Noncentral forces mediated between inclusions in a bath of active Brownian rods

TL;DR

This study uses simulations to explore how active Brownian rods with different propulsion directions mediate noncentral forces and torques between inclusions in a two-dimensional bath, revealing orientation-dependent interaction behaviors.

Contribution

It demonstrates how the direction of self-propulsion in active rods influences the range and symmetry of bath-mediated interactions and torques between inclusions.

Findings

Transverse self-propulsion enhances rod accumulation and interaction range.

Tilted propulsion axes induce asymmetric accumulation and noncentral forces.

Active rods mediate longer-range and asymmetric interactions depending on propulsion direction.

Abstract

Using Brownian Dynamics simulations, we study effective interactions mediated between two identical and impermeable disks (inclusions) immersed in a bath of identical, active (self-propelled), Brownian rods in two spatial dimensions, by assuming that the self-propulsion axis of the rods may generally deviate from their longitudinal axis. When the self-propulsion is transverse (perpendicular to the rod axis), the accumulation of active rods around the inclusions is significantly enhanced, causing a more expansive steric layering (ring formation) of the rods around the inclusions, as compared with the reference case of longitudinally self-propelling rods. As a result, the transversally self-propelling rods also mediate a significantly longer ranged effective interaction between the inclusions. The bath-mediated interaction arises due to the overlaps between the active-rod rings formed around the inclusions, as they are brought into small separations. When the self-propulsion axis is tilted relative to the rod axis, we find an asymmetric imbalance of active-rod accumulation around the inclusion dimer. This leads to a noncentral interaction, featuring an anti-parallel pair of transverse force components and, hence, a bath-mediated torque on the dimer.