Danish Technical University, October 23, 2006 Nonlinear electrokinetics at large voltages Martin Z. Bazant (Department of Mathematics, MIT) Abstract: Beginning with the recent discovery of AC electro-osmosis at micro-electrode arrays, much attention has been paid to nonlinear "induced-charge" electrokinetic phenomena. Other examples include DC or AC flows around polarizable (metal or dielectric) microstructures and induced-charge electrophoretic motion of polarizable colloids. The current theory is borrowed from classical linear electrokinetics in a dilute solution, which can only be justified for "small" surface potentials (of order kT/e = 25 mV). In all situations of interest, however, much larger voltages are applied across the double layer, up to several volts = 100 kT/e, so that the solution cannot remain dilute. As a result, the theory fails to predict crucial experimental features, such as the loss of flow at high ionic strength (> 10 mM) and flow reversal at high voltage and high frequency (in certain geometries). This talk presents a modified theory taking into account (i) steric effects for ions of finite size and (ii) nonlinearities in viscosity and permittivity, which suffices to explain some (but not all) of the experimental puzzles.