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Low-voltage dynamic control for DC electroosmotic devices
Sensors and Actuators A 153 (2009) 237–243
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Low-voltage dynamic control for DC electroosmotic devices Prachya Mruetusatorn a , Mohamed R. Mahfouz b , Jie (Jayne) Wu a,∗ a b
Department of Electrical Engineering and Computer Science, The University of Tennessee, Knoxville, TN 37996, USA Department of Mechanical, Aerospace and Biomedical Engineering, The University of Tennessee, Knoxville, TN 37996, USA
a r t i c l e
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Article history: Received 24 June 2008 Received in revised form 15 May 2009 Accepted 19 May 2009 Available online 23 May 2009 Keywords: Electroosmotic flow Field-induced electroosmosis Gate control Zeta-potential Flow reversal
a b s t r a c t Electroosmotic micropumps can find wide applications in laboratories-on-a-chip. While much progress has been accomplished, local and reconfigurable control of fluids remain an active research area in microfluidics, as this ability is essential to mixing and convection to speed up the reaction or analysis processes. This paper explores field-induced electroosmosis, also known as gate control technique, to achieve dynamic flow control. The gate-controlled electroosmotic devices are built using modified soft lithography technique. The gate electrode directly contacts the fluids, providing high efficiency in modulating zeta potential at the fluid-channel interface and hence efficient flow regulation. The flow velocity can be locally manipulated by applying low gate voltages (−2 V to +2 V). The flow rates are improved by 112% with negative gate voltages (0 V to −2 V), 0.61 L/min up to 1.29 L/min. The flow reversal is observed to start at an applied gate voltage of about +1.3 V. Moving fluid back and forth will greatly enhance mixing. The device design is scalable to accommodate various applications, portable in size, and can be readily interfaced with other microfluidic components, suitable for LOC applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Laboratories-on-a-chip (LOC) miniaturizes laboratory functions onto a single chip of no more than a few square centimeters in size and can achieve speed, efficiency, reduced sample consumption, and multiplexing detection [1]. Micropumps [2–4], as one important LOC element, can generally be divided into two categories: reciprocating displacement micropumps and continuous dynamic flow micropumps. The first category operates by the oscillatory or rotational movement of mechanical parts to exert pressure on the fluid to displace fluids; for example, microdiaphragm pumps and peristaltic micropumps. Microfluidic devices with moving mechanical elements suffer from many drawbacks, for example, they are difficult to design and fabricate as well as prone to mechanical failure during operation due to fatigue and fabrication defects [2,3]. The second category operates by transforming external energy into continuous fluid movement. Continuous flow pumping devices, such as electrohydrodynamic (EHD), electroosmotic (EO), electrochemical, and magnetohydrodynamic (MHD) pumps have been under active research and development up to present. Electroosmosis (EO) is a pumping mechanism of choice to transport fluids in disposable chip-based microfluidic system [2]. Its advantages include the simple design of EO devices ensuring robustness, reliability and trouble-free operation, sim-
∗ Corresponding author. E-mail address: [email protected] (J. Wu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.05.011
ple fabrication supporting high manufacturing reproducibility, straightforward integration with other components on microfluidic platforms, accurate delivery of minute amount of fluids, and easy further developments by various techniques. EO devices depend on the mobile charges induced at the interface between the microchannel surface and fluid to produce the electroosmotic flow (EOF). There has been extensive study on EO micropumps. For an exhaustive review on EO micropumps, the readers are referred to [5–10]. EO flows are generated by the electric field imposed parallel along the microchannel, so the fluid streamlines assume the contour of the parallel electric field lines. Therefore, EO devices lack local flow control, and mixing is difficult in such devices. Local manipulation of EO flow can be achieved by modulating zeta potential (an indicator of surface charge density at the fluidchannel interface of the inner wall of fluidic channels [1,11] or by applying a transverse electric field across the fluid-channel interface [12,13]. It has been reported that by patterning the channel surface with varied chemical modifications, the flow velocity at different locations can be individually regulated. However, the surface modification lacks the capability of dynamic control, i.e., the velocity regulation at points of interest cannot be switched on/off or changed at will. The ability to dynamically manipulate the EOF at any point is very much desired in a number of applications, such as mixing or separation of analytes in a lab-on-a-chip. On the other hand, the field-effect flow control (FEFC) method [2,3] provide an alternative approach for dynamically and electrically modifying zeta potential of the EOF by applying a transverse electric field across the microchannel wall.
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FEFC method is implemented basically by applying a voltage to an electrode (a.k.a. the gate) embedded in the channel sidewall, and FEFC has been demonstrated both with fused-silica capillaries and with microfabricated devices. In prior reports, the gate was separated from the fluid by an insulation layer, and high gate voltages were needed to modulate zeta potential at the channel sidewall. In the experiments of Sniadecki et al. [14], a parylene C layer of 1.22 m thickness was used between the fluid and the gate electrode, and the required gate voltage is on order of −120 V to +120 V to modify the channel surface charge density for observable FEFC. Schasfoort et al. [15] used 390-nm thick silicon nitride as the insulation layer with a transverse voltage of 50 V to control over the fluid flow. Another study of the FEFC technique was done with a polydimethylsiloxane (PDMS)-based microfluidic system by Buch et al. [11]. The silicon wafer served as a gate electrode with a transverse electric fields at an average of 1.50 MV/cm through a 2.0-m electrically insulating layer of silicon dioxide. It can be concluded from the prior work that FEFC control of EOF is directly proportional to the dielectric constant of the wall material and the applied transverse electric field and therefore inversely proportional to the wall thickness [2,3]. Induced charge electroosmosis, or gate control technique [2,3,16] is another flow control method for EO flows. Electrodes are deposited inside the microchannel and exposed to electrolyte directly without an insulation layer. The zeta potential at the fluid-microchannel interface is modified by the gate voltage. For polarizable electrodes, at small DC voltage, the electrode–electrolyte interface acts like a charged capacitor. Mobile charges are induced very close to the electrode in the fluid, which can be regarded as one plate of the capacitor. By changing gate voltage, this technique can modify the quantity and polarity of the mobile charges in the diffuse double layer at the gate, thus adjusting EO velocity at that location. It bears similarity to FEFC, but operates at much lower voltage, since the gate electrode is in direct contact with the fluid and can avoid the large voltage drop over the insulation layer. This work has developed this gate control technique and investigated its operating conditions to manipulate fluid flows. Our experiments demonstrated a 150% change in the local fluid velocity with ±2 V of the applied gate voltages. Higher gate voltages could induce electrolysis at the gate electrode surface. The micropumps were fabricated using modified soft lithography technique with patterned indium tin oxide (ITO) glass for gate control. Because the gate electrodes are patternable and can be individually addressed, independent fluid control can be realized at multiple points in the channel. As a result, complex streamlines can be produced to realize microfluidic functions such as mixing, without the trouble of adjusting the longitudinal electric field strength [17]. Numerical simulation using COMSOL Multiphysics was performed to model the electro-fluid flow in the channel.
(EDL), close to the microchannel wall [2]. Due to the strong electrostatic interaction, counter-ions closest to the microchannel wall is immobilized and forms a region known as the Stern layer. The counter-ions further away from the microchannel wall have more freedom to move and form a region, so called the diffuse layer. The potential over these two layers is referred to as zeta-potential. The EDL thickness inversely depends on fluid ionic concentration or conductivity. By applying a DC electric field over the microchannel, ions in the EDL are driven into motion, following Coulomb’s law. Due to a viscous coupling effect between the ions in the EDL and those in the bulk fluid, EOF is developed to drive the aqueous solution. Note the EOF strength depends on ion concentration in the EDL. Simultaneously, the electrophoretic flow of charged particles also takes place in the system. The electrophoretic flow is a net flow between the positively charged and non-charged particles moving in the direction of the EOF, and the negatively charged particles moving in the opposite direction, under the DC applied electric field [2,18]. In this work, as the EOF relies on ion concentration in the EDL, the low fluid conductivity, de-ionized (DI) water, was used to achieve EOF dominance for the net flow. The expression for EOF velocity was first derived by Smoluchowski [19], ueo = −
ε
Ex
(1)
where ε is the fluid permittivity, is the fluid viscosity, is the zeta potential indicating the ion concentration at the fluid–solid interface, and Ex is the applied longitudinal electric field, which is the voltage between the anode and the cathode divided by the channel length. The Smoluchowski equation describes the maximum fluid velocity (ueo ) at the slip plane with absence of hydrostatic pressure. The induced charge electroosmosis, or gate control technique, can manipulate the local EOF by applying a gate voltage at the fluid–solid interface to adjust the zeta potential. Fig. 1a shows schematically the change in surface charge due to an applied gate voltage (VG ). Typically negative charges will be induced in the fluid side of the EDL when there is no externally voltage applied to the electrodes or sidewalls. When a negative gate voltage is applied, a larger concentration of cations resides in the EDL (both the Stern layer and the diffuse layer) over the gate region. In this case, G > 0 ,
2. Theoretical background When fluid and microchannel wall come into contact, a potential difference develops between the two phases in most cases, and electric charge neutrality no longer holds in a single phase at the interface. Due to the preferential adsorption and desorption of certain ions, for most of the inorganic surfaces, such as silica, the microchannel wall acquires a negative surface charge resulting from the surface functional groups when immersed in fluid, such as water. However, note that various materials will have different properties of solid walls and thus different amounts of surface charges represented by the zeta potential. The microchannel wall will then attract the counter-ions or positive ions, while simultaneously repel co-ions or negative ions in the fluid, spontaneously forming a charged region, so called electrical double layer
Fig. 1. (a) Gate voltage influence within the EDL, and (b) equivalent capacitor circuit model.
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where G is the modified zeta potential by VG and 0 is the inherent zeta potential (i.e. VG = 0) at the liquid–solid interface. The increase in cation concentration leads to an increase in magnitude of zeta potential, thus leading to an enhanced EOF pumping. If a positive gate voltage is applied, cations will be driven away, and the EOF will be reduced, or reversed at an even larger positive field. To characterize the influence of gate voltage over zeta potential, a capacitor model has been proposed by researchers [20] to determine the corresponding change in zeta potential (). The equivalent capacitor circuit is shown as in Fig. 1b, G = 0 + .
(2)
The diffuse layer capacitance (CD ) and the Stern layer capacitance (CSL ) are in series. is the voltage drop across CD due to VG . Vi as the potential over the gate region due to a longitudinal electric field in this case is considered as a neutral condition or a zero potential over the gate region. Therefore, the modified zeta potential for a given gate voltage can be written as [20], G = 0 + VG
CSL (CSL + CD )
(3)
It can be seen that VG can modify zeta potential and exert control over the fluid flow. G changes almost linearly with VG , and only a part of VG will contribute towards the changes in zeta potential. The ratio of [CSL /(CSL + CD )] will be experimentally given in a later section. Since the gate/electrolyte interface can be regarded as a capacitor at low voltage, DC electric field is not supposed to pass through. As a result, the gate voltage has negligible influence on Ex , the longitudinal electric field, which drives the double layer mobile charges to form EOF. Therefore, the function of the gate voltage is limited to modifying zeta potential in our analysis. Reactions must occur for a DC voltage to reach into the fluid [21]. In the experiments, we did not observe reactions at the gate electrode, which supports the assumption that the gate voltage did not penetrate the double layer and reach into the fluid bulk. 3. Experimental setup and microfluidic modeling The EOF pumping system with the gate control component is schematically shown in Fig. 2. Modified soft lithography technique was used to fabricate the polydimethylsiloxane (PDMS) channels (SYLGARD 184 Silicone Elastomer Kit, Dow Corning, Midland, MI) on indium tin oxide coated glass. Channel dimensions could be well controlled, with length (L) = 2 cm, width (W) = 1 mm, and height (H) = 50 m. The ITO layer coated on glass was patterned and etched to form the gate electrode. The gate covers 50% of the channel on the cathode side, as this ratio makes it easier to compare the flow velocities over the non-gate and gate regions and derive the change in zeta potential by the gate voltage. No other gate coverage ratio was investigated, since the focus of this work is to investigate the local flow control rather than pumping performance by the gate control. The seal between the PDMS film and the substrate was reversible, so
Fig. 2. Experimental setup for the gate-controlled microchannel. The gate component covers 50% length area on the channel bottom on the cathodic side.
Fig. 3. At 20 V/cm longitudinal electric field, (a) numerical simulation results of EOF velocity extracted over the gate region against the experimental data with the curve fits in a single PDMS-based microchannel, and (b) applied gate voltage vs. change in zeta potential with the curve fit.
the devices were easily cleaned between tests. Inlet/outlet reservoir accesses were drilled and PVC tubes served as reservoirs. The aqueous fluid has a conductivity of 2.80 S/cm. A DC voltage of 40 V was applied to Pt reservoir electrodes to generate the longitudinal electric field (20 V/cm) across the 2-cm long channel, as Ex in Eq. (1), for the EOF generation. The gate voltages (VG ) were directly applied to the ITO gate electrode on the channel bottom and the grounding wire was attached to the glass substrate part of ITO slide to modulate counter ions at the surface channel, resulting in the EOF regulation. The applied gate voltages were varied from −2 to +2 V. The current was monitored through measurement of voltage drop across a resistance of 1 k. Flow velocities over gate region were indicated using 3-m latex tracer particles (Fluka Chemica) and measured by micro-particle imaging velocimetry (micro-PIV) technique. Tracer particle motion was recorded by Nikon LV 100D microscope equipped with a CCD camera, and fluid velocity were then extracted using Image Pro AMS 6.0 software [www.mediacy.com, Cybernetics Inc.]. COMSOL Multiphysics was used to simulate fluid flows in the channel. Electrostatic (ES) and Incompressible Navier–Stokes (NS) modules were applied. Because the flow rate was too low to produce an appreciable migration of charges to affect the electric field, ES and NS modules can be regarded as decoupled. The electric field distribution was first solved for with the ES module, then the solution was used to solve for the velocity field using the NS module. The model was built with the actual dimensions of height/length (H/L), 50 m/2 cm. For the boundary conditions, the inlet and outlet electrodes were given a fixed electric potential of 40 V and ground, respectively. All other boundaries used zero charge/electric insulation conditions. A relative permittivity of 78.5 was used for the fluid (water). In the NS module, no pressure gradient was considered, so only the EOF was simulated in the channel. A viscosity of 0.001 N s m−2 and a density of 1000 kg m−3 were used for the fluid (water). The boundaries were given the slip velocities as defined by
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Fig. 4. Numerical simulation of velocity field distribution with VG = −2 V using COMSOL Multiphysics: (a) overview of velocity field distribution with arrows indicating field directions and relative magnitudes via arrow size, (b) velocity field distribution profile at channel centerline from inlet to outlet, and (c) cross-sectional velocity field distribution profiles at channel midpoint.
Eq. (1) to calculate the flow fields. The zeta potential was extracted from Fig. 3b, G = −0.146 + 0.096·VG . As discussed before, for the gate region, even though an electric potential is applied, the appropriate boundary condition is charge conservation: the electric field induces an increase of the stored charges in the electric double layer, which screens the gate electric field from the bulk electrolyte [22]. Therefore, the simulation needs to account for the influence of the induced charges. As it is almost impossible to analytically determine the charge density, we experimentally determined the zeta potential by monitoring the local fluid velocity over the gate region, and used it to set the boundary condition for the gate.
4. Results and discussions 4.1. Experiment When negative gate voltages (−VG ) arranging from −2 V to 0 V were applied, the flow velocity from anode to cathode was improved from 203.28 m/s (0.61 L/min) at VG = 0 V up to 430.25 m/s (1.29 L/min) at VG = −2 V because more counter ions (cations) were attracted into the EDL, hereby resulting in flow velocity improvement. In contrast, applying the positive gate voltages (+VG ) drove away cations and eventually attracted anions into the EDL. The flow velocity was reduced to 25.21 m/s (0.076 L/min)
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Fig. 5. Numerical simulation of velocity field distribution with VG = +2 V using COMSOL Multiphysics: (a) Overview of velocity field distribution with arrows indicating field directions and relative magnitudes via arrow size, (b) velocity field distribution profile at channel centerline from inlet to outlet, and (c) cross-sectional velocity field distribution profiles at channel midpoint.
with VG = +1 V. The flow reversal (the flow direction towards the anode) started to be observed between VG of +1 V and +1.5 V. When applying VG = +1.5 V, the reverse flow velocity was 10.58 m/s (0.032 L/min) and then increased up to 97.71 m/s (0.29 L/min) at VG = +2.0 V. In comparison with previous work, Fu et al.’s group [23] studied influence of applying a step change in zeta potential on the EOF in a rectangular channel. Their results indicated change in the velocity field over the region of step change in zeta potential, however, with no observed flow reversal. A study by Wouden et al.’s [24] demonstrated that the EOF could be locally manipulated by applying gate fields (<10 MV/cm) over a 200–300 nm thick silica layer. Although the study estimated that the EOF could be
completely stopped by an applied gate field of 1.7 MV/cm under a longitudinal electric field of 150 V/cm, realization of flow reversal was not claimed. The experimental data under the conditions described above are plotted in Fig. 3a. The fluid velocity exhibits a linear relationship with the applied gate voltages. There existed small deviations from the linear relationship, which is probably due to the pressure head difference built up from the pumped fluid, generating the pressure-driven flow in the channel. Based on the experiments, the EOF can be evidently and easily manipulated by locally modulating zeta potential at the channel walls with the applied gate voltages lower than ±2 V. The applied gate voltage should
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not exceed 2 V to avoid gas bubble formation due to electrolysis. Zeta potentials could be calculated by using Eq. (1) with known fluid velocities, so under the longitudinal electric field of 20 V/cm with no gate voltage application (VG = 0), zeta potential ( 0 ) was about −0.146 V. Fig. 3b shows change in zeta potential () as a function of the applied gate voltage (VG ) over the gate region, which can be approximately curve-fitted as = 0.096·VG . Revoking Eq. (2), the modified zeta potential ( G ) as a function of an applied gate voltage could be written as G = 0 + 0.096·VG = −0.146 + 0.096·VG . According to Eq. (3), the term of [CSL /(CSL + CD )] could be estimated as 0.096. This relationship helps predict changes in zeta potential over the gate region by applied gate voltages, thus leading to the modified zeta potential and flow velocity prediction. It should be recognized that the tracer particles could experience electrophoretic force. Even though the tracer particles are charge-neutral, they still show electrophoresis due to the specific molecular structure of water at their surface. The electrophoretic mobility depends on particle size and fluid conductivity. According to Ref. [25], ∼2 m latex particles exhibited an electrophoretic mobility of −2 m cm/V s at 10−4 S/m. For the experimental condition in this work, electrophoresis would move particles from the cathode to anode at about 60 m/s, which was opposite to the EOF and made the EOF appear slower, however, it would not affect the change in zeta potential by the gate voltage.
positive gate voltage, the velocity field became much negatively stronger near the gate surface region, thus generating the reverse flow. The flow fields with other applied gate voltages were also simulated. The simulated EO velocities over the gate region were extracted and plotted against the experimental data in Fig. 3a. The simulation data well supported the experimental data.
5. Conclusions A low voltage dynamic control for EO micropumps has been investigated. The experimental and simulation results confirmed the ability of modulating zeta potential by varying gate voltages to enable independent, dynamic control over the EOF in the channel, without adjusting any longitudinal electric fields. The low gate voltages, ranging from −2 V to +2 V, were applied with no observable electrochemical reactions in the channel. Comparing with the no gate DC EO pump, the flow rates were enhanced with the negative gate voltages while reduced with the positive gate voltages under the same longitudinal electric field of 20 V/cm. The flow reversal towards the inlet reservoir could be observed at the gate voltage of around +1.3 V. The results can be readily used to produce effective mixing on LOC devices, with the appropriate gate voltage application. In addition, the micropumps and its gate control were scalable to accommodate various LOC applications with portable size and easy integration with other microfluidic elements.
4.2. Numerical simulation A longitudinal electric field strength of 20 V/cm, i.e. 40 V applied over the 2-cm long channel, was uniformly applied throughout the channel. A set of varied gate voltages (VG ) were applied ranging from −2 V to + 2 V. After applying the gate voltages, the EDL is in quasi-equilibrium, and there is little electric field into the fluid when a DC potential is applied. The gate voltages contribute towards the EOF by inducing more charges in the EDL over the gate electrode, which is represented by change in zeta potential. Therefore, overall electric field distribution was considered uniform throughout the channel in our work. Fig. 4a shows the simulated fluid velocity field when VG = −2 V. Arrows in the figure indicate field directions and relative magnitudes via arrow size. The velocities over the gate region become positively higher. Fig. 4b shows a step change in the fluid velocity at the channel centerline from inlet to outlet with VG = −2 V. The non-uniform distribution of flow velocities was due to the back pressure from regions with lower zeta potential. The velocity field at downstream (outlet) was enhanced. Fig. 4c demonstrates the cross-sectional velocity distribution profiles at channel midpoint. At channel midpoint, the velocity field became stronger over the gate surface region and decreased almost linearly towards the other channel wall, which is a superposition of faster EOF above the gate and the back pressure driven flow from the slower-moving nongate region. Because the flow rate has to remain the same at any cross-section of the channel, the faster fluid velocity at the channel bottom is compensated by slower or even reverse flow. Overall, the flow rate of the pump improved due to the extra charges induced by the negative VG . On the other hand, by applying VG = +2 V, the flow field became reversed over the gate region as more anions were induced and eventually dominated over cations in the EDL, as shown in Fig. 5a. The reverse flow induced by cations over the gate region competed with the forward flow over the other channel surface region, leading to a pressure driven flow profile in the non-gate region. Fig. 5b shows a step change in EO velocity at the channel centerline from inlet to outlet, where the flow direction reversed after crossing over to the gate. At the channel midpoint in Fig. 5c, as co-ions in the EDL were significantly and sufficiently increased due to the applied
References [1] A. Manz, N. Graber, H.M. Widmer, Miniaturized total chemical analysis systems: a novel concept for chemical sensing, Sens. Actuators B 1 (1990) 244–248. [2] L. Chen, S. Lee, J. Choo, E.K. Lee, Continuous dynamic flow micropumps for microfluid manipulation, J. Micromech. Microeng. 18 (2008) 013001–13022. [3] C.H. Wu, J.K. Chen, R.J. Yang, Electrokinetically driven flow control using bare electrodes, Microfluid. Nanofluid. 3 (2007) 485–494. [4] D.J. Laser, J.G. Santiago, A review of micropumps, J. Micromech. Microeng. 14 (2004) 35–64. [5] L.X. Chen, J.P. Ma, Y.F. Guan, Study of an electroosmotic pump for liquid delivery and its application in capillary column liquid chromatography, J. Chromatogr. A 1028 (2004) 219–226. [6] S.L. Zeng, C.-H. Chen, J.C. Mikkelsen, J.G. Santiago, Fabrication and characterization of electroosmotic micropumps, Sens. Actuators B 79 (2001) 107–114. [7] Y.J. Kang, S.C. Tan, C. Yang, X.Y. Huang, Electrokinetic pumping using packed microcapillary, Sens. Actuators A 133 (2007) 375–382. [8] S.L. Zeng, C.-H. Chen, J.G. Santiago, J.-R. Chen, R.N. Zare, J.A. Tripp, F. Svec, J.M.J. Frechet, Electroosmotic flow pumps with polymer frits, Sens. Actuators B 82 (2002) 209–212. [9] G.S. Chirica, V.T. Remcho, Fritless capillary columns for HPLC and CEC prepared by immobilizing the stationary phase in an organic polymer matrix, Anal. Chem. 72 (2000) 3605–3610. [10] I.M. Lazar, B.L. Karger, Multiple open-channel pumping system for microfluid sample handling, Anal. Chem. 74 (2002) 6259–6268. [11] J.S. Buch, P.C. Wang, D.L. Devoe, C.S. Lee, Field-effect flow control in a polydimethylsiloxane-based microfluidic system, Electrophoresis 22 (2001) 3902–3907. [12] C.M. Brotherton, R.H. Davis, Electroosmotic flow in channels with step changes in zeta potential and cross section, J. Colloid Interface Sci. 270 (2004) 242–246. [13] J.S.H. Lee, C.L. Ren, D. Li, Effects of surface heterogeneity on flow circulation in electroosmotic flow in microchannels, Anal. Chim. Acta 530 (2005) 273–282. [14] N.J. Sniadecki, C.S. Lee, P. Sivanesan, D.L. DeVoe, Induced pressure pumping in polymer microchannels via field-effect flow control, Anal. Chem. 76 (2004) 1942–1947. [15] R.B.M. Schasfoort, S. Schlautmann, J. Hendrikse, A. van den Berg, Field-effect flow control for microfabricated fluidic networks, Science 286 (1999) 942–945. [16] T.M. Squires, M.Z. Bazant, Induced-charge electro-osmosis, J. Fluid Mech. 509 (2004) 217–252. [17] K. Seibel, L. Schöler, H. Schäfer, M. Böhm, A programmable planar electroosmotic micropump for lab-on-a-chip applications, J. Micromech. Microeng. 18 (2008) 025008–25017. [18] S.A. Mirbozorgi, H. Niazmand, M. Renksizbulut, Electro-osmotic flow in reservoir-connected flat microchannels with non-uniform zeta potential, J. Fluids Eng. 128 (2006) 1133–1143. [19] R.S. Ramsey, J.M. Ramsey, Generating electrospray from microchip devices using electroosmotic pumping, Anal. Chem. 69 (1997) 1174–1178. [20] C.S. Lee, D. McManigill, C.-T. Wu, B. Patel, Factors affecting direct control of electroosmosis using an external electric field in capillary electrophoresis, Anal. Chem. 63 (1991) 1519–1523.
P. Mruetusatorn et al. / Sensors and Actuators A 153 (2009) 237–243 [21] J. Wu, Interactions of electrical fields with fluids: laboratory-on-a-chip applications, IET Nanobiotechnol. 2 (2008) 14–27. [22] N.G. Green, A. Ramos, A. Gonzalez, H. Morgan, A. Castellanos, Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. III. Observation of streamlines and numerical simulation, Phys. Rev. E. 66 (2002) 026305–26311. [23] L.M. Fu, J.Y. Lin, R.J. Yang, Analysis of electroosmotic flow with step change in zeta potential, J. Colloid Interface Sci. 258 (2003) 266–275. [24] E.J. van der Wouden, T. Heuser, D.C. Hermes, R.E. Oosterbroek, J.G.E. Gardeniers, A. van den Berg, Field-effect control of electro-osmotic flow in microfluidic network, Colloids Surf. A 267 (2003) 110–116. [25] I. Ermolina, H. Morgan, The electrokinetic properties of latex particles: comparison of electrophoresis and dielectrophoresis, J. Colloid Interface Sci. 285 (2005) 419–428.
Biographies Prachya Mruetusatorn received his B.E. degree in Electronics Engineering from King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok, Thailand, in 2002. In 2004, he received his first M.S. degree in Industrial and Engineering Technology from Murray State University, Kentucky, USA. He received his second M.S. degree in 2007 and is currently working towards his Ph.D. degree in Electrical Engineering at the University of Tennessee at Knoxville, USA. He is a member of the Electromechanics and Transducer Research for Bio-Nanoengineering Laboratory and a member of
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the Center for Musculoskeletal Research. His research interests involve manipulation of fluids and particles using electrokinetics (EK) techniques. Mohamed R. Mahfouz received his Ph.D. degree in Biomedical Engineering from Colorado School of Mines, Colorado, USA, in 2002. He is an Associate Professor of Biomedical Engineering, University of Tennessee at Knoxville, USA. He is also Technical Director of Center for Musculoskeletal Research, Computational Sciences and Engineering Division Oak Ridge National Laboratory, Oak Ridge, Tennessee. His research interests include biomedical instrumentation, medical imaging and enhancement, surgical navigation, advanced visualization, orthopedic dynamic modeling, 3D bone and tissue reconstruction, vascular computational fluid dynamics, engineering analysis of surgical techniques and outcomes, and anthropomorphic classification. Jie (Jayne) Wu received her Ph.D. degree in Applied Physics from the Chinese Academy of Sciences, Beijing, China, in 1999, and her Ph.D. degree in Electrical Engineering from the University of Notre Dame, Indiana, USA, in 2004. She is currently an Assistant Professor with the Department of Electrical Engineering and Computer Science, the University of Tennessee at Knoxville, USA. From August 2003 to July 2004, she conducted post-doctoral research at the Center for Microfluidics and Medical Diagnostics, Department of Chemical and Bio-molecular engineering, University of Notre Dame. Her research interests are solid-state device physics, biomicroelectromechanical systems (bio-MEMS), microfluidics.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Low-voltage dynamic control for DC electroosmotic devices Prachya Mruetusatorn a , Mohamed R. Mahfouz b , Jie (Jayne) Wu a,∗ a b
Department of Electrical Engineering and Computer Science, The University of Tennessee, Knoxville, TN 37996, USA Department of Mechanical, Aerospace and Biomedical Engineering, The University of Tennessee, Knoxville, TN 37996, USA
a r t i c l e
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Article history: Received 24 June 2008 Received in revised form 15 May 2009 Accepted 19 May 2009 Available online 23 May 2009 Keywords: Electroosmotic flow Field-induced electroosmosis Gate control Zeta-potential Flow reversal
a b s t r a c t Electroosmotic micropumps can find wide applications in laboratories-on-a-chip. While much progress has been accomplished, local and reconfigurable control of fluids remain an active research area in microfluidics, as this ability is essential to mixing and convection to speed up the reaction or analysis processes. This paper explores field-induced electroosmosis, also known as gate control technique, to achieve dynamic flow control. The gate-controlled electroosmotic devices are built using modified soft lithography technique. The gate electrode directly contacts the fluids, providing high efficiency in modulating zeta potential at the fluid-channel interface and hence efficient flow regulation. The flow velocity can be locally manipulated by applying low gate voltages (−2 V to +2 V). The flow rates are improved by 112% with negative gate voltages (0 V to −2 V), 0.61 L/min up to 1.29 L/min. The flow reversal is observed to start at an applied gate voltage of about +1.3 V. Moving fluid back and forth will greatly enhance mixing. The device design is scalable to accommodate various applications, portable in size, and can be readily interfaced with other microfluidic components, suitable for LOC applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Laboratories-on-a-chip (LOC) miniaturizes laboratory functions onto a single chip of no more than a few square centimeters in size and can achieve speed, efficiency, reduced sample consumption, and multiplexing detection [1]. Micropumps [2–4], as one important LOC element, can generally be divided into two categories: reciprocating displacement micropumps and continuous dynamic flow micropumps. The first category operates by the oscillatory or rotational movement of mechanical parts to exert pressure on the fluid to displace fluids; for example, microdiaphragm pumps and peristaltic micropumps. Microfluidic devices with moving mechanical elements suffer from many drawbacks, for example, they are difficult to design and fabricate as well as prone to mechanical failure during operation due to fatigue and fabrication defects [2,3]. The second category operates by transforming external energy into continuous fluid movement. Continuous flow pumping devices, such as electrohydrodynamic (EHD), electroosmotic (EO), electrochemical, and magnetohydrodynamic (MHD) pumps have been under active research and development up to present. Electroosmosis (EO) is a pumping mechanism of choice to transport fluids in disposable chip-based microfluidic system [2]. Its advantages include the simple design of EO devices ensuring robustness, reliability and trouble-free operation, sim-
∗ Corresponding author. E-mail address: [email protected] (J. Wu). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.05.011
ple fabrication supporting high manufacturing reproducibility, straightforward integration with other components on microfluidic platforms, accurate delivery of minute amount of fluids, and easy further developments by various techniques. EO devices depend on the mobile charges induced at the interface between the microchannel surface and fluid to produce the electroosmotic flow (EOF). There has been extensive study on EO micropumps. For an exhaustive review on EO micropumps, the readers are referred to [5–10]. EO flows are generated by the electric field imposed parallel along the microchannel, so the fluid streamlines assume the contour of the parallel electric field lines. Therefore, EO devices lack local flow control, and mixing is difficult in such devices. Local manipulation of EO flow can be achieved by modulating zeta potential (an indicator of surface charge density at the fluidchannel interface of the inner wall of fluidic channels [1,11] or by applying a transverse electric field across the fluid-channel interface [12,13]. It has been reported that by patterning the channel surface with varied chemical modifications, the flow velocity at different locations can be individually regulated. However, the surface modification lacks the capability of dynamic control, i.e., the velocity regulation at points of interest cannot be switched on/off or changed at will. The ability to dynamically manipulate the EOF at any point is very much desired in a number of applications, such as mixing or separation of analytes in a lab-on-a-chip. On the other hand, the field-effect flow control (FEFC) method [2,3] provide an alternative approach for dynamically and electrically modifying zeta potential of the EOF by applying a transverse electric field across the microchannel wall.
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FEFC method is implemented basically by applying a voltage to an electrode (a.k.a. the gate) embedded in the channel sidewall, and FEFC has been demonstrated both with fused-silica capillaries and with microfabricated devices. In prior reports, the gate was separated from the fluid by an insulation layer, and high gate voltages were needed to modulate zeta potential at the channel sidewall. In the experiments of Sniadecki et al. [14], a parylene C layer of 1.22 m thickness was used between the fluid and the gate electrode, and the required gate voltage is on order of −120 V to +120 V to modify the channel surface charge density for observable FEFC. Schasfoort et al. [15] used 390-nm thick silicon nitride as the insulation layer with a transverse voltage of 50 V to control over the fluid flow. Another study of the FEFC technique was done with a polydimethylsiloxane (PDMS)-based microfluidic system by Buch et al. [11]. The silicon wafer served as a gate electrode with a transverse electric fields at an average of 1.50 MV/cm through a 2.0-m electrically insulating layer of silicon dioxide. It can be concluded from the prior work that FEFC control of EOF is directly proportional to the dielectric constant of the wall material and the applied transverse electric field and therefore inversely proportional to the wall thickness [2,3]. Induced charge electroosmosis, or gate control technique [2,3,16] is another flow control method for EO flows. Electrodes are deposited inside the microchannel and exposed to electrolyte directly without an insulation layer. The zeta potential at the fluid-microchannel interface is modified by the gate voltage. For polarizable electrodes, at small DC voltage, the electrode–electrolyte interface acts like a charged capacitor. Mobile charges are induced very close to the electrode in the fluid, which can be regarded as one plate of the capacitor. By changing gate voltage, this technique can modify the quantity and polarity of the mobile charges in the diffuse double layer at the gate, thus adjusting EO velocity at that location. It bears similarity to FEFC, but operates at much lower voltage, since the gate electrode is in direct contact with the fluid and can avoid the large voltage drop over the insulation layer. This work has developed this gate control technique and investigated its operating conditions to manipulate fluid flows. Our experiments demonstrated a 150% change in the local fluid velocity with ±2 V of the applied gate voltages. Higher gate voltages could induce electrolysis at the gate electrode surface. The micropumps were fabricated using modified soft lithography technique with patterned indium tin oxide (ITO) glass for gate control. Because the gate electrodes are patternable and can be individually addressed, independent fluid control can be realized at multiple points in the channel. As a result, complex streamlines can be produced to realize microfluidic functions such as mixing, without the trouble of adjusting the longitudinal electric field strength [17]. Numerical simulation using COMSOL Multiphysics was performed to model the electro-fluid flow in the channel.
(EDL), close to the microchannel wall [2]. Due to the strong electrostatic interaction, counter-ions closest to the microchannel wall is immobilized and forms a region known as the Stern layer. The counter-ions further away from the microchannel wall have more freedom to move and form a region, so called the diffuse layer. The potential over these two layers is referred to as zeta-potential. The EDL thickness inversely depends on fluid ionic concentration or conductivity. By applying a DC electric field over the microchannel, ions in the EDL are driven into motion, following Coulomb’s law. Due to a viscous coupling effect between the ions in the EDL and those in the bulk fluid, EOF is developed to drive the aqueous solution. Note the EOF strength depends on ion concentration in the EDL. Simultaneously, the electrophoretic flow of charged particles also takes place in the system. The electrophoretic flow is a net flow between the positively charged and non-charged particles moving in the direction of the EOF, and the negatively charged particles moving in the opposite direction, under the DC applied electric field [2,18]. In this work, as the EOF relies on ion concentration in the EDL, the low fluid conductivity, de-ionized (DI) water, was used to achieve EOF dominance for the net flow. The expression for EOF velocity was first derived by Smoluchowski [19], ueo = −
ε
Ex
(1)
where ε is the fluid permittivity, is the fluid viscosity, is the zeta potential indicating the ion concentration at the fluid–solid interface, and Ex is the applied longitudinal electric field, which is the voltage between the anode and the cathode divided by the channel length. The Smoluchowski equation describes the maximum fluid velocity (ueo ) at the slip plane with absence of hydrostatic pressure. The induced charge electroosmosis, or gate control technique, can manipulate the local EOF by applying a gate voltage at the fluid–solid interface to adjust the zeta potential. Fig. 1a shows schematically the change in surface charge due to an applied gate voltage (VG ). Typically negative charges will be induced in the fluid side of the EDL when there is no externally voltage applied to the electrodes or sidewalls. When a negative gate voltage is applied, a larger concentration of cations resides in the EDL (both the Stern layer and the diffuse layer) over the gate region. In this case, G > 0 ,
2. Theoretical background When fluid and microchannel wall come into contact, a potential difference develops between the two phases in most cases, and electric charge neutrality no longer holds in a single phase at the interface. Due to the preferential adsorption and desorption of certain ions, for most of the inorganic surfaces, such as silica, the microchannel wall acquires a negative surface charge resulting from the surface functional groups when immersed in fluid, such as water. However, note that various materials will have different properties of solid walls and thus different amounts of surface charges represented by the zeta potential. The microchannel wall will then attract the counter-ions or positive ions, while simultaneously repel co-ions or negative ions in the fluid, spontaneously forming a charged region, so called electrical double layer
Fig. 1. (a) Gate voltage influence within the EDL, and (b) equivalent capacitor circuit model.
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where G is the modified zeta potential by VG and 0 is the inherent zeta potential (i.e. VG = 0) at the liquid–solid interface. The increase in cation concentration leads to an increase in magnitude of zeta potential, thus leading to an enhanced EOF pumping. If a positive gate voltage is applied, cations will be driven away, and the EOF will be reduced, or reversed at an even larger positive field. To characterize the influence of gate voltage over zeta potential, a capacitor model has been proposed by researchers [20] to determine the corresponding change in zeta potential (). The equivalent capacitor circuit is shown as in Fig. 1b, G = 0 + .
(2)
The diffuse layer capacitance (CD ) and the Stern layer capacitance (CSL ) are in series. is the voltage drop across CD due to VG . Vi as the potential over the gate region due to a longitudinal electric field in this case is considered as a neutral condition or a zero potential over the gate region. Therefore, the modified zeta potential for a given gate voltage can be written as [20], G = 0 + VG
CSL (CSL + CD )
(3)
It can be seen that VG can modify zeta potential and exert control over the fluid flow. G changes almost linearly with VG , and only a part of VG will contribute towards the changes in zeta potential. The ratio of [CSL /(CSL + CD )] will be experimentally given in a later section. Since the gate/electrolyte interface can be regarded as a capacitor at low voltage, DC electric field is not supposed to pass through. As a result, the gate voltage has negligible influence on Ex , the longitudinal electric field, which drives the double layer mobile charges to form EOF. Therefore, the function of the gate voltage is limited to modifying zeta potential in our analysis. Reactions must occur for a DC voltage to reach into the fluid [21]. In the experiments, we did not observe reactions at the gate electrode, which supports the assumption that the gate voltage did not penetrate the double layer and reach into the fluid bulk. 3. Experimental setup and microfluidic modeling The EOF pumping system with the gate control component is schematically shown in Fig. 2. Modified soft lithography technique was used to fabricate the polydimethylsiloxane (PDMS) channels (SYLGARD 184 Silicone Elastomer Kit, Dow Corning, Midland, MI) on indium tin oxide coated glass. Channel dimensions could be well controlled, with length (L) = 2 cm, width (W) = 1 mm, and height (H) = 50 m. The ITO layer coated on glass was patterned and etched to form the gate electrode. The gate covers 50% of the channel on the cathode side, as this ratio makes it easier to compare the flow velocities over the non-gate and gate regions and derive the change in zeta potential by the gate voltage. No other gate coverage ratio was investigated, since the focus of this work is to investigate the local flow control rather than pumping performance by the gate control. The seal between the PDMS film and the substrate was reversible, so
Fig. 2. Experimental setup for the gate-controlled microchannel. The gate component covers 50% length area on the channel bottom on the cathodic side.
Fig. 3. At 20 V/cm longitudinal electric field, (a) numerical simulation results of EOF velocity extracted over the gate region against the experimental data with the curve fits in a single PDMS-based microchannel, and (b) applied gate voltage vs. change in zeta potential with the curve fit.
the devices were easily cleaned between tests. Inlet/outlet reservoir accesses were drilled and PVC tubes served as reservoirs. The aqueous fluid has a conductivity of 2.80 S/cm. A DC voltage of 40 V was applied to Pt reservoir electrodes to generate the longitudinal electric field (20 V/cm) across the 2-cm long channel, as Ex in Eq. (1), for the EOF generation. The gate voltages (VG ) were directly applied to the ITO gate electrode on the channel bottom and the grounding wire was attached to the glass substrate part of ITO slide to modulate counter ions at the surface channel, resulting in the EOF regulation. The applied gate voltages were varied from −2 to +2 V. The current was monitored through measurement of voltage drop across a resistance of 1 k. Flow velocities over gate region were indicated using 3-m latex tracer particles (Fluka Chemica) and measured by micro-particle imaging velocimetry (micro-PIV) technique. Tracer particle motion was recorded by Nikon LV 100D microscope equipped with a CCD camera, and fluid velocity were then extracted using Image Pro AMS 6.0 software [www.mediacy.com, Cybernetics Inc.]. COMSOL Multiphysics was used to simulate fluid flows in the channel. Electrostatic (ES) and Incompressible Navier–Stokes (NS) modules were applied. Because the flow rate was too low to produce an appreciable migration of charges to affect the electric field, ES and NS modules can be regarded as decoupled. The electric field distribution was first solved for with the ES module, then the solution was used to solve for the velocity field using the NS module. The model was built with the actual dimensions of height/length (H/L), 50 m/2 cm. For the boundary conditions, the inlet and outlet electrodes were given a fixed electric potential of 40 V and ground, respectively. All other boundaries used zero charge/electric insulation conditions. A relative permittivity of 78.5 was used for the fluid (water). In the NS module, no pressure gradient was considered, so only the EOF was simulated in the channel. A viscosity of 0.001 N s m−2 and a density of 1000 kg m−3 were used for the fluid (water). The boundaries were given the slip velocities as defined by
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Fig. 4. Numerical simulation of velocity field distribution with VG = −2 V using COMSOL Multiphysics: (a) overview of velocity field distribution with arrows indicating field directions and relative magnitudes via arrow size, (b) velocity field distribution profile at channel centerline from inlet to outlet, and (c) cross-sectional velocity field distribution profiles at channel midpoint.
Eq. (1) to calculate the flow fields. The zeta potential was extracted from Fig. 3b, G = −0.146 + 0.096·VG . As discussed before, for the gate region, even though an electric potential is applied, the appropriate boundary condition is charge conservation: the electric field induces an increase of the stored charges in the electric double layer, which screens the gate electric field from the bulk electrolyte [22]. Therefore, the simulation needs to account for the influence of the induced charges. As it is almost impossible to analytically determine the charge density, we experimentally determined the zeta potential by monitoring the local fluid velocity over the gate region, and used it to set the boundary condition for the gate.
4. Results and discussions 4.1. Experiment When negative gate voltages (−VG ) arranging from −2 V to 0 V were applied, the flow velocity from anode to cathode was improved from 203.28 m/s (0.61 L/min) at VG = 0 V up to 430.25 m/s (1.29 L/min) at VG = −2 V because more counter ions (cations) were attracted into the EDL, hereby resulting in flow velocity improvement. In contrast, applying the positive gate voltages (+VG ) drove away cations and eventually attracted anions into the EDL. The flow velocity was reduced to 25.21 m/s (0.076 L/min)
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Fig. 5. Numerical simulation of velocity field distribution with VG = +2 V using COMSOL Multiphysics: (a) Overview of velocity field distribution with arrows indicating field directions and relative magnitudes via arrow size, (b) velocity field distribution profile at channel centerline from inlet to outlet, and (c) cross-sectional velocity field distribution profiles at channel midpoint.
with VG = +1 V. The flow reversal (the flow direction towards the anode) started to be observed between VG of +1 V and +1.5 V. When applying VG = +1.5 V, the reverse flow velocity was 10.58 m/s (0.032 L/min) and then increased up to 97.71 m/s (0.29 L/min) at VG = +2.0 V. In comparison with previous work, Fu et al.’s group [23] studied influence of applying a step change in zeta potential on the EOF in a rectangular channel. Their results indicated change in the velocity field over the region of step change in zeta potential, however, with no observed flow reversal. A study by Wouden et al.’s [24] demonstrated that the EOF could be locally manipulated by applying gate fields (<10 MV/cm) over a 200–300 nm thick silica layer. Although the study estimated that the EOF could be
completely stopped by an applied gate field of 1.7 MV/cm under a longitudinal electric field of 150 V/cm, realization of flow reversal was not claimed. The experimental data under the conditions described above are plotted in Fig. 3a. The fluid velocity exhibits a linear relationship with the applied gate voltages. There existed small deviations from the linear relationship, which is probably due to the pressure head difference built up from the pumped fluid, generating the pressure-driven flow in the channel. Based on the experiments, the EOF can be evidently and easily manipulated by locally modulating zeta potential at the channel walls with the applied gate voltages lower than ±2 V. The applied gate voltage should
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not exceed 2 V to avoid gas bubble formation due to electrolysis. Zeta potentials could be calculated by using Eq. (1) with known fluid velocities, so under the longitudinal electric field of 20 V/cm with no gate voltage application (VG = 0), zeta potential ( 0 ) was about −0.146 V. Fig. 3b shows change in zeta potential () as a function of the applied gate voltage (VG ) over the gate region, which can be approximately curve-fitted as = 0.096·VG . Revoking Eq. (2), the modified zeta potential ( G ) as a function of an applied gate voltage could be written as G = 0 + 0.096·VG = −0.146 + 0.096·VG . According to Eq. (3), the term of [CSL /(CSL + CD )] could be estimated as 0.096. This relationship helps predict changes in zeta potential over the gate region by applied gate voltages, thus leading to the modified zeta potential and flow velocity prediction. It should be recognized that the tracer particles could experience electrophoretic force. Even though the tracer particles are charge-neutral, they still show electrophoresis due to the specific molecular structure of water at their surface. The electrophoretic mobility depends on particle size and fluid conductivity. According to Ref. [25], ∼2 m latex particles exhibited an electrophoretic mobility of −2 m cm/V s at 10−4 S/m. For the experimental condition in this work, electrophoresis would move particles from the cathode to anode at about 60 m/s, which was opposite to the EOF and made the EOF appear slower, however, it would not affect the change in zeta potential by the gate voltage.
positive gate voltage, the velocity field became much negatively stronger near the gate surface region, thus generating the reverse flow. The flow fields with other applied gate voltages were also simulated. The simulated EO velocities over the gate region were extracted and plotted against the experimental data in Fig. 3a. The simulation data well supported the experimental data.
5. Conclusions A low voltage dynamic control for EO micropumps has been investigated. The experimental and simulation results confirmed the ability of modulating zeta potential by varying gate voltages to enable independent, dynamic control over the EOF in the channel, without adjusting any longitudinal electric fields. The low gate voltages, ranging from −2 V to +2 V, were applied with no observable electrochemical reactions in the channel. Comparing with the no gate DC EO pump, the flow rates were enhanced with the negative gate voltages while reduced with the positive gate voltages under the same longitudinal electric field of 20 V/cm. The flow reversal towards the inlet reservoir could be observed at the gate voltage of around +1.3 V. The results can be readily used to produce effective mixing on LOC devices, with the appropriate gate voltage application. In addition, the micropumps and its gate control were scalable to accommodate various LOC applications with portable size and easy integration with other microfluidic elements.
4.2. Numerical simulation A longitudinal electric field strength of 20 V/cm, i.e. 40 V applied over the 2-cm long channel, was uniformly applied throughout the channel. A set of varied gate voltages (VG ) were applied ranging from −2 V to + 2 V. After applying the gate voltages, the EDL is in quasi-equilibrium, and there is little electric field into the fluid when a DC potential is applied. The gate voltages contribute towards the EOF by inducing more charges in the EDL over the gate electrode, which is represented by change in zeta potential. Therefore, overall electric field distribution was considered uniform throughout the channel in our work. Fig. 4a shows the simulated fluid velocity field when VG = −2 V. Arrows in the figure indicate field directions and relative magnitudes via arrow size. The velocities over the gate region become positively higher. Fig. 4b shows a step change in the fluid velocity at the channel centerline from inlet to outlet with VG = −2 V. The non-uniform distribution of flow velocities was due to the back pressure from regions with lower zeta potential. The velocity field at downstream (outlet) was enhanced. Fig. 4c demonstrates the cross-sectional velocity distribution profiles at channel midpoint. At channel midpoint, the velocity field became stronger over the gate surface region and decreased almost linearly towards the other channel wall, which is a superposition of faster EOF above the gate and the back pressure driven flow from the slower-moving nongate region. Because the flow rate has to remain the same at any cross-section of the channel, the faster fluid velocity at the channel bottom is compensated by slower or even reverse flow. Overall, the flow rate of the pump improved due to the extra charges induced by the negative VG . On the other hand, by applying VG = +2 V, the flow field became reversed over the gate region as more anions were induced and eventually dominated over cations in the EDL, as shown in Fig. 5a. The reverse flow induced by cations over the gate region competed with the forward flow over the other channel surface region, leading to a pressure driven flow profile in the non-gate region. Fig. 5b shows a step change in EO velocity at the channel centerline from inlet to outlet, where the flow direction reversed after crossing over to the gate. At the channel midpoint in Fig. 5c, as co-ions in the EDL were significantly and sufficiently increased due to the applied
References [1] A. Manz, N. Graber, H.M. Widmer, Miniaturized total chemical analysis systems: a novel concept for chemical sensing, Sens. Actuators B 1 (1990) 244–248. [2] L. Chen, S. Lee, J. Choo, E.K. Lee, Continuous dynamic flow micropumps for microfluid manipulation, J. Micromech. Microeng. 18 (2008) 013001–13022. [3] C.H. Wu, J.K. Chen, R.J. Yang, Electrokinetically driven flow control using bare electrodes, Microfluid. Nanofluid. 3 (2007) 485–494. [4] D.J. Laser, J.G. Santiago, A review of micropumps, J. Micromech. Microeng. 14 (2004) 35–64. [5] L.X. Chen, J.P. Ma, Y.F. Guan, Study of an electroosmotic pump for liquid delivery and its application in capillary column liquid chromatography, J. Chromatogr. A 1028 (2004) 219–226. [6] S.L. Zeng, C.-H. Chen, J.C. Mikkelsen, J.G. Santiago, Fabrication and characterization of electroosmotic micropumps, Sens. Actuators B 79 (2001) 107–114. [7] Y.J. Kang, S.C. Tan, C. Yang, X.Y. Huang, Electrokinetic pumping using packed microcapillary, Sens. Actuators A 133 (2007) 375–382. [8] S.L. Zeng, C.-H. Chen, J.G. Santiago, J.-R. Chen, R.N. Zare, J.A. Tripp, F. Svec, J.M.J. Frechet, Electroosmotic flow pumps with polymer frits, Sens. Actuators B 82 (2002) 209–212. [9] G.S. Chirica, V.T. Remcho, Fritless capillary columns for HPLC and CEC prepared by immobilizing the stationary phase in an organic polymer matrix, Anal. Chem. 72 (2000) 3605–3610. [10] I.M. Lazar, B.L. Karger, Multiple open-channel pumping system for microfluid sample handling, Anal. Chem. 74 (2002) 6259–6268. [11] J.S. Buch, P.C. Wang, D.L. Devoe, C.S. Lee, Field-effect flow control in a polydimethylsiloxane-based microfluidic system, Electrophoresis 22 (2001) 3902–3907. [12] C.M. Brotherton, R.H. Davis, Electroosmotic flow in channels with step changes in zeta potential and cross section, J. Colloid Interface Sci. 270 (2004) 242–246. [13] J.S.H. Lee, C.L. Ren, D. Li, Effects of surface heterogeneity on flow circulation in electroosmotic flow in microchannels, Anal. Chim. Acta 530 (2005) 273–282. [14] N.J. Sniadecki, C.S. Lee, P. Sivanesan, D.L. DeVoe, Induced pressure pumping in polymer microchannels via field-effect flow control, Anal. Chem. 76 (2004) 1942–1947. [15] R.B.M. Schasfoort, S. Schlautmann, J. Hendrikse, A. van den Berg, Field-effect flow control for microfabricated fluidic networks, Science 286 (1999) 942–945. [16] T.M. Squires, M.Z. Bazant, Induced-charge electro-osmosis, J. Fluid Mech. 509 (2004) 217–252. [17] K. Seibel, L. Schöler, H. Schäfer, M. Böhm, A programmable planar electroosmotic micropump for lab-on-a-chip applications, J. Micromech. Microeng. 18 (2008) 025008–25017. [18] S.A. Mirbozorgi, H. Niazmand, M. Renksizbulut, Electro-osmotic flow in reservoir-connected flat microchannels with non-uniform zeta potential, J. Fluids Eng. 128 (2006) 1133–1143. [19] R.S. Ramsey, J.M. Ramsey, Generating electrospray from microchip devices using electroosmotic pumping, Anal. Chem. 69 (1997) 1174–1178. [20] C.S. Lee, D. McManigill, C.-T. Wu, B. Patel, Factors affecting direct control of electroosmosis using an external electric field in capillary electrophoresis, Anal. Chem. 63 (1991) 1519–1523.
P. Mruetusatorn et al. / Sensors and Actuators A 153 (2009) 237–243 [21] J. Wu, Interactions of electrical fields with fluids: laboratory-on-a-chip applications, IET Nanobiotechnol. 2 (2008) 14–27. [22] N.G. Green, A. Ramos, A. Gonzalez, H. Morgan, A. Castellanos, Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. III. Observation of streamlines and numerical simulation, Phys. Rev. E. 66 (2002) 026305–26311. [23] L.M. Fu, J.Y. Lin, R.J. Yang, Analysis of electroosmotic flow with step change in zeta potential, J. Colloid Interface Sci. 258 (2003) 266–275. [24] E.J. van der Wouden, T. Heuser, D.C. Hermes, R.E. Oosterbroek, J.G.E. Gardeniers, A. van den Berg, Field-effect control of electro-osmotic flow in microfluidic network, Colloids Surf. A 267 (2003) 110–116. [25] I. Ermolina, H. Morgan, The electrokinetic properties of latex particles: comparison of electrophoresis and dielectrophoresis, J. Colloid Interface Sci. 285 (2005) 419–428.
Biographies Prachya Mruetusatorn received his B.E. degree in Electronics Engineering from King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok, Thailand, in 2002. In 2004, he received his first M.S. degree in Industrial and Engineering Technology from Murray State University, Kentucky, USA. He received his second M.S. degree in 2007 and is currently working towards his Ph.D. degree in Electrical Engineering at the University of Tennessee at Knoxville, USA. He is a member of the Electromechanics and Transducer Research for Bio-Nanoengineering Laboratory and a member of
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the Center for Musculoskeletal Research. His research interests involve manipulation of fluids and particles using electrokinetics (EK) techniques. Mohamed R. Mahfouz received his Ph.D. degree in Biomedical Engineering from Colorado School of Mines, Colorado, USA, in 2002. He is an Associate Professor of Biomedical Engineering, University of Tennessee at Knoxville, USA. He is also Technical Director of Center for Musculoskeletal Research, Computational Sciences and Engineering Division Oak Ridge National Laboratory, Oak Ridge, Tennessee. His research interests include biomedical instrumentation, medical imaging and enhancement, surgical navigation, advanced visualization, orthopedic dynamic modeling, 3D bone and tissue reconstruction, vascular computational fluid dynamics, engineering analysis of surgical techniques and outcomes, and anthropomorphic classification. Jie (Jayne) Wu received her Ph.D. degree in Applied Physics from the Chinese Academy of Sciences, Beijing, China, in 1999, and her Ph.D. degree in Electrical Engineering from the University of Notre Dame, Indiana, USA, in 2004. She is currently an Assistant Professor with the Department of Electrical Engineering and Computer Science, the University of Tennessee at Knoxville, USA. From August 2003 to July 2004, she conducted post-doctoral research at the Center for Microfluidics and Medical Diagnostics, Department of Chemical and Bio-molecular engineering, University of Notre Dame. Her research interests are solid-state device physics, biomicroelectromechanical systems (bio-MEMS), microfluidics.