# Understanding dense active nematics from microscopic models

## TL;DR

This paper derives hydrodynamic equations for dense active nematics from microscopic models, revealing how defect dynamics and configurations depend on parameters, and providing a detailed theoretical understanding of their behavior.

## Contribution

It extends the Boltzmann-Ginzburg-Landau approach to dense active nematics, connecting microscopic models with continuum descriptions and analyzing defect dynamics.

## Key findings

- Qualitative agreement between particle and continuum models.
- Identification of parameter-dependent defect dynamics.
- Discovery of 'arch' solutions leading to defect-ordered states.

## Abstract

We study dry, dense active nematics at both particle and continuous levels. Specifically, extending the Boltzmann-Ginzburg-Landau approach, we derive well-behaved hydrodynamic equations from a Vicsek-style model with nematic alignment and pairwise repulsion. An extensive study of the phase diagram shows qualitative agreement between the two levels of description. We find in particular that the dynamics of topological defects strongly depends on parameters and can lead to ``arch'' solutions forming a globally polar, smectic arrangement of N\'eel walls. We show how these configurations are at the origin of the defect ordered states reported previously. This work offers a detailed understanding of the theoretical description of dense active nematics directly rooted in their microscopic dynamics.