Active phase separation by turning toward regions of higher density

TL;DR

This paper introduces a novel active phase separation mechanism driven by orientational torques in self-propelled particles, supported by theory and experiments with Janus colloids, revealing fluid clusters with high particle turnover.

Contribution

It uncovers a new class of active phase separation caused by orientational interactions, expanding understanding beyond alignment and repulsion-driven behaviors.

Findings

Particles reorient toward high-density regions due to non-reciprocal torques.

Clusters are fluid with rapid particle exchange, unlike jammed self-trapped states.

Interfaces are wide and span entire clusters, lacking internal orientational order.

Abstract

Studies of active matter, from molecular assemblies to animal groups, have revealed two broad classes of behavior: a tendency to align yields orientational order and collective motion, whereas particle repulsion leads to self-trapping and motility-induced phase separation. Here, we report a third class of behavior: orientational interactions that produce active phase separation. Combining theory and experiments on self-propelled Janus colloids, we show that stronger repulsion on the rear than on the front of these particles produces non-reciprocal torques that reorient particle motion toward high-density regions. Particles thus self-propel toward crowded areas, which leads to phase separation. Clusters remain fluid and exhibit fast particle turnover, in contrast to the jammed clusters that typically arise from self-trapping, and interfaces are sufficiently wide that they span entire clusters. Overall, our work identifies a torque-based mechanism for phase separation in active fluids, and our theory predicts that these orientational interactions yield coexisting phases that lack internal orientational order.