A classical nucleation theory description of active colloid assembly

TL;DR

This paper demonstrates that classical nucleation theory can effectively describe the kinetics of phase separation in active colloids, bridging nonequilibrium dynamics with equilibrium thermodynamic frameworks.

Contribution

It introduces an effective free energy approach to model active colloid phase separation, capturing key kinetic features despite the system's nonequilibrium nature.

Findings

The theory accurately predicts the binodal and nucleation rates.

Cluster size distributions match theoretical predictions below the binodal.

Discrepancies reveal additional physics in early crystal formation stages.

Abstract

Non-aligning self-propelled particles with purely repulsive excluded volume interactions undergo athermal motility-induced phase separation into a dilute gas and a dense cluster phase. Here, we use enhanced sampling computational methods and analytic theory to examine the kinetics of formation of the dense phase. Despite the intrinsically nonequilibrium nature of the phase transition, we show that the kinetics can be described using an approach analogous to equilibrium classical nucleation theory, governed by an effective free energy of cluster formation with identifiable bulk and surface terms. The theory captures the location of the binodal, nucleation rates as a function of supersaturation, and the cluster size distributions below the binodal, while discrepancies in the metastable region reveal additional physics about the early stages of active crystal formation. The success of the theory shows that a framework similar to equilibrium thermodynamics can be obtained directly from the microdynamics of an active system, and can be used to describe the kinetics of evolution toward nonequilibrium steady states.