(371d) Induced Charge Electrokinetics Over 'controllably Contaminated' Surfaces: The Effects of Dielectric Thin Films and Surface Chemistry

Microfluidic devices have sparked a renewed interest in electrokinetic phenomena for transporting fluids on small scales, and subjected classical electrokinetic theories and understanding to new challenges. For example, AC electric potentials applied to and across metallic surfaces have been shown to drive a time-averaged flow, which could provide an on-chip means to drive high pressures with low voltage. The central physical effect is Induced-charge electro-osmosis, which is a non-linear electrokinetic effect in which an externally applied electric field both induces and drives a layer of charged fluid near an electrically conductive surface[1]. Experimental data on ICEO and related phenomena have shown that the standard theory consistently overpredicts observed slip velocities by up to three orders of magnitude under a variety of conditions[2], suggesting an as yet unknown mechanism for electrokinetic flow supression. Here we presents experiments and theory in which we deliberately and controllably `contaminate' the metallic surface with a thin dielectric film, and derive a theory for ICEO that incorporates both dielectric effects and surface chemistry, both of which act to decrease the slip velocity relative to a `clean' metal. Experimental data for over a thousand combinations of electric field strength and frequency, electrolyte compositions, dielectric thicknesses and surface chemistries show essentially unprecedented quantitative agreement with our theory.

[1]Squires and Bazant. Journal of Fluid Mechanics 2004.

[2]Bazant, et al. arXiv. 0903.4790