Nonequilibrium critical Casimir interactions in binary fluids

TL;DR

This paper investigates how critical Casimir forces between colloids in a binary fluid mixture behave under non-equilibrium conditions, revealing significant effects near the critical point through fluid particle dynamics simulations.

Contribution

It introduces a fluid particle dynamics approach to study non-equilibrium critical Casimir forces, extending understanding beyond equilibrium scenarios.

Findings

Drag force on moving colloids is significantly affected near the critical point.

Relative motion of colloids is influenced by non-equilibrium CCFs.

Critical slowing down amplifies non-equilibrium effects.

Abstract

Colloids immersed in a critical binary liquid mixture are subject to critical Casimir forces (CCFs) because they confine its concentration fluctuations and influence the latter via effective surface fields. To date, CCFs have only been studied in thermodynamic equilibrium. However, due to the critical slowing down, the order parameter around a particle can easily be perturbed by any motion of the colloid or by solvent flow. This leads to significant but largely unexplored changes in the CCF. Here we study the drag force on a single colloidal particle moving in a near-critical fluid mixture and the relative motion of two colloids due to the CCF acting on them. In order to account for the kinetic couplings among the order parameter field, the solvent velocity field, and the particle motion, we use a fluid particle dynamics method. These studies extend the understanding of CCFs from thermal equilibrium to non-equilibrium processes, which are relevant to current experiments, and show the emergence of significant effects near the critical point.