# Analytical structure, dynamics, and coarse-graining of a kinetic model of an active fluid

## TL;DR

This paper develops a simplified yet accurate kinetic and Q-tensor model for active suspensions of extensile particles, capturing complex flow and defect dynamics with reduced computational effort.

## Contribution

It introduces the BQ-tensor theory, a novel coarse-grained model that efficiently describes active nematic suspensions and their boundary-driven defect dynamics.

## Key findings

- The BQ-tensor model accurately reproduces complex suspension dynamics.
- Novel defect production and absorption observed in strong nematic alignment regimes.
- Boundary effects induce a Fredericks-like transition in bi-concave domains.

## Abstract

We analyze one of the simplest active suspensions with complex dynamics: a suspension of immotile "Extensor" particles that exert active extensile dipolar stresses on the fluid in which they are immersed. This is relevant to several experimental systems, such as recently studied tripartite rods that create extensile flows by consuming a chemical fuel. We first describe the system through a Doi-Onsager kinetic theory based on microscopic modeling. This theory captures the active stresses produced by the particles that can drive hydrodynamic instabilities, as well as the steric interactions of rod-like particles that lead to nematic alignment. This active nematic system yields complex flows and disclination defect dynamics very similar to phenomenological Landau-deGennes Q-tensor theories for active nematic fluids, as well as by more complex Doi-Onsager theories for polar microtubule/motor-protein systems. We apply the quasi-equilibrium Bingham closure, used to study suspensions of passive microscopic rods, to develop a non-standard Q-tensor theory. We demonstrate through simulation that this "BQ-tensor" theory gives an excellent analytical and statistical accounting of the suspension's complex dynamics, at a far reduced computational cost. Finally, we apply the BQ-tensor model to study the dynamics of Extensor suspensions in circular and bi-concave domains. In circular domains, we reproduce previous results for systems with weak nematic alignment, but for strong alignment find novel dynamics with activity-controlled defect production and absorption at the boundaries of the domain. In bi-concave domains, a Fredericks-like transition occurs as the width of the neck connecting the two disks is varied.