Nonlinear electrokinetics at large voltages Citation

The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose doublelayer voltage is of order the thermal voltage, kBT/e = 25 mV. In nonlinear ‘induced-charge’ electrokinetic phenomena, such as ac electro-osmosis, several volts ≈100kBT/e are applied to the double layer, and the theory breaks down and cannot explain many observed features. We argue that, under such a large voltage, counterions ‘condense’ near the surface, even for dilute bulk solutions. Based on simple models, we predict that the double-layer capacitance decreases and the electro-osmotic mobility saturates at large voltages, due to steric repulsion and increased viscosity of the condensed layer, respectively. The former suffices to explain observed high-frequency flow reversal in ac electroosmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking. Electrically driven flows in ionic solutions are finding many new applications in microfluidics [1]. The theory of electro-osmosis [2] was developed for slip past a surface in chemical equilibrium, whose double-layer voltage is typically of order kBT/e = 25 mV. However, the discovery of ac electro-osmosis (ACEO) at micro-electrodes [3, 4] has shifted attention to a new regime, where the induced double-layer voltage is ≈100 kBT/e, oscillating 5 Author to whom any correspondence should be addressed. New Journal of Physics 11 (2009) 075016 1367-2630/09/075016+09$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft