(185b) Alternating Current Electro-Osmotic Pumping at Asymmetrically Metallized Porous Membranes

J. Beharic and C. K. Harnett

Department of Electrical and Computer Engineering, University of Louisville

The goal of this project is to investigate AC electro-osmotic pumping through asymmetrically metallized membrane nanopores for bubble-free, electrically driven fluid transport in compact lab-on-a-chip systems that require higher backpressure than can be achieved with microscale pumping features [1]. Induced Charge Electroosmosis (ICEO) is an electrokinetic phenomenon at the interface of polarized conductive surfaces and electrolytes in the presence of an electric field. The polarized conductor attracts counterions, forming a double layer at the surface. With continuous application of an electric field, the ions in the double layer move with a slip velocity that can create bulk flow. ICEO flow can persist even in alternating current (AC) electric fields [2]. Most work in this area uses microfabricated pillars, in contrast to the flow-through format described here. We have created several different types of metallized membranes and tested them under different conditions. To validate our experimental results and to optimize our parameters we have also performed computational fluid dynamics (CFD) simulations. The work is motivated by the idea that nanoscale pore geometry can potentially exceed the pressure limitations of current ICEO and alternating current electroosmosis (ACEO) microscale systems. Nanoporous materials made either by self-assembly, or by â??top-downâ? nanolithography, will open the possibility of using ICEO in new applications including lab-on-a-chip sample preparation.

Methods

Fabrication

We have produced flow-through ICEO structures using both top-down photolithography for optical microscopy experiments, and track-etched nanoporous polymer membranes for pressure measurements. Unlike traditional micropillar-based ICEO devices, the fabrication of these asymmetrically metallized membranes is a simple process. Nanoporous ICEO structures are fabricated by sputtering gold onto off-the-shelf track-etched nanoporous polycarbonate membranes. The sputtering process coats only the top and side walls of the membrane, breaking the symmetry of the metal coating. Track-etched pores are commercially available at nanoscale diameters (80 nm- 1 micron) which are not achievable using photolithography. Although the track-etched membranes provide nanoscale features for backpressure generation, their random distribution and small size makes it difficult to optically image the flow pattern at individual pores. Therefore, larger and more uniform pores are needed for optical studies. These larger pores were created using SU-8 epoxy-based photoresist and standard cleanroom lithography processes. The membranes were created by first depositing a sacrificial layer of styrene onto a wafer, followed by standard SU-8 lithography and release in toluene. The released SU-8 membranes were transferred to a silicone carrier and metallized by sputtering.

Measurements

Pressure measurements were performed with a MPXV7002DP pressure sensor which outputs a voltage proportional to pressure difference across the membrane. The voltage was collected using LabVIEW and analyzed in MATLAB. The pressure measurement device consisted of two reservoirs connected by a microfluidic channel. Electrical contact was made using platinum wires immersed in the reservoirs. The membrane was placed above one end of the connecting channel. By using an indium tin oxide slide as a transparent electrode and substrate for the microfluidic channel, the membrane could be polarized while tracer particles were observed using inverted fluorescence microscopy. The flow was recorded using an INFINITY3-3ur video camera and analyzed using MATLAB Open PIV.

Simulations

Simulations were performed in ANSYS Fluent CFD software using a moving wall in an axisymmetric domain, representing electroosmotic slip along the pore walls. The surface charge density along the wall was calculated using ANSYS Maxwell. The resulting charge density distribution was then imported into the ANSYS Fluent mode to calculate flow velocity vectors. The goal was to determine the net flow, which combines ICEO-driven flow with pressure-driven backflow, and the maximum pressure and flow rates attained by a given geometry. Maximum velocity was calculated by leaving the inlet and outlet open. Using these simulations allows us to determine the optimal parameters to optimize our device performance.

Results

Out-of-plane tracer particle motions were observed during fluorescence microscopy of active pores in AC electric fields. Pressure measurements depended on several variables:

• Voltage. Increasing the voltage (0-50Vpp) resulted in an increase in pressure, with typical maximum pressures in the 120 Pa range for a single membrane and higher for stacked membranes.
• Frequency. Higher driving frequencies (> 1000 Hz) quenched the ICEO effect as the ions in the fluid could not diffuse fast enough to respond to charges on the metal surface.
• Electrolyte conductivity. Increasing KCl concentration in the solution diminished the ICEO effect. The drop-off in ICEO slip velocity with increased conductivity is consistent with the literature, and is usually ascribed to ion crowding [1,4].
• Pore diameter. The pressure output of the devices increased with decreasing pore diameters as predicted by CFD simulations. Optimal performance was found at 400 nm.

Conclusions

We have successfully created metallzed membranes for induced charge electroosmotic flow. Flow has been validated by fluorescence video microscopy at membranes using fluorescent tracer beads. The flow response to the AC driving signal has been validated by pressure readings. Pressure ranges are comparable to most conventional ICEO devices, with simpler fabrication.

[1] C. C. Huang, M. Z. Bazant and T. Thorsen, Ultrafast high- pressure AC electro-osmotic pumps for portable biomedical microfluidics, Lab Chip, 2009, 10(1), 80â??85.

[2] Bazant, M. Z., & Squires, T. M. (2004). Induced-charge electrokinetic phenomena: theory and microfluidic applications. Physical Review Letters,92(6), 066101.

[3] C. K.Harnett, Jeremy Templeton, Katherine A.Dunphy-Guzman, Yehya M.Senousy, and Michael P.Kanouff, Lab on a Chip, 8 , Pages 565-572, 2008

[4] Bazant, M. Z., Kilic, M. S., Storey, B. D., & Ajdari, A. (2009). Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions. Advances in Colloid and Interface Science, 152(1-2), 48-88.