Towards a liquid-state theory for active matter

TL;DR

This paper develops a theoretical framework extending the virial expansion to active matter, capturing phase separation phenomena in dilute, weakly-active systems from microscopic principles.

Contribution

It introduces a first-principles derivation of the coarse-grained density dynamics for active particles without relying on equilibrium assumptions.

Findings

Identifies a spinodal instability indicating motility-induced phase separation.

Derives hydrodynamic coefficients explicitly dependent on microscopic interactions.

Shows the approach captures key features of active matter beyond equilibrium conditions.

Abstract

In equilibrium, the collective behaviour of particles interacting via steep, short-ranged potentials is well captured by the virial expansion of the free energy at low density. Here, we extend this approach beyond equilibrium to the case of active matter with self-propelled particles. Given that active systems do not admit any free-energy description in general, our aim is to build the dynamics of the coarse-grained density from first principles without any equilibrium assumption. Starting from microscopic equations of motion, we obtain the hierarchy of density correlations, which we close with an ansatz for the two-point density valid in the dilute regime at small activity. This closure yields the nonlinear dynamics of the one-point density, with hydrodynamic coefficients depending explicitly on microscopic interactions, by analogy with the equilibrium virial expansion. This dynamics admits a spinodal instability for purely repulsive interactions, a signature of motility-induced phase separation. Therefore, although our approach should be restricted to dilute, weakly-active systems a priori, it actually captures the features of a broader class of active matter.