Concentration polarization, surface currents, and bulk advection in a microchannel

TL;DR

This paper provides a detailed numerical and analytical study of salt transport and overlimiting currents in microchannels, revealing strong coupling between bulk advection and surface currents, and extending models of overlimiting current.

Contribution

It introduces a comprehensive analysis combining simulations and analytical models to understand the coupling effects in microchannel salt transport, extending existing theories of overlimiting current.

Findings

Strong coupling between bulk advection and surface current.

Bulk advection is suppressed without surface current.

Extended analytical model of overlimiting current including diffusion and advection.

Abstract

We present a comprehensive analysis of salt transport and overlimiting currents in a microchannel during concentration polarization. We have carried out full numerical simulations of the coupled Poisson-Nernst-Planck-Stokes problem governing the transport and rationalized the behaviour of the system. A remarkable outcome of the investigations is the discovery of strong couplings between bulk advection and the surface current; without a surface current, bulk advection is strongly suppressed. The numerical simulations are supplemented by analytical models valid in the long channel limit as well as in the limit of negligible surface charge. By including the effects of diffusion and advection in the diffuse part of the electric double layers, we extend a recently published analytical model of overlimiting current due to surface conduction.