(475b) The Effects of Surface Chemistry On Induced Charge Electrokinetics

Recent work with microfluidic devices and labs-on-a-chip have highlighted the importance of efficiently transporting fluids at the micron scale. This has lead to a resurgence of interest in electrokinetic phenomena, which scale favorably with the small channel dimensions encountered in microfluidics. For example, induced charge electro-omosis (ICEO), a nonlinear electrokinetic effect in which an applied electric field both induces and drives a layer of charged fluid near an electrically conductive surface, has been shown to produce time-averaged flows with AC electric fields and may provide an on-chip means of generating high pressure flows with low applied voltage [1]. Experimental studies of ICEO and related phenomena have shown that the standard theory generally overpredicts the observed slip velocity, sometimes by orders of magnitude [2]. Recently, we have reported experimental data over electrodes coated with silica that show quantitative agreement over a wide range of experimental parameters with a theory that includes the effects surface chemistry at the silica/electrode interface. Here, we further explore the effects of surface chemistry, which enter through an additional 'buffer' capacitance in series with the double layer, on the ICEO slip velocity. Specifically, we present the effects of electrolyte pH and chemical surface modification of the silica surface on the buffer capacitance and ICEO slip velocity.

[1]Squires, T. M., and M. Z. Bazant. J. Fluid Mech. 560, 1 (2006) [2]Bazant, M. Z., M. S. Kilic, B. D. Storey, and A. Ajdari. Adv. Colloid Interfac. 152, 1-2 (2009) [3]Pascall, A. J., and T. M. Squires. Phys. Rev. Lett. 104, 088301 (2010)