Contact Forces in Motility-Regulated Active Matter

TL;DR

This paper investigates how short-range repulsive forces influence the collective behaviors of active particles undergoing motility regulation, revealing their role in phase transitions and density regulation in dense biological assemblies.

Contribution

It demonstrates that short-range forces can significantly alter phase behavior in motility-regulated active matter, a novel insight into dense biological systems.

Findings

Repulsive forces oppose condensate formation during chemotactic collapse.

Short-range forces can increase liquid density in phase-separated systems.

Repulsive interactions stabilize coexistence of finite-density phases.

Abstract

Long-range interactions are ubiquitous in nature, where they are mediated by diffusive fields at the cellular scale or by visual cues for groups of animals. Short-range forces, which are paradigmatic in physics, can thus often be neglected when modeling the collective behaviors of biological systems induced by mediated interactions. However, when self-organization leads to the emergence of dense phases, we show that excluded-volume interactions play an important and versatile role. We consider assemblies of active particles that undergo either condensation or phase-separation due to motility regulation and show that short-range repulsive forces can induce opposite effects. When motility regulation triggers an absorbing phase transition, such as a chemotactic collapse, repulsive forces opposes the formation of condensates and stabilize the coexistence between finite-density phases. In contrast, when motility regulation induces liquid-gas coexistence, repulsive forces can, counterintuitively, lead to a significant increase in the liquid density.