Casimir force in dense confined electrolytes

TL;DR

This paper investigates the Casimir-like fluctuation-induced forces in dense electrolytes confined between surfaces, revealing how these forces depend on ion concentration and fluctuation modes, with implications for understanding experimental observations.

Contribution

It introduces an analytical framework for the Casimir force in electrolytes considering density fluctuations, highlighting the effects of ion concentration and excluded volume.

Findings

Force decays exponentially with a length half of the bulk correlation length in the RPA.

At high concentrations, the force becomes exponentially damped oscillatory.

The analysis suggests fluctuation-induced forces could influence surface interactions in electrolytes.

Abstract

Understanding the force between charged surfaces immersed in an electrolyte solution is a classic problem in soft matter and liquid-state theory. Recent experiments showed that the force decays exponentially but the characteristic decay length in a concentrated electrolyte is significantly larger than what liquid-state theories predict based on analysing correlation functions in the bulk electrolyte. Inspired by the classical Casimir effect, we consider an alternative mechanism for force generation, namely the confinement of density fluctuations in the electrolyte by the walls. We show analytically within the random phase approximation, which assumes the ions to be point charges, that this fluctuation-induced force is attractive and also decays exponentially, albeit with a decay length that is half of the bulk correlation length. These predictions change dramatically when excluded volume effects are accounted for within the mean spherical approximation. At high ion concentrations the Casimir force is found to be exponentially damped oscillatory as a function of the distance between the confining surfaces. Our analysis does not resolve the riddle of the anomalously long screening length observed in experiments, but suggests that the Casimir force due to mode restriction in density fluctuations could be an hitherto under-appreciated source of surface-surface interaction.