
and 180 8, respectively. c) The work of adhesion (open symbol) and
contact angle (solid symbol) as a function of the drop volume on the PDMS
with flat surface (square), 10 2 array (circle), and 10 6 array (triangle).
The work of adhesion was obtained directly from the contact-angle
measurements. The insets show a cross-sectional view of the SEM images
of the microlens array with different heights. d) Schematic image of
thepossiblestaticanddynamicbehaviorofadropletatmicrolens-arrayed
interfaces according to the observations.
equilibrium configurations of droplets on rough substrates[9]
and therefore avoids the indiscriminate problems that occur
with randomly rough surfaces.
Figure 4a and b shows the behavior of the water droplet
when the PDMS microlens-arrayed substrate is tilted
vertically or turned upside down. It was noted that the water
droplet does not slide and the contact angle is larger at the
front (advancing angle, uA) than at the rear (receding angle,
uR; Figure 4a). This contact-angle hysteresis (uA–uR) generates
a force that opposes the weight of the drop (and is able, if the
drop is small enough, to balance it): the liquid is pinned.[23] As
indicated above, even though the height of the microlens was
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increased, the surface could not reach a state in which the
contact angle was larger than 150 8; this is the case where the
contact-angle hysteresis (i.e., a strong adhesive force) might be
influenced in the transition of a surface from a hydrophobic to
a superhydrophobic. Here, it is ascribed to the Wenzel model
in which the droplet is able to wet the surface, thus resulting in
a high-contact-angle hysteresis (Figure 4d). The microlensarrayed surface thus becomes ‘‘sticky’’. The work of adhesion
is a method used to investigate the physical force (i.e., adhesive
force or adhesion) at interfaces between solid substances and
water.[24] It is generally defined by the following equation:[25]
W ¼ g LV ð1 þ cos uÞ
where W is the work of adhesion, and g LV and u refer to the
surface tension of the liquid–vapor and apparent contact
angle, respectively. Therefore, the work of adhesion and
contact angle of the PDMS flat and microlens-arrayed surfaces
was measured, and the comparison is shown in Figure 4c. The
work of adhesion decreased with an increase of the contact
angle for these three types of surface. The structure of the
microlens array improved the adhesion at the interfaces. The
work of adhesion was obviously decreased by introducing
microlens arrays onto the surface. Additionally, the adhesive
force on the 10 6 microlens-arrayed surface was smaller
than that on the 10 2 surface but it was still strong. It was
then concluded that the flat PDMS showed a higher work of
adhesion and the surface topography could greatly influence
the adhesive force.
For a nanotube film and an artificial lotus-leaf surface, it
has been suggested that the high adhesive force results from
the nanotube density[26] and the force acting between the
micro-orifices and wall roughness.[27] It is proposed here that
the high adhesive force results from the microlens-shaped
array geometry. Considering that the contact angle and
contact-angle hysteresis can be controlled by tailoring the
surface topography of the solid substrate, PDMS microbowl
arrays were fabricated using PDMS microlens arrays as a
templates (lock-and-key domains). Figure 5a shows the
surface topography of microbowl arrays. The bowl-like shape
and nanoscale wall notches are clearly visible. The bowls are
arranged in a regular hexagonal array. This is a perfect holearrayed surface with numerous notches and sharp nano-apexes
(this is why the influence of release agent on the wettability is
ignored). The ‘‘substrate’’ is mainly composed of air, which
eventually leads to a strong reduction or elimination of the
contact-angle hysteresis. The inset in Figure 5a indicates that
the bowl-shaped structure has a hollow that looks like a
capillary tube. For this geometry, its size-dependent superhydrophobic behavior has been demonstrated both experimentally and theoretically using a photoresist material as an
example.[19] Given reasonable dimensions, the wettability
follows the Cassie–Baxter model in which the air trapping
mode is dominant.[28] As expected, the 10 6 PDMS
microbowl-arrayed surface exhibits perfect superhydrophobicity with a contact angle of approximately 164.6 8 (Figure 5b)
owing to the hollow structures in the surface. It is much better
than that of the AZ9260 photoresist microbowl-arrayed
surface with the same dimensions (143.5 8).[19] This is because
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Figure 5. The superhydrophobic behavior of a 10 6 PDMS microbowlarrayed surface. a) SEM image of a 10 6 PDMS surface morphology with
a microbowl-arrayed structure. The inset shows a high-magnification
SEM image of the bowl-shaped structure. The scale bar is 2 mm. b) The
shape of a water droplet on the microbowl-arrayed surface, indicating its
superhydrophobicity with a contact angle of 164.6 8. c) The contact angle
measured at different locations for a PDMS microbowl-arrayed sample,
and the comparison of adhesive force obtained directly from contactangle measurements from a PDMS microlens-arrayed surface.
d) Schematic image of the possible static and dynamic behavior
of a droplet at microbowl-arrayed interfaces according to the observations.
the AZ9260 photoresist is a weak hydrophilic material,
uY ¼ 78.5 8. Since the contact-angle hysteresis originates in
the defects of the solid substrate, only a very small hysteresis is
expected on this microtextured superhydrophobic surface.
Next, in order to determine the contact-angle hysteresis of
the surfaces, the sample was fixed on an optical bench below
the syringe, which could be moved freely. A droplet was first
formed and maintained with the syringe. When the droplet was
in the exact contact state, the substrate was slowly moved with
a micrometric screw. It is noted that the water droplet does not
come to rest and can be slipped effortlessly on this surface
using a syringe; the receding contact angle is almost equal to
the advancing contact angle (Figure S1), indicating a very
small contact-angle hysteresis or sliding angle. Based on the
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understanding that the contact-angle hysteresis or sliding
angle is a direct measure of the adhesive force, the adhesive
force of a PDMS microbowl-arrayed structure should be small
and a droplet should easily roll off the surface. This situation is
rationalized by considering the trapped air below the droplet
(Cassie–Baxter state), which has an elastic effect on the water
drop (Figure 5d). The slippage is reinforced; thus, the
microbowl-arrayed surface becomes ‘‘slippy’’. Further confirmation that the switching of adhesive force is caused by the
geometry of the microtextures on a solid is found by measuring
the sliding angle of an h-PDMS microlens-arrayed surface,
that is, the same geometry but using different materials
(Figure S2). A water drop could not be separated from the
surface of the sample. The shape of the water droplet changed
significantly, implying a very high contact-angle hysteresis.
Figure 5c shows the contact angle measured in different
locations for a PDMS microbowl-arrayed sample and a
comparison of the adhesive force with a PDMS microlensarrayed surface obtained directly from the contact-angle
measurements. Firstly, there were no obvious variations, even
when the contact angles were measured numerous times at
twelve sites, further demonstrating the uniform surface.
Secondly, Figure 5c clearly confirms the small adhesion of
the PDMS microbowl-structured surface. The work of
adhesion at a PDMS microbowl-arrayed intersurface is onetenth of that at a PDMS microlens-arrayed intersurface. As
discussed above, it is important to design textures that not only
induce air trapping, but that also create a more stable state
than the Wenzel state.
In summary, the transition between hydrophobic and
superhydrophobic and the switching of the strong adhesive
force and anti-adhesive force were experimentally realized
using the lock-and-key geometry effect of patterned surfaces
without silanization. The microlens-arrayed surfaces showed a
dimension-dependent hydrophobic behavior. Their wettability showed a low contact angle and a high adhesive force
following the Wenzel state: the liquid droplet retains contact at
all points with the solid surface below it. The microbowlarrayed surfaces exhibited a high contact angle and antiadhesive behavior following the Cassie–Baxter state where a
drop rests on the peaks of the surface protrusions and bridges
the air gaps in between. The contact-angle hysteresis
microscopically results in the adhesive force, while the contact
angle microscopically results from such a force. Thus, it is
unlikely that a patterned surface can be constructed with both
a high contact angle and a high adhesive force. This case is
entirely different from the hierarchical rough surfaces[27] and
aligned nanotube films[26] fabricated using pure chemical
methods. The results finally lead to another conclusion that if
the surface of the microtexture containing the lock aspect of
some geometries shows hydrophobic behavior, the other
94 www.small-journal.com
surface containing the matching key should exhibit a superhydrophobic behavior even though more time is needed to
demonstrate this contribution. It is believed that the present
work will provide good guidance in understanding the real
superhydrophobic state and designing many functional
surfaces with different geometries.
Keywords:
adhesive forces . geometry effects . superhydrophobicity .
switches
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ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: May 6, 2008
Published online: November 28, 2008
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