Thermodynamically consistent coarse-graining of polar active fluids

TL;DR

This paper presents a new closure model for polar active fluids that accurately captures kinetic theory properties and reproduces complex turbulent behaviors in simulations, advancing understanding of active matter dynamics.

Contribution

The paper introduces a thermodynamically consistent closure model based on quasi-equilibrium approximation, improving upon existing models by closely matching kinetic theory properties and turbulence phenomena.

Findings

Model reproduces linear stability and entropy balance of kinetic theory.

Simulations show complex multiscale turbulence with defects.

Model outperforms traditional closure models in qualitative behavior.

Abstract

We introduce a closure model for coarse-grained kinetic theories of polar active fluids. Based on a quasi-equilibrium approximation of the particle distribution function, the model closely captures important analytical properties of the kinetic theory, including its linear stability and the balance of entropy production and dissipation. Nonlinear simulations show the model reproduces the qualitative behavior and nonequilibrium statistics of the kinetic theory, unlike commonly used closure models. We use the closure model to simulate highly turbulent suspensions in both two and three dimensions in which we observe complex multiscale dynamics, including large concentration fluctuations and a proliferation of polar and nematic defects.