Tuning Nonequilibrium Phase Transitions with Inertia

TL;DR

This paper shows that increasing particle inertia in active systems can suppress nonequilibrium behaviors and restore equilibrium-like states, providing a way to tune phase transitions in such systems.

Contribution

It demonstrates that inertia can be used to control and tune nonequilibrium phase transitions in active matter, revealing a pathway to effective equilibrium states.

Findings

Inertia suppresses motility-induced phase separation.

Active systems can exhibit equilibrium-like crystallization with increasing inertia.

Density-dependent effective temperature emerges in the inertial regime.

Abstract

In striking contrast to equilibrium systems, inertia can profoundly alter the structure of active systems. Here, we demonstrate that driven systems can exhibit effective equilibrium-like states with increasing particle inertia, despite rigorously violating the fluctuation-dissipation theorem. Increasing inertia progressively eliminates motility-induced phase separation and restores equilibrium crystallization for active Brownian spheres. This effect appears to be general for a wide class of active systems, including those driven by deterministic time-dependent external fields, whose nonequilibrium patterns ultimately disappear with increasing inertia. The path to this effective equilibrium limit can be complex, with finite inertia sometimes acting to accentuate nonequilibrium transitions. The restoration of near equilibrium statistics can be understood through the conversion of active momentum sources to passive-like stresses. Unlike truly equilibrium systems, the effective temperature is now density dependent, the only remnant of the nonequilibrium dynamics. This density-dependent temperature can in principle introduce departures from equilibrium expectations, particularly in response to strong gradients. Our results provide additional insight into the effective temperature ansatz while revealing a mechanism to tune nonequilibrium phase transitions.