ac electro-osmotic micropump by asymmetric electrode polarization

JOURNAL OF APPLIED PHYSICS 103, 024907 共2008兲

ac electro-osmotic micropump by asymmetric electrode polarization Jie 共Jayne兲 Wua兲 Department of Electrical and Computer Engineering, The University of Tennessee, Knoxville, Tennessee 37996, USA

共Received 16 May 2007; accepted 18 November 2007; published online 18 January 2008兲 ac electro-osmosis 共ACEO兲 has emerged recently as a promising strategy for fluid transport at microscale. With an array of planar interdigital electrodes immersed in an electrolyte, different charging mechanisms at electrode/electrolyte interface and electrokinetic surface flows can be induced by nonuniform electrical fields. To implement ACEO micropump, asymmetry in an electrode pair is essential to generate net flow, which has been typically achieved through asymmetric electrode geometries. This work proposes asymmetric electrode polarization processes to break the electrode symmetry. A dc bias is superimposed onto ac potentials, so that the two electrodes in a pair undergo capacitive charging or Faradaic charging separately. Applying such signals, pumping action has been demonstrated with only a few volts of applied voltage and a power consumption in the range of milliwatts. Pumping velocity by asymmetric electrode polarization exhibits an exponential dependency on voltage. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2832624兴 I. INTRODUCTION

Microfluidic devices are instrumental to the realization or improvement of miniature bio/medical/chemical diagnostic kits, high performance liquid chromaographs, fuel cells, ion exchange devices, chip and microcircuit cooling, biochips for drug screening, etc. An important microfluidic function is to transport and mix fluid at microscale with reliability and efficiency. As surface/volume ratio increases at reduced device scale, electrically driven flows such as electro-osmosis 共EO兲 offer advantages over the more familiar pressure-driven flows. EO can be applied with dc or ac electric sources. dc EO has a long history of development, being investigated and applied extensively. However, dc EO suffers from high voltage operation 共several kVs兲 and consequently excessive electrochemical reactions and electrolysis at the electrodes. In the last few years, ac electro-osmosis 共ACEO兲 receives increasing research interest as it has demonstrated great potential for microfluidic actuation. Compared with dc EO, ACEO has the following advantages and features: 共1兲 low operating voltage 共less than 10 Vrms versus several kilovolts兲, 共2兲 alternating electric fields, minimizing electrolysis and chemical reactions, and 共3兲 nonuniform streamlines, which can be used to convect and mix fluids. As a result, ACEO has been under intensive research in recent years for microfluidic applications, and ACEO pumps have shown great potential for lab-on-a-chip applications with its low-voltage operation, planar electrode structure, and capability for local flow control. However, ACEO tends to produce recirculating flows as reported in the literature. In order to produce net flow, it is necessary to break ac electric field symmetry. So far, this objective has been achieved by various asymmetric electrode geometries. The examples include asymmetric interdigitated electrodes,1–3 orthogonal electrodes,4 and three-dimensional ACEO pump.5 a兲

Electronic mail: [email protected].

0021-8979/2008/103共2兲/024907/5/$23.00

Here, we report a different type of ACEO and its basic characteristics for fluid transport or mixing. The strategy is to induce asymmetry polarization of the electrodes in a pair by applying biased ac signals, so as to generate unidirectional flow of fluids.6 When a voltage is applied over the electrodes in electrolytes, electrode surfaces will become populated with net charges and the process is known as polarization. The process is predominately induced or capacitive charging7 at relative low potentials 共⬍1 V兲, and electrochemical reaction or Faradaic charging at higher potentials 共2 – 3 V兲. The two types of electrode polarizations generate opposite surface charges for EO surface flows. By applying asymmetric electric signals to the electrodes, the two electrodes in a pair will exhibit different polarizations and strength of the double layer at the electrode/electrolyte interface. As a result, a unidirectional Maxwell force will be exerted on the fluid, leading to throughflow pumping. The mechanism of asymmetric polarization ACEO will be discussed in details in the following section. II. MECHANISMS

Electro-osmosis is the fluid motion induced by the movement of surface charges at the solid/liquid interface under the influence of electric fields. ACEO is implemented by applying ac electric potentials over the electrodes that are immersed in an electrolyte. Electric fields will induce a nanometer layer of charges/ions 共also known as double layer兲 at the electrode surface, which is typically of opposite sign to the excitation voltage 共i.e., capacitive charging process兲. Under the influence of electric fields that are parallel to the surface, the surface charges in the double layer will migrate, which in turn produces fluid motion due to fluid viscosity. Very similar to dc EO, ACEO velocity can also be expressed as uEO ⬀ ␳Et, where Et is the electric field parallel to the solid surface and ␳ is the induced charge density at the electrode surface.

103, 024907-1

© 2008 American Institute of Physics

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FIG. 1. 共Color online兲 Electric field distribution above a pair of coplanar electrodes, with four counter-rotating vortices formed above the electrodes due to changes in tangential electric fields.

A common configuration of ACEO electrodes is the “side-by-side” planar electrode pairs, as shown in Fig. 1. For the electrode configuration shown here, the electric fields at the electrode surface have both tangential and normal components. The normal E-fields induce charges/ions in the nanometer thick double layer, and the tangential E-fields drive the ions along the electrode surfaces. Because charges in the double layer change signs with electric fields, the flow directions are maintained over ac cycles. ACEO induced by a pair of symmetric electrodes exhibits mirror symmetry, so that counter-rotating local vortices are produced above the electrodes. There are two major vortices close to the inner edges of the electrodes and two minor vortices at the outer edges. The minor vortices are caused by fringing fields and are noticeably weaker than the major vortices. This phenomenon was experimentally demonstrated in Refs. 8 and 9. To use ACEO for pumping, it is essential to break the symmetry of electric fields within an electrode pair, so that the major vortex on one electrode will overcome the resistance from the other to produce a unidirectional flow. This can be achieved by spatial asymmetry in electrode design as people typically do, or by polarization asymmetry as presented in this paper. With asymmetric geometries, the flow direction is predetermined by electrode design. This paper demonstrated the net transport of fluid based solely on asymmetric electrode polarization 共A-P兲. Symmetric interdigitated electrodes are used in A-P ACEO pump, so the pumping direction can be easily changed or programed, which is advantageous when constructing a reconfigurable fluidic network. A-P ACEO adds another degree of flexibility to electrokinetic techniques for fluid manipulation. When an electric potential is applied to the electrodes that are exposed to electrolyte, the electrode surface becomes polarized with charges. The electrodes can attract counterions from the electrolyte to screen the electrode potential 共also known as capacitive or induced charging兲, or can generate coions from electrochemical reactions at the electrodes following Faraday’s law 共also known as Faradaic charging兲. With ac fields, electrode polarization by capacitive charging has been widely recognized as the mechanism responsible for ACEO, while the polarization by Faradaic charging was reported only recently based on our experimental results.10,11 Our previous experiments verified that capacitive charging and Faradaic charging coexist and compete for dominance at ac frequencies around or below the inverse RC time

J. Appl. Phys. 103, 024907 共2008兲

of the device. When capacitive charging dominates the electrode polarization 共e.g., at low voltages兲, counterions migrate under the influence of the electric fields and induce microflows going from the edges toward the inside of the electrodes, as schematically shown in Fig. 1. The strength of Faradaic polarization has an exponential dependence on voltage. As the voltage at the electrode/electrolyte increases, the rate of electrochemical reactions will increase to produce coions at the electrode surface, quickly exceeding that of capacitive polarization. Consequently, the electro-osmotic flows at the electrode surface will reverse their directions, opposite to those from capacitive charging. There are several factors contributing to the unequal Faradaic processes between the two electrodes. First, by adding a bias to the ac signals, the energy barriers for Faradaic reactions are changed. The rate constants of reduction and oxidation processes, kre and kox, depend on the applied voltage V and activation free energy, ⌬Gre and ⌬Gox, as kre ⬀ exp关共−⌬Gre / RT兲共−␣V / kBT兲兴, and kox ⬀ exp关共−⌬Gox / RT兲 ⫻共共1 − ␣兲V / kBT兲兴, where ␣ is called the transfer coefficient, indicating how the activation free energy is influenced by the voltage. As a result, oxidation reaction becomes dominant at the positively biased electrode. Reduction process is the dominant reaction at the negatively biased electrode; however, due to insufficient energy in overcoming ⌬Gre it still does not generate sufficient anions to reverse the polarity of the capacitive charging. Further, the reaction rate also depends on the mass transport of the products, as it affects the local concentration of the species. The mobility of H+ 共36.3⫻ 10−8 m2 V−1 s−1兲 is almost twice as much as that of OH−, slowing down reduction process even more, augmenting the difference in reaction rates between the oxidation and reaction. Capacitive and Faradaic reaction charging have the following distinct features that are the foundation of the concept of asymmetric polarization ACEO. Capacitive charging cannot produce a polarization exceeding the equilibrium charge density on the electrode side, while Faradaic charging can produce charge densities orders of magnitude beyond equilibrium values. Also, the two polarizations have different dependences on the applied voltage, which leads to difference in the charge density ␳ of the double layer. ␳ ⬃ ⌬␰ for capacitive charging 共⌬␰ is zeta potential兲 and ␳ ⬃ exp共⌬␰兲 for Faradaic charging, respectively.12 As a result, the EO velocities produced by the two polarizations have different dependencies on the applied voltage. Since ⌬␰, Et ⬀ V for capacitive charging, ut ⬃ V2, while for Faradaic charging, ut ⬃ exp共V兲. The microflows from Faradaic charging can become stronger than that from capacitive charging. Capacitive charging predominates at low voltages, while Faradaic charging takes over at higher voltages with its exponential dependence on potential. Asymmetric polarized or biased ACEO capitalizes on the alternating dominance of capacitive and Faradaic charging to break electrode polarization symmetry and consequently realize directed flow motion. Asymmetric electrode polarization is realized by applying ac signals with dc offsets over electrode pairs, with the potential of the electrolyte floating, as shown in Fig. 2. Consequently, two electrodes have different electrical potentials

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J. Appl. Phys. 103, 024907 共2008兲

FIG. 2. 共Color online兲 Signal application for asymmetric polarization ac electro-osmosis.

with respect to the electrolyte. With a biased ac signal impressed over the electrodes, the negatively biased electrode 关−共Vdc / 2 − Vac / 2兲cos ␻␶, left兴 is always negative and subject to capacitive charging, inducing cations. The positively biased electrode 关共VDC / 2 + VAC / 2兲cos ␻t, right兴 is always positive and more prone to Faradaic charging. It will induce anions from capacitive charging at low voltages and transit to cations from Faradaic charging as the applied voltage increases. When the voltage exceeds the threshold for reaction, the vortices above the two electrodes will lose their reflection symmetry as the two major vortices will connect and form a large recirculation flow, as schematically shown in Fig. 3共a兲. The flows are still not strong enough to overcome the resistance from the fluid outside the electrodes, so the flows only provide local transport of substances, which is often indicated by the movement of tracer particles from the negatively biased electrode to the positively biased one in microfluidic experiments. At higher voltages, the Faradaic polarization becomes sufficiently strong, and throughflow pumping to the left is realized, as shown in Fig. 3共b兲. The advantage of asymmetric polarization over asymmetric electrode ACEO pumping includes 共1兲 higher efficiency as counterflows are minimized with our technique, 共2兲 flexible pumping directions since it is easy to change electrical signals, and this feature can be used for active microvalves 共two counter-rotating flows兲, hence programable microfluidic network, 共3兲 effective microscale mixing with the same electrode array, which is a desirable feature for biochemical analysis, and 共4兲 capability for electrophoretical separation of bioparticles since it has dc component, and this feature make it a unique candidate to realize on-chip bioparticle separation. III. EXPERIMENTS

The flow motions generated by A-P AC EO are examined using microfabricated arrays of electrode pairs on silicon substrate. Au/ Ti 共100 nm/ 5 nm兲 electrodes were fabricated by lift-off process. Ti is the adhesion layer between the substrate and Au, and Au is in contact with electrolytes. The

FIG. 3. 共Color online兲 Schematics of fluid manipulation by asymmetric polarization ac electroosmosis. 共a兲 At an appropriately biased ac potential, streamlines from capacitive charging and Faradaic charging become connected, forming a large local vortex over the electrode pair. Substances such as particles can be moved from the left to the right electrode. 共b兲 At higher potentials, flow motion by Faradaic charging on the left electrode predominates, and a net flow to the right is produced.

electrodes are 80 ␮m wide and 40 ␮m apart. Deionized 共DI兲 water was used as testing solution with suspended polystyrene spheres of 3 ␮m diameter 共Fluka Chemica兲 and at a concentration of 106 particles/ ml.

A. Surface flow manipulation

In experiments, the fluid motion is monitored and extracted by following tracer particles that are seeded in the fluid. It will be shown in this section that ACEO flows from capacitive charging will converge on the electrodes and collect particles into lines, while the flows from Faradaic charging are diverging and will disperse the particle lines.10,11 Electric signals in the form Vappl = VO共1 + cos 2␲ ft兲 are applied over the electrode pair, with f being the frequency from 25 to 500 Hz. By increasing VO from 0 to 4 V, the left electrode experienced capacitive polarization, intermediate to strong Faradaic polarization sequentially, which led to various surface flows. The process is shown in Figs. 4共a兲–4共d兲. Figures 4共a兲 and 4共b兲 show the particle movement when VO is at 1.2 V over the 20 s interval after the field is turned on. At this voltage, electrochemical reactions at the left electrode are weak, so four capacitive vortices are developed in

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Jie 共Jayne兲 Wu

J. Appl. Phys. 103, 024907 共2008兲

FIG. 4. Fluid motion on electrode surface by A-P ACEO, as indicated by particle movement. The right members in an electrode pair was positively biased. The electrodes are 80 ␮m wide. 共a兲 Initial distribution of latex particles over the electrodes. 共b兲 Two lines of concentrated latex particles on the electrodes by capacitive charging at low potentials 共Vmax = 1.2 V兲, before the applied potential is able to induce Faradaic polarization. 共c兲 Particles moving from the left to the right electrodes at medium potentials 共Vmax = 2.2 V兲. 共d兲 Particles starting to be swept off their stagnation lines at Vmax = 3.4 V.

Fig. 4共b兲 as with unbiased ac forcing, which leads to two lines of particles assembled on the electrodes. At increased voltages, appreciable Faradaic reactions take place at the positively biased electrodes, and asymmetric vortices are formed above two electrodes. At an intermediate voltage 兩VO兩 = 2.2 V, particles were moved from the left to the right electrode, as shown in Fig. 4共c兲, which corresponds to the localized fluid motion above the electrodes. As the voltage becomes higher, the Faradaic polarization becomes sufficiently strong that a net flow to the left is produced, as shown in Fig. 4共d兲. Within 5 s after the potential was increased to 兩VO兩 = 3.4 V, the particle lines on the electrode surface shift from the right to the left, indicating a directional surface flow. A surface flow velocity up to 300 ␮m / s was demonstrated. Because the top part of the electrodes is next to the sidewall of the microfluidic chamber, fluid motion in the vicinity of the sidewalls was subject to no-slip boundary condition and became too weak to carry particles.

dition 共u = 0兲 needs to be applied for the top and sidewalls of the channel. A lower channel will pose a less drag for the EO flows, and a lower voltage is required for the onset of pumping action. Bubble generation is not observed until VO = 6.0 V, which is roughly five times the threshold voltage for noticeable electrolysis at dc. As discussed earlier, ACEO velocity by Faradaic polarization should have exponential dependency on voltage, u ⬃ exp共V兲, while that by capacitive polarization has quadratic dependence. The two sets of velocity data in Fig. 5 were curve fitted well with u ⬃ exp共V / 0.77兲. As a comparison, the

B. A-P ACEO micropump

For pump experiments, polydimethylsiloxane molded microchannels were used at two heights of 350 and 100 ␮m. The fluid velocity was characterized as a function of applied voltage. The fluid velocity data were taken at electrode surface and averaged over a length of two electrode periods. Figure 5 gives the fluid velocity 共fluid conductivity ␴ = 2.1 mS/ m兲 at applied voltages from 2.2 to 4.5 V. It can be seen that the surface flow velocity is consistently higher with a lower channel. This can be understood since no-slip con-

FIG. 5. 共Color online兲 Surface flow velocity of A-P ACEO pump as a function of voltage at 500 Hz.

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Jie 共Jayne兲 Wu

velocity data were also compared with quadratic curves, which fit the experimental data at low voltage but are significantly lower at higher voltage. Experiments on capacitive charging ACEO by other groups2,13 yielded u ⬃ V2 to u ⬃ V at large voltage 共e.g., 5 V兲. Comparing with the velocity data presented here, it can be deduced that the pumping originates from Faradaic polarization. Net displacement of the fluid has also been demonstrated. The fluid channel was filled with DI water to half of its length. After applying voltage, the fluid front 共i.e., the fluid/air interface兲 was observed to advance through the electrode pairs. For a microchannel with a cross-sectional area of 100⫻ 500 ␮m2 and 2 mm length, a pressure head of ⬃11 mPa is generated at an applied voltage of 3 – 4 Vpeak. A surface flow rate of ⬃100 ␮m / s was maintained at a power consumption of 3.5 mW for 30 min, without noticeable electrode deterioration. IV. SUMMARY

An ACEO micropump based on asymmetric polarization of electrodes has been demonstrated. Pumping and mixing of fluids can hence be realized by adjusting the magnitude of bias and ac signals. The pumping action requires only a few volts of applied voltage and a power consumption in the range of milliwatts, making it suitable for on-chip implementation. It is fortuitous that this net flow is also sufficiently precise to be able to concentrate and sweep particles into a

J. Appl. Phys. 103, 024907 共2008兲

specific location on the electrode to allow detection. Such a versatile pump and trap combination with an easily fabricated array should prove useful in diagnostic technology. ACKNOWLEDGMENTS

The project has been supported by the U.S. National Science Foundation under Grant No. ECS-0448896. 1

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