Predicting the phase behavior of mixtures of active spherical particles

TL;DR

This paper demonstrates that the phase behavior of binary mixtures of torque-free active Brownian particles can be accurately predicted using measurements of pressure and reservoir densities, confirming simple coexistence rules in active matter thermodynamics.

Contribution

It introduces a method to predict phase coexistence in active Brownian particles by measuring pressure and reservoir densities, confirming mechanical and chemical equilibrium conditions.

Findings

Phase coexistences are in mechanical equilibrium with equal pressures.

Coexisting phases are in chemical equilibrium with identical reservoir densities.

Phase boundaries can be constructed from properties measured within individual phases.

Abstract

An important question in the field of active matter is whether or not it is possible to predict the phase behavior of these systems. Here, we study the phase coexistence of binary mixtures of torque-free active Brownian particles, for both systems with purely repulsive interactions and systems with attractions. Using Brownian dynamics simulations, we show that phase coexistences can be predicted quantitatively for these systems by measuring the pressure and "reservoir densities". Specifically, in agreement with previous literature, we find that the coexisting phases are in mechanical equilibrium, i.e. the two phases have the same pressure. Importantly, we also demonstrate that the coexisting phases are in chemical equilibrium by bringing each phase into contact with particle reservoirs, and showing that for each species these reservoirs are characterized by the same density for both phases. Using this requirement of mechanical and chemical equilibrium we accurately construct the phase boundaries from properties which can be measured purely from the individual coexisting phases. This result highlights that torque-free active Brownian systems follow simple coexistence rules, thus shedding new light on their thermodynamics.