Active crystallization from power functional theory

TL;DR

This paper uses power functional theory to analyze phase behavior of active Brownian particles, revealing critical points, metastability, and the role of swim speed, aligning with recent simulations.

Contribution

It introduces a power functional approach to describe phase transitions and metastability in active matter, extending understanding of nonequilibrium phase behavior.

Findings

Motility-induced phase separation starts at a critical point.

Active freezing occurs above a nonequilibrium triple point.

The mean swim speed is a key state variable.

Abstract

We address the gas, liquid, and crystal phase behaviour of active Brownian particles in three dimensions. The nonequilibrium force balance at coexistence leads to equality of state functions for which we use power functional approximations. Motility-induced phase separation starts at a critical point and quickly becomes metastable against active freezing for P\'eclet numbers above a nonequilibrium triple point. The mean swim speed acts as a state variable, similar to the density of depletion agents in colloidal demixing. We obtain agreement with recent simulation results and correctly predict the strength of particle number fluctuations in active fluids.