Pattern Formation in Chemically Interacting Active Rotors with Self-Propulsion

TL;DR

This paper explores how chemically signaling active rotors, like bacteria near walls, form patterns through a nonlinear instability, with rotation speed influencing whether they cluster or form traveling waves, revealing new control mechanisms for active matter self-assembly.

Contribution

It introduces a novel mechanism where active rotations combined with chemotaxis lead to distinct pattern formation and phase behaviors in active matter.

Findings

Slow rotations cause transient uniform states followed by clustering and phase separation.

Faster rotations prevent phase separation, resulting in traveling wave patterns.

Rotation speed can be used to control self-assembly and coarsening in active systems.

Abstract

We demonstrate that active rotations in chemically signalling particles, such as autochemotactic {\it E. coli} close to walls, create a route for pattern formation based on a nonlinear yet deterministic instability mechanism. For slow rotations, we find a transient persistence of the uniform state, followed by a sudden formation of clusters contingent on locking of the average propulsion direction by chemotaxis. These clusters coarsen, which results in phase separation into a dense and a dilute region. Faster rotations arrest phase separation leading to a global travelling wave of rotors with synchronized roto-translational motion. Our results elucidate the physics resulting from the competition of two generic paradigms in active matter, chemotaxis and active rotations, and show that the latter provides a tool to design programmable self-assembly of active matter, for example to control coarsening.