From single-particle to collective effective temperatures in an active fluid of self-propelled particles

TL;DR

This paper investigates effective temperatures in an active fluid of self-propelled particles, revealing that a single temperature characterizes dense regimes near glass transition, while multiple temperatures are needed at lower densities.

Contribution

It demonstrates the density-dependent nature of effective temperatures in active fluids and identifies the emergence of a unique temperature near the glass transition.

Findings

No universal effective temperature across all densities.

Lengthscale-dependent effective temperatures in dilute and moderate densities.

Emergence of a unique effective temperature near the glass transition.

Abstract

We present a comprehensive analysis of effective temperatures based on fluctuation-dissipation relations in a model of an active fluid composed of self-propelled hard disks. We first investigate the relevance of effective temperatures in the dilute and moderately dense fluids. We find that a unique effective temperature does not in general characterize the non-equilibrium dynamics of the active fluid over this broad range of densities, because fluctuation-dissipation relations yield a lengthscale-dependent effective temperature. By contrast, we find that the approach to a non-equilibrium glass transition at very large densities is accompanied by the emergence of a unique effective temperature shared by fluctuations at all lengthscales. This suggests that an effective thermal dynamics generically emerges at long times in very dense suspensions of active particles due to the collective freezing occurring at non-equilibrium glass transitions.