Novel Configuration for an AC Electroosmotic Pump Driven by AC Voltage with DC Voltage Bias for Bi-Directionality and Increased Volumetric Flow Rates

Abstract

This paper discusses the principle of AC electroosmosis and its use to move the bulk of an electrically conducting fluid in a micro-channel as an alternative to mechanical pumping methods. Previous EO driven flow research [1-3] has looked at the effect of electrode asymmetry and transverse traveling wave forms on the performance of electroosmotic pumps. This paper presents an analysis that was conducted to assess the effect of combining an AC signal with a DC bias when generating the electric field needed to impart electroosmosis within a micro-channel [4]. The analysis was done using COMSOL 3.5a in which previously developed equations [1-2] were embedded and used to evaluate the effects of the frequency of excitation, electrode array geometry, and the AC signal with a DC bias on the flow imparted on an electrically conducting fluid. A single type of fluid was simulated to date. For the AC driven flow, the simulation results indicate the existence of an optimized frequency of excitation and an optimum geometry that lead to the maximum net forward flow of the pump. For a specified set of constants [electric conductivity (2.1 mS/m), lagging electrode width (220 µm), micro-channel height (200 µm), applied AC voltage (0.25 V), electrode array gap (290 µm), and etc], the optimum frequency was 250 Hz and the optimum geometry consisted of a preceding electrode width of 60 μm with an inter electrode gap of 30 μm. No relevant net flows were generated with the asymmetric electrode arrays with a constant magnitude of AC voltage applied to both electrodes. However, superimposing a DC signal over the AC signal on the same asymmetric electrode array lead to a noticeable net forward flow of 18.70 μL/min. Experimental flow measurements were performed on several pump configurations manufactured using typical MEMS fabrication techniques. The experimental results are in good agreement with the simulation data. They confirm that using an asymmetric electrode array excited by an AC signal with a DC bias leads to a significant improvement in flow rates in comparison to the flow rates obtained in an asymmetric electrode array configuration excited just with an AC signal. [1] Fluid Flow Induced by Nonuniform AC Electric Fields in Electrolytes on Microelectrodes. II. A Linear Double-Layer Analysis. Gonzalez, A., et al. 4, April 2000, Physical Review E, Vol. 61, pp. 4019-4028. [2] Novel Systems for Configurable AC Electroosmotic Pumping. Loucaides, N., Ramos, A. and Georghiou, G.E. 2007, Microfluid Nanofluid, Vol. 3, pp. 709-714. [3] Pumping of liquids with ac voltages applied to asymmetric pairs of microelectrodes. Ramos, A., et al. 2003, Physical Review E 67 (5-2), pp. 056302/1-056302/11. [4] Biased AC Electroosmosis Micropump for Water Management in PEM Fuel Cells. Islam, N. Boston, Massachusetts, USA : ASME, 2008. 2008 ASME International Mechanical Engineering Congress and Exposition. pp. 1-4.