Nonadditivity of Critical Casimir Forces

TL;DR

This paper provides experimental evidence that critical Casimir forces among colloids are nonadditive, with three-body effects depending on proximity to the critical point and surface properties, advancing understanding of fluctuation-induced interactions.

Contribution

It experimentally demonstrates the nonadditivity of critical Casimir forces and characterizes the dependence of three-body effects on criticality and surface preferences.

Findings

Three-body critical Casimir forces differ from pairwise sums.

Magnitude and range of effects depend on proximity to critical point.

Surface adsorption preferences influence force nonadditivity.

Abstract

In soft and condensed matter physics, effective interactions often emerge as a result of the spatial confinement of a fluctuating field. For instance, microscopic particles in a binary liquid mixture are subject to critical Casimir forces whenever their surfaces confine the thermal fluctuations of the order parameter of this kind of solvent, the range of which diverges upon approaching the critical demixing point. Critical Casimir forces are predicted to be nonadditive on a particular large scale. However, a direct experimental evidence of this fact is still lacking. Here, we fill in this gap by reporting the experimental measurement of the associated many-body effects. In particular, we focus on three colloidal particles in optical traps and observe that the critical Casimir force exerted on one of them by the other two colloids differs from the sum of the forces they exert separately. The magnitude and range of this three-body effect turn out to depend sensitively on the distance from the critical point of the solvent and on the preferential adsorption at the surfaces of the colloids for the two components of the mixture.