Active colloidal suspensions: Clustering and phase behavior

TL;DR

This paper reviews recent experimental, numerical, and theoretical studies on active colloidal suspensions, highlighting their clustering behavior and phase separation phenomena in two-dimensional systems.

Contribution

It provides a comprehensive overview of experimental setups, propelling mechanisms, numerical models, and a mean-field theory explaining phase separation in active colloids.

Findings

Active colloids exhibit clustering and phase separation without attractive interactions.

Experimental and simulation results show a clear separation into dense clusters and dilute gas phases.

A mean-field theoretical framework explains the density instability leading to phase separation.

Abstract

We review recent experimental, numerical, and analytical results on active suspensions of self-propelled colloidal beads moving in (quasi) two dimensions. Active colloids form part of the larger theme of active matter, which is noted for the emergence of collective dynamic phenomena away from thermal equilibrium. Both in experiments and computer simulations, a separation into dense aggregates, i.e., clusters, and a dilute gas phase has been reported even when attractive interactions and an alignment mechanism are absent. Here, we describe three experimental setups, discuss the different propelling mechanisms, and summarize the evidence for phase separation. We then compare experimental observations with numerical studies based on a minimal model of colloidal swimmers. Finally, we review a mean-field approach derived from first principles, which provides a theoretical framework for the density instability causing the phase separation in active colloids.