A microscopic flow model based on Brownian dynamics for simulating ionic diffusion in a 2D-channel geometry

A microscopic flow model is proposed and computed in order to qualitatively reproduce experimental behaviors as mixed alkali effect and inhomogeneous ionic concentration often observed in ionic glasses or melts. The model takes into account a stochastic force and an ion-ion repulsion force. The simulation mimics the dynamics of spherical particles moving in a nanoscopic 2D-channel geometry delimited by walls from which the particles are elastically back-scattered. Influence of the ion density, of the geometry of the 2D-channel and of the amplitude of the stochastic force is reported. The dynamics of a system containing two types of ions, having different intrinsic velocities, are also investigated. The simulation shows (i) a sub-diffusive regime (≈0.8) characterized by a time shift dependent on the parameters of the simulation, (ii) the occurrence of spatial heterogeneity characterized by ion rich domains and (iii) a mixture effect characterized by a non-linear evolution of the diffusion coefficient as a function the molar fraction when two types of ions are mixed.