Self-propelled rods exhibit a novel phase-separated state characterized by the presence of active stresses and the ejection of polar clusters

TL;DR

This paper uncovers a new phase-separated state in self-propelled rods characterized by active stresses, dynamic aggregates, and polar cluster ejection, revealing complex coupling between density, stresses, and order.

Contribution

It introduces a novel phase-separated state in self-propelled rods, highlighting active stresses and mass ejection phenomena not previously observed.

Findings

Formation of dynamic high-density aggregates surrounded by gas.

Active stresses cause continuous ejection of polar clusters.

Absence of global orientational order in the thermodynamic limit.

Abstract

We study collections of self-propelled rods (SPR) moving in two dimensions for packing fractions less than or equal to 0.3. We find that in the thermodynamical limit the SPR undergo a phase transition between a disordered gas and a novel phase-separated system state. Interestingly, (global) orientational order patterns -- contrary to what has been suggested -- vanish in this limit. In the found novel state, the SPR self-organize into a highly dynamical, high-density, compact region - which we call aggregate - which is surrounded by a disordered gas. Active stresses build inside aggregates as result of the combined effect of local orientational order and active forces. This leads to the most distinctive feature of these aggregates: constant ejection of polar clusters of SPR. This novel phase-separated state represents a novel state of matter characterized by large fluctuations in volume and shape, related to mass ejection, and exhibits positional as well as orientational local order. SPR systems display new physics unseen in other active matter systems due to the coupling between density, active stresses, and orientational order (such coupling cannot be reduced simply to a coupling between speed and density).