Deriving hydrodynamic equations from dry active matter models in three dimensions

TL;DR

This paper derives three-dimensional hydrodynamic equations for dry active matter models from microscopic rules, revealing key differences in phase transitions and solutions between polar and nematic cases.

Contribution

It extends the derivation of hydrodynamic equations from 2D to 3D for Vicsek-style models, including explicit transport coefficients and stability analysis.

Findings

Polar case equations similar to 2D counterparts.

Active nematics show discontinuous transition and bistability.

Cholesteric solutions analyzed for stability.

Abstract

We derive hydrodynamic equations from Vicsek-style dry active matter models in three dimensions (3D), building on our experience on the 2D case using the Boltzmann-Ginzburg-Landau approach. The hydrodynamic equations are obtained from a Boltzmann equation expressed in terms of an expansion in spherical harmonics. All their transport coefficients are given with explicit dependences on particle-level parameters. The linear stability analysis of their spatially-homogeneous solutions is presented. While the equations derived for the polar case (original Vicsek model with ferromagnetic alignment) and their solutions do not differ much from their 2D counterparts, the active nematics case exhibits remarkable differences: we find a true discontinuous transition to order with a bistability region, and cholesteric solutions whose stability we discuss.