Collective behavior of colloids due to critical Casimir interactions

TL;DR

This paper reviews how critical Casimir forces influence colloid behavior near solvent critical points, enabling tunable self-assembly and phase transitions, with implications for controlling colloidal systems.

Contribution

It provides an overview of experimental and theoretical advances in understanding critical Casimir forces and their role in colloidal self-assembly and phase behavior.

Findings

Critical Casimir forces induce reversible colloid aggregation.

Experimental evidence of phase transitions driven by CCFs.

Theoretical models vary in describing ternary solvent-colloid systems.

Abstract

If colloidal solute particles are suspended in a solvent close to its critical point, they act as cavities in a fluctuating medium and thereby restrict and modify the fluctuation spectrum in a way which depends on their relative configuration. As a result effective, so-called critical Casimir forces (CCFs) emerge between the colloids. The range and the amplitude of CCFs depend sensitively on the temperature and the composition of the solvent as well as on the boundary conditions of the order parameter of the solvent at the particle surfaces. These remarkable, moreover universal features of the CCFs provide the possibility for an active control over the assembly of colloids. This has triggered a recent surge of experimental and theoretical interest in these phenomena. We present an overview of current research activities in this area. Various experiments demonstrate the occurrence of thermally reversible self-assembly or aggregation or even equilibrium phase transitions of colloids in the mixed phase below the lower consolute points of binary solvents. We discuss the status of the theoretical description of these phenomena, in particular the validity of a description in terms of effective, one-component colloidal systems and the necessity of a full treatment of a ternary solvent-colloid mixture. We suggest perspectives on the directions towards which future research in this field might develop.