Nonlinear electrokinetics at large voltages
Author(s)Bazant, Martin Z.; Kilic, Mustafa Sabri; Storey, Brian D.; Ajdari, Armand
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The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose double-layer 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 electro-osmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking.
DepartmentMassachusetts Institute of Technology. Department of Chemical Engineering; Massachusetts Institute of Technology. Department of Mathematics
New Journal of Physics
Martin Z Bazant et al. "Nonlinear electrokinetics at large voltages ." 2009 New J. Phys. 11 075016 © 2009 IOP Publishing
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