Electroviscous drag on squeezing motion in sphere-plane geometry

TL;DR

This paper combines theoretical modeling and experimental measurements to analyze electroviscous effects in nanoscale capillaries, revealing how charge-flow coupling influences electroviscous drag and electrostatic interactions in confined water films.

Contribution

It introduces a theoretical framework based on Poisson-Boltzmann theory and provides experimental validation for electroviscous effects in sphere-plane geometry at the nanoscale.

Findings

Electroviscous coupling parameter peaks when film thickness is comparable to screening length.

The theory accurately predicts electroviscous drag and electrostatic repulsion as functions of film thickness.

Charge regulation effects become significant at very small distances.

Abstract

Theoretically and experimentally, we study electroviscous phenomena resulting from charge-flow coupling in a nanoscale capillary. Our theoretical approach relies on Poisson-Boltzmann mean-field theory and on coupled linear relations for charge and hydrodynamic flows, including electro-osmosis and charge advection. With respect to the unperturbed Poiseuille flow, we define an electroviscous coupling parameter $\xi$, which turns out to be maximum where the film thickness $h_0$ is comparable to the screening length $\lambda$. We also present dynamic AFM data for the visco-elastic response of a confined water film in sphere-plane geometry; our theory provides a quantitative description for the electroviscous drag coefficient and the electrostatic repulsion as a function of the film thickness, with the surface charge density as the only free parameter. Charge regulation sets in at even smaller distances.