Analysis of electroconvective instability of aqueous electrolytes under oscillatory voltages

It was reported that electroconvective instability (ECI) can be generated in aqueous electrolytes near flat electrodes under AC voltages (Kim, Davidson and Mani 2017). Similar to ECI under DC electric fields, strong nonlinearity and enhanced mixing by large-scale vortices characterize ECI under AC voltages. In this study, a series of statistical and spectral analysis are conducted to understand the underlying physics of ECI. Solutions of direct numerical simulation of the Poisson--Nernst--Planck and Stokes equations are analyzed. The critical voltage above which ECI is amplified is a strong function of oscillation frequency. At frequencies higher than the intrinsic RC frequency, the critical voltage is higher than 200 thermal volts. Above the critical voltage, ion depletion outside the Debye layer and the transverse instability of extended space charge layers are intensified, with velocity fluctuations two to three orders of magnitude higher than the diffusion velocity. As frequency is lowered to 1/16 of the RC frequency, an optimal response in terms of velocity and current density is obtained. The bulk electrolyte loses equilibrium, and its salt concentration fluctuates at the oscillation frequency. The maximum current density is doubled compared with the corresponding 1D setup.