Multiscale response of ionic systems to a spatially varying electric field

TL;DR

This study investigates how ionic systems respond to spatially varying electric fields, revealing different behaviors in low and high screening regimes through theoretical modeling and molecular dynamics simulations.

Contribution

The paper introduces a new constitutive relation for ionic flux due to electric potential gradients and analyzes the response functions across screening regimes.

Findings

Response functions are similar in low screening, indicating common physical processes.

In high screening, response functions differ, showing different governing processes.

The results explain the failure of the Nernst-Einstein relation in certain regimes.

Abstract

In this paper the response of ionic systems subjected to a spatially varying electric field is studied. Following the Nernst-Planck equation, two forces driving the mass flux are present, namely, the concentration gradient and the electric potential gradient. The mass flux due to the concentration gradient is modelled through Fick's law, and a new constitutive relation for the mass flux due to the potential gradient is proposed. In the regime of low screening the response function due to the potential gradient is closely related to the ionic conductivity. In the large screening regime, on the other hand, the response function is governed by the charge-charge structure. Molecular dynamics simulations are conducted and the two wave vector dependent response functions are evaluated for models of a molten salt and an ionic liquid. In the low screening regime the response functions show same wave vector dependency, indicating that it is the same underlying physical processes that govern the response. In the screening regime the wave vector dependency is very different and, thus, the overall response is determined by different processes. This is in agreement with the observed failure of the Nernst-Einstein relation.