Collective motion of active particles exhibiting non-reciprocal orientational interactions

TL;DR

This study uses Brownian dynamics simulations to explore how non-reciprocal orientational interactions influence collective motion in active particles, revealing conditions under which flocking behavior emerges despite disruptive steric interactions.

Contribution

It demonstrates that non-reciprocal orientational interactions can induce flocking in active particles, even when steric interactions tend to disrupt clustering, highlighting new mechanisms of collective behavior.

Findings

Flocking occurs when non-reciprocal interaction range is similar to steric interaction range.

Flock structures include dense bands and dynamic stripes.

Flocking order is weakly dependent on flock structure.

Abstract

We present a Brownian dynamics study of a 2d bath of active particles interacting among each other through usual steric interactions and, additionally, via non-reciprocal avoidant orientational interactions. We motivate them by the fact that the two flagella of the alga Chlamydomonas interact sterically with nearby surfaces such that a torque acts on the alga. As expected, in most cases such interactions disrupt the motility-induced particle clustering in active baths. Surprisingly, however, we find that the active particles can self-organize into collectively moving flocks if the range of non-reciprocal interactions is close to that of steric interactions. We observe that the flocking motion can manifest itself through a variety of structural forms, spanning from single dense bands to multiple moderately-dense stripes, which are highly dynamic. The flocking order parameter is found to be only weakly dependent on the underlying flock structure. Together with the variance of the local-density distribution, one can clearly group the flocking motion into the two separate band and dynamic-stripes states.