Brownian Dynamics Simulations of Inclusions in an Active Fluid Bath

TL;DR

This study uses two-dimensional Brownian dynamics simulations to explore how rigid inclusions behave in an active fluid bath, revealing enhanced diffusion and rotation influenced by active particle properties and fluid phase transitions.

Contribution

It introduces a detailed simulation framework for inclusions in active fluids, highlighting how activity and fluid order affect inclusion transport and orientation.

Findings

Inclusion diffusion is significantly enhanced by active fluid forces.

Chiral inclusions exhibit constant rotational drift in active baths.

Transport properties are modulated by active particle swimming speed.

Abstract

We carry out two-dimensional Brownian dynamics simulations of the behavior of rigid inclusion particles immersed in an active fluid bath. The active fluid is modeled as a collection of self-propelled circular disks interacting via a soft repulsive potential and a nematic alignment interaction. The fluid is characterized by its nematic order, polar order and orientational correlation length. The active fluid bath transitions from the isotropic to the nematic phase with increasing number density, increasing nematic interaction strength or increasing P\'eclet number. The inclusion particles are modeled as rigid assemblies of passive circular disks. Four types of inclusions are considered: a rod-like $I$ shape, a boomerang-like $L$ shape, and stair-like shapes $Z$ and $Z^*$, with opposite handedness. When inclusions are introduced into the active fluid bath, their diffusion is significantly enhanced by the force and torque exerted by the active fluid particles and the chiral inclusion particles exhibit constant rotational drift. These diffusion and rotation enhancements increase as the swimming speed of the active fluid particles increases. The translational motion of the inclusion particles also couples with their orientational motion, and the correlation is modulated by the active fluid particles' swimming speed. This work paves the way for future simulations of inclusions in active fluid baths and suggests potential avenues for controlling transport properties in active materials.