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The venerable theory of electrokinetic phenomena rests on the hypothesis of adilute solution of point-like ions near a weakly charged surface, whosepotential relative to the bulk is of order the thermal voltage ($kT/e \approx25$ mV at room temperature). In nonlinear electrokinetic phenomena, such as ACor induced-charge electro-osmosis (ACEO, ICEO) and induced-chargeelectrophoresis (ICEP), several Volts $\approx 100 kT/e$ are applied topolarizable surfaces in microscopic geometries, and the resulting electricfields and induced surface charges are large enough to violate the assumptionsof the classical theory. In this article, we review the literature, highlightdiscrepancies between theory and experiment, introduce possible modificationsof the theory, and analyze their consequences. We argue that, in response to alarge applied voltage, the "compact layer" and "shear plane" effectivelyadvance into the liquid, due to the crowding of counter-ions. Using simplecontinuum models, we predict two general trends, each enhanced by dielectricresponse: (i) ionic crowding against a blocking surface expands the diffusedouble layer and thus decreases its differential capacitance, and (ii) acharge-induced viscosity increase near the surface reduces the electro-osmoticmobility. The first effect is able to predict high-frequency flow reversal inACEO pumps, while the second may explain the decay of ICEO flow with increasingsalt concentration. Through examples, such as ICEP of an uncharged metal spherein an asymmetric electrolyte, we show that ICEO flows are ion-specific. Similarissues arise in nanofluidics (due to confinement) and ionic liquids (due to thelack of solvent), so the paper concludes with a general framework of modifiedelectrokinetic equations for finite-sized ions.

Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions