INCREASING THE FLUID FLOW VELOCITY IN A MICROCHANNEL
USING 3D NON-METALLIC ELECTRODES
Hamza A. Rouabah1*, Benjamin Y. Park2, Rabih B. Zaouk2 , Marc J. Madou2, Hywel Morgan1 and
Nicolas G. Green1
1
2
School of Electronics and Computer Science, University of Southampton, UK and
Department of Mechanical & Aerospace & Biomedical Engineering, University of California Irvine, USA
ABSTRACT
Handling and manipulating fluid using AC-electroosmosis pumping has recently shown some success. However most
AC-electroosmosis pumps studied previously were fabricated using metals such as gold and titanium, which restrict the
geometry of electrodes to planar shapes. Previously we have shown that conductive polymers can be used to fabricate 3D
planar and high aspect ratio electrodes to drive fluid inside microchannels. This paper presents experimental testing of
two designs of AC electroosmotic micropumps for different applied voltages and fluid conductivities.
KEYWORDS: Pumping, AC-electroosmosis, high-aspect-ratio electrodes, AC-electrokinetics, microfluidics, CMEMS, 3D –electrodes
INTRODUCTION
MicroTAS, Lab-on-a-chip, integrated Microsystems, all require integrated fluid handling systems. The fast evolution
in microelectronics fabrication processes have allowed the miniaturization and integration of a range of different
micropumps. One useful type of micropump in lab-on-a-chip applications is the AC Electroosmotic micropump, since
the operating voltage is low (and therefore negligible Joule heating which may destroy bio-particles), the fabrication
requirements are simple and highly scalable, and system integration is straightforward.
AC electroosmosis is the motion of the Electrical Double Layers (EDL) on microelectrode surfaces, induced and
driven into motion by the non-uniform AC electric field generated by the electrodes [1]. In an AC Electroosmotic
micropump, specially designed electrode arrays use this effect to induce imbalanced electrical forces on the fluid and, as
a result, pumping. Since the electrodes are the core of the AC-electroosmotic micropump, any variation on the structure
or the properties of the electrodes may affect pumping. Usually the electrodes are made from metals such as gold, which
are subjected to corrosion, electrolysis and may contaminate the fluid inside the channel. In addition the use of metals
makes the fabrication of 3D-electrodes complicated due to difficulties in electroplating processes. Alternatively
conductive materials converted from polymers using Carbon-MEMS technology [2], have allowed these problems to be
overcome, making chemically resistive electrodes that are capable of generating AC-electroosmosis flow similarly to
metallic electrodes, and opened the doors to allow complex 3D structures to be easily fabricated for 3D ACelectoosmotic micropumping.
In this paper the fluid flow velocity measurements as a function of voltage and variation of fluid conductivities is
reported for AC-electroosmosis micropumps with two different electrode configurations.
ELECTRODE DESIGN
AC-electroosmosis micropumps comprised of two different electrode configurations were fabricated employing CMEMS technology. The fabrication process of the micropumps and its functionality has been reported previously [3, 4].
The first device consists of 5µm thick planar asymmetric electrodes and the second combines both 50µm thick 3D-highaspect-ratio (3D-HAR) electrodes built on top of 5µm thick planar asymmetric electrodes as shown in Figure 1. The
widths of the electrodes are about 20 and 60 µm. Both designs of electrode array are embedded in a microfluidic channel
which has a width of 500 µm, with the electrodes extending across the entire channel. The dimensions of the pillar
electrodes (3D high aspect ratio electrodes), shown in Figure 1, are 20 x 20 µm on the narrower electrodes and 20 µm x
60 µm on the wider electrode. In the microfluidic channel, the array was aligned so that vertical pillar electrodes were
flush with the side walls of the channels. An SEM of the fabricated 3D-electrodes is also shown in Figure 1.
978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS
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14th International Conference on
Miniaturized Systems for Chemistry and Life Sciences
3 - 7 October 2010, Groningen, The Netherlands
Figure 1: (Left) images “a” and “c” represent the 3D-Planar electrodes, images “b” and “d” represent the
combination of the 3D-Planar + high aspect ratio (HAR) electrodes. (Right) SEM image of the High-aspect-ratio and
planar electrodes.
EXPERIMENTAL
The fluid velocity was measured as a function of applied voltage in different electrolyte conductivities. The
electrolyte was made from different concentrations of potassium chloride (KCl) in ultrapure water. The voltage was
applied using a standard TTI signal generator for a fixed frequency of f=1 kHz for 10 values of AC voltages, linearly
distributed between 1 Vpp and 20 Vpp. To define a pumping direction (since the flow can be in both directions) forward
(positive) pumping is defined to be the direction from the small electrode across the narrow gap to the large electrode.
The duration of the experiments varied but for each data point, the micropump was operated for at least two hours
continuously.
RESULTS AND DISCUSSION
The velocity data is presented as a function of the applied potential (peak to peak) for several concentrations of
electrolyte (concentration).
Figure 2: Fluid flow velocity as a function of voltage for different electrolyte concentrations in the range 10
10 mM for 3D planar electrodes (left) and for 3D-planar + 3D high aspect ration electrodes (right).
μM to
For the planar electrode array, and for the lowest medium concentration (10 µM) and small applied voltages (<
3Vpp), a slight reverse pumping was observed. Increasing the voltage results in a positive fluid flow velocity increasing
with voltage up to 13 Vpp, where a maximum forward fluid flow velocity of 26 µm/s was observed. At higher voltages,
the flow velocity reverses, and becomes increasingly negative with increasing voltage. Reversed flow was observed for
all voltages over 14 Vpp with a maximum value of -55 µm/s at an applied voltage of 20 Vpp, the limit of the generator.
For the higher medium concentration of 0.1mM, the micropump also experiencing reverse pumping for applied
voltages below 5 Vpp. Above the applied voltage beyond 5 Vpp, leads to an increase in the forward flow velocity.
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Although fluid flow velocity slope rises slower than for the 10µM, but no reverse pumping can be seen after 5 Vpp,
where a maximum forward fluid flow velocity of 27 µm/s was reached for an applied voltage of 20 Vpp. For higher
medium conductivities with concentrations of 1 mM and 10 mM, no fluid flow velocity can be seen for a range of the
applied voltages between 1-20 Vpp.
For the 3D HAR electrode array and for the lowest medium concentration, the fluid flow velocity increases almost
exponentially to up to 160 µm/s, with a slight decrease at very high voltages (20 Vpp). For the next highest medium
concentration, the behavior is similar only with a lower magnitude: for applied voltages below 10 Vpp the velocity
increases slowly, above this voltage, the velocity increases exponentially to approximately 120 µm/s and then drops after
increasing the voltage to 20 Vpp. For medium concentration of 1 mM, in this case the fluid flow velocity can be driven by
3D HAR electrodes unlike planar ones. Although the fluid flow slope was very slow, a maximum velocity, of 10 µm/s
was achieved at 19 Vpp, and then it decreases at 20Vpp. For the highest medium concentration (10 mM), no fluid flow
motion was observed.
The reverse pumping phenomena was therefore only observed in the 3D-planar structures at low voltages below 5
Vpp and then at high voltages more than 14 Vpp. The magnitude of the velocity during reversal pumping is observed to
be higher than the peak velocities prior reversal flow. The high voltage behavior is similar to that reported previously but
the low voltage reversal has not been reported. This phenomena is not well understood but the reliable demonstration of
reverse pumping at high voltages, agreed with the hypothesis that there are two charging mechanism on the electrodes
surface: capacitive and Faradaic charging mechanisms [5]. The capacitive charging mechanism results from the electrical
double layer capacitor charging, whereas Faradaic charging is due to the generation of the co-ions from the
electrochemical reactions of the electrodes at certain applied voltages. For higher conductivities, no reversal pumping
can be seen as this can be due to ion crowding effect in the diffuse layer [6].
CONCLUSION
Measurements of the fluid flow velocity as function of voltage for different electrolyte conductivities demonstrated a
steady fluid flow velocity inside the channel at low conductivities as shown in Figure 2. In contrast at the lowest medium
conductivity (10µM) the highest fluid flow can be seen in both designs with the 3D-HAR configuration demonstrating an
increase in fluid flow velocity of approximately 3 times that for the 3D-planar one. For the 3D-planar device, reverse
pumping occurred at voltages above 12V but in the 3D-HAR device no reverse pumping was observed.
ACKNOWLEDGEMENTS
The authors would like to thank the INRF team for their support and collaboration, the Royal Academy of
Engineering and the School of Electronics and Computer Science at the University of Southampton for research funding.
REFERENCES
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