Casimir forces in binary liquid mixtures

TL;DR

This paper explores how critical fluctuations in binary liquid mixtures near their demixing transition generate Casimir-like forces between bodies, comparable to van der Waals forces, with implications for experiments and force measurements.

Contribution

It demonstrates that critical Casimir forces in binary liquid mixtures can be significant and comparable to dispersion forces, providing analytical and Monte Carlo estimates for these effects.

Findings

Critical Casimir forces can match van der Waals forces in magnitude.

Analytical estimates align with Monte Carlo results in three dimensions.

Modification of force measurements and wetting layer thicknesses is detectable.

Abstract

If two ore more bodies are immersed in a critical fluid critical fluctuations of the order parameter generate long ranged forces between these bodies. Due to the underlying mechanism these forces are close analogues of the well known Casimir forces in electromagnetism. For the special case of a binary liquid mixture near its critical demixing transition confined to a simple parallel plate geometry it is shown that the corresponding critical Casimir forces can be of the same order of magnitude as the dispersion (van der Waals) forces between the plates. In wetting experiments or by direct measurements with an atomic force microscope the resulting modification of the usual dispersion forces in the critical regime should therefore be easily detectable. Analytical estimates for the Casimir amplitudes Delta in d=4-epsilon are compared with corresponding Monte-Carlo results in d=3 and their quantitative effect on the thickness of critical wetting layers and on force measurements is discussed.