Phase coexistence of active Brownian particles

TL;DR

This paper develops an analytical nonequilibrium theory for phase coexistence in active Brownian particles, validated by simulations, revealing how internal force fields and pressures drive motility-induced phase separation.

Contribution

It introduces a novel power functional theory-based analytical framework for understanding nonequilibrium phase coexistence in active matter systems.

Findings

Internal force fields include isotropic and interfacial drag, superadiabatic pressure, and quiet life forces.

Balance of quiet life and adiabatic forces determines bulk coexistence conditions.

Phase transition driven by nonequilibrium repulsion, with active particles exhibiting different repulsive behaviors.

Abstract

We investigate motility-induced phase separation of active Brownian particles, which are modeled as purely repulsive spheres that move due to a constant swim force with freely diffusing orientation. We develop on the basis of power functional concepts an analytical theory for nonequilibrium phase coexistence and interfacial structure. Theoretical predictions are validated against Brownian dynamics computer simulations. We show that the internal one-body force field has four nonequilibrium contributions: (i) isotropic drag and (ii) interfacial drag forces against the forward motion, (iii) a superadiabatic spherical pressure gradient and (iv) the quiet life gradient force. The intrinsic spherical pressure is balanced by the swim pressure, which arises from the polarization of the free interface. The quiet life force opposes the adiabatic force, which is due to the inhomogeneous density distribution. The balance of quiet life and adiabatic forces determines bulk coexistence via equality of two bulk state functions, which are independent of interfacial contributions. The internal force fields are kinematic functionals which depend on density and current, but are independent of external and swim forces, consistent with power functional theory. The phase transition originates from nonequilibrium repulsion, with the agile gas being more repulsive than the quiet liquid.