Theory for the Anomalous Phase Behavior of Inertial Active Brownian Particles

TL;DR

This paper develops a mechanical theory to describe the phase behavior of inertial active Brownian particles, revealing how inertia influences motility-induced phase separation and identifying key factors like particle softness.

Contribution

It introduces a comprehensive non-equilibrium theoretical framework for inertial active matter, capturing phase coexistence and the effects of inertia without relying on equilibrium assumptions.

Findings

Inertia affects the phase diagram and critical points of active particles.

Particle softness, not inertia, causes MIPS reentrance effects.

The theory qualitatively matches simulation results for phase behavior.

Abstract

In contrast to equilibrium systems, inertia can profoundly impact the phase behavior of active systems. This has been made particularly evident in recent years, with motility-induced phase separation (MIPS) exhibiting several intriguing dependencies on translational inertia. Here we report extensive simulations characterizing the phase behavior of inertial active matter and develop a mechanical theory for the complete phase diagram without appealing to equilibrium notions. Our theory qualitatively captures all aspects of liquid-gas coexistence, including the critical value of inertia above which MIPS ceases. Notably, our findings highlight that particle softness, and not inertia, is responsible for the MIPS reentrance effect at the center of a proposed active refrigeration cycle.