Institute of Physics Publishing doi:10.1088/1742-6596/34/1/058
Journal of Physics: Conference Series 34 (2006) 356–361 International MEMS Conference 2006
Microfluidic Transport by AC Electroosmosis NAZMUL ISLAM and JIE WU Department of Electrical and Computer Engineering, The University of Tennessee, Knoxville, TN 37996, USA.
[email protected] Abstract. AC Electro-osmosis (ACEO) is a newly developed microfluidic mechanism, which can transport fluids and particles with low voltage that is suitable for integration with IC circuitry. We observed line formation of particles on electrodes by ACEO, leading to better understanding of ACEO microflows. Also by identifying a new ACEO mechanism from electrochemical reaction, we developed Asymmetric Polarization (A-P) ACEO technique, adding another dimension to ACEO. Both simulation and experiments have been performed to promote the understanding and optimization of ACEO devices. The experimental result of A-P ACEO is also presented in this paper. Keywords: electrokinetics, AC Electro-osmosis (ACEO); electrode polarization; microfluidics.
1. Introduction
During recent years, AC electro-osmosis (ACEO) has emerged as a promising microfluidic mechanism, drawing increasing attention from researchers. The advantages of ACEO include flexible fluid manipulation by electrode design, compatibility with IC technology, suppressed electrochemical reactions, etc, and those properties are desirable for integration into lab-on-a-chip. Our group has been investigating particle concentrator, micro-pumps and mixers by ACEO. Electro-osmosis typically refers to using DC potential to moving fluids through a porous medium, which is a well-established technique and finds wide applications in biochemical analysis, civil engineering, etc. Both DC EO and ACEO are based on the ion migration within a nanometer layer of charges/ions at the interfaces of electrolytes and solids (a.k.a. double layer). This layer of charges will migrate under electric fields tangential to the interface, and because of fluid viscosity, the ion movement carries along its surrounding fluids, leading to fluid motion. In ACEO, the charges in the double layer are induced by AC potentials, and tangential E-fields are also from the same voltage source. Therefore, the changes of polarities in charges and field directions are simultaneous and cancelled out, maintaining steady ion migration and fluid motion. Ramos et al have provided a rather comprehensive review [1] on various forces acting on microsize particles on microelectrode arrays when ACEO electrodes are energized with ac voltages over a wide range of frequency. In a subsequent work, Ramos et al [2] presented a RC model describing the frequency dependence of ACEO flow velocity by capacitive charging of the electrodes. Our research © 2006 IOP Publishing Ltd
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emphasizes on the charging processes of electrodes. By studying surface EO flows with respect to AC potential, we identified ACEO induced by electrochemical reactions (i.e. Faradaic charging), and we have developed a new ACEO technique—asymmetric polarization ACEO. Asymmetric polarization of electrodes in a pair is achieved by combining the DC bias into AC signals over electrode pairs. For adding DC bias the reflection symmetry of electrode charging is broken, leading to asymmetric surface flow and net-flow [7]. By adjusting the amplitude and frequency of AC signals, a variety of directed surface flows are produced on electrodes to manipulate and transport particles. This paper first studied the electric field distribution around an isolated pair of electrodes, which clarified the microfluidic motions in the proximity of electrodes. Four counter-rotating vortices were predicted and experimentally verified. Then noting that ACEO originates from electrochemistry at the interface of electrode and fluids [5], and the size of the electrode could affect fluid velocity, four different dimensions of electrodes were used to study their effects. ACEO tends to produce recirculating flows, and the key to produce net flow is to break AC electric field symmetry. This goal can be realized by asymmetric electrode design, or by asymmetric excitation signals, a.k.a. asymmetric-polarization (A-P) in this paper. The experimental result of the A-P ACEO fluid flow and it dependence on the frequency is also explained.
2. Experimental Result
ACEO flow is examined using microfabricated arrays of electrode pairs on silicon substrate. Au/Cr (90nm/10nm thickness) electrodes were fabricated by lift-off procedure in IC processing. Cr is the adhesion layer between the substrate and Au, and Au is in contact with electrolytes. The electrodes were 20 mm long, 0.1 Pm think, 160 Pm wide with a 40 Pm separation (denoted as 160/40). Microfluidic chambers were formed by sealing silicone microchambers (PC8R-0.5, Grace Bio-Labs, Inc.) over the wafer, which have a height of 500 Pm. Polystyrene spheres (3 Pm diameter; Fluka Chemica) seeded in DI water was used to track fluid motion. The electrode impedances were measured with an Agilent 4294A impedance analyzer from 40 Hz to 5 MHz at an open oscillation level of 500 mVrms.
(a)
(b) 1/¥2 of electrode width (null point)
Fig. 1 (a) FEMLAB simulation for the Electric field distribution above a pair of planar electrodes with voltage of +1V & -1V in two electrodes (160/40micron). (b) Four counter-rotating vortices are formed above the electrodes due to changes in tangential electric fields, which facilitates particles aggregation on electrodes. We have used FEMLab to simulate the electric field distribution above a pair of planar electrodes (160micron width and 40 micron separation between the electrodes, with infinitesimal thickness). As shown in Fig. 1a, tangential electric fields change directions over one electrode, which indicates that two counter-rotating vortices exist on one electrode, as schematically drawn in Fig. 1b.
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Figure 2a shows the initial distribution of particles when no signals are applied over the electrodes. Figure 2b shows that particles accumulated from both sides into lines at approximately 1 2 of electrode width [4]. This corroborates the theoretic prediction, since fluid velocity reduces at the null points of electric fields, and particles become trapped to the electrodes due to surface forces of between particles and electrodes.
1/¥2 of electrode width (null point)
(a)
(b)
Fig. 2 (a) Particle without the supply voltage (b) Experimental picture of the particles accumulating at the 1/¥2 electrode width; ACEO can transport the particles from a large region in the bulk fluid to the electrode surface. The flow velocity is important for optimizing the micropump and particle transportation. In contrast to electrophoretic and dielectrophoretic (DEP) velocity, which are typically limited to less than 20 microns per second [2], the ACEO velocity exceed 100 micron/sec. The trapped particles also change the impedance of the electrode and hence their presence can be detected by using the trapping electrode as impedance sensors. Impedance Variation with particle Impedance (ohm)
1.50E+06 1.25E+06 1.00E+06 7.50E+05 5.00E+05 2.50E+05 0.00E+00 1.00E+02
1.00E+03
1.00E+04
1.00E+05
Frequency (Hz) Particle Concentration
Electrode Pair in water
Fig. 3 Impedance measurement with and without the particle We use information from impedance measurement to optimize the operating condition (signal frequency, magnitude etc.) of ACEO. Figure 3 shows the plot of the impedance variation after adding the particles and it clearly shows the impedance differences at the lower frequencies. At first we measured the impedance of the two electrodes in the tap water. Then we measured the impedance of the electrodes for the particles (106 particles/ml) suspended in DI water. From the figure we can see that the difference of the impedance with and without the particle is more in frequency below 1KHz. So we adopted signal frequency range between 100 Hz to 1 kHz. The experiment was done at 500 mVrms oscillation level. Same characteristics were obtained at 1 Vrms. So the maximum peak-to-peak (p-p) voltage we used in our experiment is 2.8 Vp-p. Our goal is to determine a higher velocity electrode pattern with the polystyrene particle, so both experiments
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and calculation were performed to determine the optimum signal magnitude for these four sizes of electrode pair (160/40, 160/20, 80/40, 80/20). Particle displacement is also affected by the gravity, dielectrophoresis (DEP) and Brownian motion, so we also calculated gravity and DEP that is more for the high voltage level. We have used following equations to calculate the force for our simplified microelectrode structure. Gravity
0 .2
Dielectrophoresis ( DEP )
a2 Um g
0.03
K
(1)
t
a 2H cV 2 t K r3
(2)
k BTt 3SaK
(3)
BrownianDisplacement
DEP displacement is inversely proportional to the r3 (where, r = spacing between the electrodes, characteristic length). For this reason 80/20 configuration produces 8 times stronger DEP than does 80/40 configuration. For higher DEP, 80/20 has rotating trapped particles at the edge that are the trade-off between ACEO and DEP force. The particle size, a, is the considering factor for the gravity. Gravity is proportional to the a2 but Brownian displacement is inversely proportional to ¥2. To decrease the gravitational displacement we certainly need to move to the smaller particles, though the Brownian motion increases. There comes the efficiency of the optical microscopy, as there is a chance to get the measurement error for dealing with smaller particles. The value of gravity and Brownian displacement in a sec is 0.202 and 0.0128 micron, for 80/20 electrode configuration, which is negligible compare to the experimental ACEO displacement per second.
Particle Velocity (micron/sec)
Comparison of the Velocities for different Geomatry
100 80 60 40 20 50
250
450
650
850
1050
Frequency (Hz) 2 V (160/40)
2 V (80/40)
2 V (80/20)
2 V (160/20)
Fig. 4 Microfluidic Experiment result & comparison of four types of Electrode geometry The lower frequency is suitable for concentrating the particles (3 μm) in the electrode pair. Figure 4 gives the comparative analysis of the microfluidic velocity generated using four types of Au electrodes. The 80/20 configuration give the highest velocity, which is suitable for the pumping application. As we have used the 3 μm particle, the displacement due to Brownian motion (Eqn. 3) is negligible. But the gravitational force is more dominant (Eqn. 1), of course we can neglect it at our optimized operating range for ACEO.
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A-P ACEO is realized by applying biased AC signals over electrode pairs, leaving the electrolyte floating; therefore, two electrodes have different electrical potentials relative to the electrolyte. With a biased AC signal, Vapplied=V0(1+cosȦt) over the electrodes, the left electrode is always positive and more prone to Faradaic charging, while the other is always negative and subject to capacitive charging. The fluid velocity generated by A-P ACEO is summarized in figure 5. When the voltage exceeds the threshold for reaction, asymmetric vortices are formed above two electrodes as Faradaic reactions take place at the positively biased electrodes. At an appropriately biased AC potential, streamlines from capacitive charging and faradaic charging become connected, forming a large vortex over the electrode pair, and eventually formed a uni-directional flow. From our earlier experiments we learned that 80/20 configuration provides higher particle velocity, we have used this electrode configuration in our A-P ACEO experiments. Our experimental result (Fig 5) suggests that the velocity from asymmetric biased electrodes is much higher than that from symmetric biased electrodes.
Particle Velocity (micron/sec)
Plot for Biased and Unbiased Source 180 160 140 120 100 80 60 40 Biased
20
W ithout Bias
0 100
1000
10000
Frequency (Hz)
Fig. 5 Flow Velocity Comparison for the ACEO with DC bias of 1.5V and without bias For the A-P experiments, applied voltage exceeds the threshold for reactions at V0=1.5V (i.e. high level & low level of biased voltage is 3V & 0V respectively). At the same voltage, the maximum flow velocity shifts to higher frequency compared with symmetric AC signals. This is because Faradaic polarization becomes suppressed at high frequency. Beyond 500Hz, microflows from capacitive charging are much stronger than those from Faradaic charging, so that the stagnation point on the left electrode disappears. At 100Hz, streamlines from capacitive charging and Faradaic charging become connected, forming a large vortex over the electrode pair and the particles aligned on left electrode. 3. Conclusions
This paper investigates new ACEO mechanisms for particle transport. A net transport of the particles can be produced from the biased AC signal (A-P ACEO) on symmetric electrodes. This net flow is also sufficiently precise to be able to transport particles into a specific location on the electrode to allow detection. Our work also shows that the fluid velocity for biased AC signals is higher than that for unbiased AC signals, which helps to advance the understanding of ACEO and promote the development of lab-on-a-chip. References [1]
Ramos, H. Morgan, N. G. Green and A. Castellanos, AC electrokinetics: a review of forces in microelectrode structures, J. Phys. D: Appl. Phys. 31(1998) 2338–53
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[2] [3] [4] [5] [6] [7]
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