# Statistical Mechanics and Hydrodynamics of Self-Propelled Hard Spheres

## TL;DR

This paper develops a statistical mechanics framework for self-propelled hard spheres, deriving key properties like pressure and phase separation thresholds, validated by simulations, advancing understanding of active matter systems.

## Contribution

It introduces a novel kinetic theory approach to model interacting active particles with excluded volume, linking microscopic parameters to macroscopic behavior.

## Key findings

- Derived steady state pressure of active fluid from microscopic parameters.
- Identified critical density for motility-induced phase separation.
- Validated theoretical results with overdamped simulations.

## Abstract

Starting from a microscopic model of self-propelled hard spheres we use tools of non-equilibrium statistical mechanics and the kinetic theory of hard spheres to derive a Smoluchowski equation for interacting Active Brownian particles. We illustrate the utility of the statistical mechanics framework developed with two applications. First, we derive the steady state pressure of the hard sphere active fluid in terms of the microscopic parameters and second, we identify the critical density for the onset of motility-induced phase separation in this system. We show that both these quantities agree well with overdamped simulations of active Brownian particles with excluded volume interactions given by steeply repulsive potentials. The results presented here can be used to incorporate excluded volume effects in diverse models of self-propelled particles.