AC-Driven Electro-Osmotic Flow in Charged Nanopores

TL;DR

This paper develops a theoretical model for AC-driven electro-osmotic flow in charged nanopores, revealing how frequency-dependent phase shifts can be used to measure ion diffusion coefficients in membranes.

Contribution

The study introduces a novel theoretical framework for analyzing AC-driven electro-osmotic flow, including the frequency response and phase shift, to quantify ion diffusion in charged nanopores.

Findings

Frequency-dependent phase shift characterizes electro-osmotic flow response.

The method allows quantification of apparent ion diffusion coefficients.

Frequency window is suitable for experimental measurements.

Abstract

In this paper we report the theory describing the electro-osmotic flow in charged nanopores with constant radius and charge density driven by alternating current. We solve the ion and solution transport in unsteady conditions as described by the Navier-Stokes and Nernst-Planck equations considering the electrical potential inside the charged nanopore uniform in the radial direction (Uniform Potential model approximation). We derive the transport equation system in the case in which the pore is connected to two boundary diffusion layers and the cations and anions have different diffusion coefficients. This approach allows the theoretical description of the characteristic frequency dependence of the phase shift between the applied current density and the electro-osmotic flow. Additionally we show how the analysis of the dynamic response of the electro-osmotic coupling factor versus the AC frequency allows us to quantify the apparent ion diffusion coefficient in membranes. Notably, the frequency window where the phase-shift is predicted is well inside the commonly used sampling rate for electro-osmotic flow experiments $f\in\left[10^{-4},10^1\right]$ Hz, hence the proposed method can be useful to determine the apparent diffusion coefficient of ions (such as redox complex for flow batteries) in charged membranes.