A microscopically reversible kinetic theory of flocking

TL;DR

This paper develops a kinetic theory for active-passive particle interactions with reactive collisions, revealing conditions under which flocking behavior emerges due to a sign change in the momentum damping coefficient.

Contribution

It introduces a reversible kinetic model for reactive collisions in active matter, linking microscopic reversibility to flocking transitions.

Findings

Flocking transition occurs when the momentum damping coefficient changes sign.

Reactive collisions can be modeled as energy converting processes in active matter.

A chemostat drives the system out of equilibrium, enabling flocking.

Abstract

We formulate a kinetic theory of two species of hard spheres undergoing reactive collisions that convert chemical energy into kinetic energy. The model describes an active species interacting with a passive background, labeled as birds and air respectively, with the reactive collisions representing of self-propulsion. Microscopic reversibility of the reactive dynamics is imposed, and a chemostat is introduced to drive the system out of equilibrium. When the chemostat is sufficiently strong and one restricts to grazing interspecies collisions, we find that the bird momentum damping coefficient can change sign, giving rise to a flocking transition.