(320c) Direct Numerical Simulation of Electrokinetic Chaos Near Ion-Selective Surfaces

Many electrochemical cells involve aqueous electrolytes with charge transport through ion-selective surfaces, such as membranes or electrodes. Using direct numerical simulation (DNS) of the coupled Poisson-Nernst-Planck and Navier-Stokes equations in a multidimensional domain, we analyze the transport processes driving electrohydrodynamic chaos near these surfaces. Relevant model problems that we explore include an aqueous electrolyte between an ion-selective membrane and a stationary reservoir, and an aqueous electrolyte between electrodes. Electrode surfaces are modeled with realistic Butler-Volmer reaction kinetics and account for the Stern-layer capacitance. Our results indicate that as the voltage drop in the electrolyte exceeds ~10-100 thermal voltages, the system transitions to a regime involving fully chaotic vortices. In contrast to the previously known vortices due to electro-osmotic instability, the chaotic vortices are highly unsteady, with a multilayer structure involving multiple vortex sizes and broadband velocity spectra. The current-voltage predictions from our simulations successfully predict an over-limiting current regime consistent with recent experimental observations. The practical implications of these vortices include: enhanced mixing, enhanced transport beyond diffusion limitation, and reduced overpotential due to concentration gradients. We will discuss the relevance of turbulence theory tools for mathematical modeling of such regimes. Namely, we show that turbulent-diffusivity-type models are promising for prediction of the net transport rate and average concentration profiles in such systems.