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

Mahmoud Sebtosheikh,Ali Naji

doi:10.1038/s41598-021-02295-y

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.

Highlights

Our numerical inspections indicate that the repulsive humps in the force profiles in Fig. 8b arise as a result of yet another effect; namely, the transversally self-propelling phantom rods can get horizontally trapped in the wedge-shaped gaps between the inclusions, when the gap size is in the range σ < d < l (between the width of the rod and its length)
We introduce our model and methods in “Model and methods” and discuss our simulation results for the distribution of the active Brownian rods in “Results for spatial distribution of active rods”, followed by an analysis of the bath-mediated interaction force on the inclusions in “Results for effective interaction force”
Any net force on the inclusions that may arise at small intersurface separations can be interpreted as the effective interaction force mediated by the active bath between the two inclusions

Summary

Introduction

Our numerical inspections indicate that the repulsive humps in the force profiles in Fig. 8b arise as a result of yet another effect; namely, the transversally self-propelling phantom rods can get horizontally trapped in the wedge-shaped gaps between the inclusions, when the gap size is in the range σ < d < l (between the width of the rod and its length).

Results

Conclusion