Induced Charge Electrokinetics Over ``Controllably Contaminated'' Surfaces: The Effects of Dielectric Thin Films and Surface Chemistry on Slip Velocity

Microfluidics has renewed interest in utilizing electrokinetics (EK) for transporting fluids on small scales, and has subjected EK theories and understanding to new challenges. For example, induced-charge electro-osmosis (ICEO), a non-linear EK effect in which an externally applied AC electric field both induces and drives a layer of charged fluid near an electrically conductive surface, could provide an on-chip means to drive high pressures with low voltage [1]. Experimental data on ICEO and related phenomena have shown that the standard theory consistently overpredicts slip velocities by up to a factor of 1000[2]. Here we present experiments in which we controllably ``contaminate'' the metallic surface with a thin dielectric film or Au-thiol self assembled monolayer, and derive a theory for ICEO that incorporates both dielectric effects and surface chemistry, which both act to decrease the slip velocity relative to a `clean' metal. Data for over a thousand combinations of electric field strength and frequency, electrolyte composition, dielectric thickness and surface chemistry show essentially unprecedented quantitative agreement with our theory. [1] Squires \& Bazant. J. Fluid Mech. 2004 [2] Bazant, et al. arXiv. 0903.4790