Shear dispersion of multispecies electrolyte solutions in the channel domain

Abstract: In multispecies electrolyte solutions, even in the absence of an external electric field, differences in ion diffusivities induce an electric potential and generate additional fluxes for each species. This electro-diffusion process is well-described by the advection-Nernst-Planck equation. This study aims to analyze the long-time behavior of the governing equation under electroneutrality and zero current conditions and investigate how the diffusion-induced electric potential and shear flow enhance the effective diffusion coefficients of each species in channel domains. The some exact solutions of the effective equation and the asymptotic analyses for ions with large diffusivity discrepancies are presented. Furthermore, there are several interesting properties of the effective equation. First, it is a generalization of the Taylor dispersion, with a nonlinear diffusion tensor replacing the scalar diffusion coefficient. Second, the effective equation exhibits a scaling relation, revealing that the system with a weak flow is equivalent to the system with a strong flow under scaled physical parameters. Third, in the case of injecting a lower-concentration electrolyte solution into a pre-existing solution with the same ion species, the effective equation simplifies to a multidimensional diffusion equation. However, when introducing the electrolyte solution into a channel filled with deionized water, the ion-electric interaction results in several phenomena do not present in the advection-diffusion equation, including upstream migration of some species, spontaneous separation of ions, and non-monotonic dependence of the effective diffusivity on P\'eclet numbers. Last, the dependence of effective diffusivity on concentration and ion diffusivity suggests a method to infer the concentration ratio of each component and ion diffusivity by measuring the effective diffusivity.