The propagation of active-passive interfaces in bacterial swarms

TL;DR

This paper investigates how active-passive interfaces in bacterial swarms evolve, revealing that their propagation is governed by flow-dependent mechanical properties and coupling to local curvature, extending classical interface theories to active matter.

Contribution

It introduces a model for active-passive interface dynamics in bacterial swarms, demonstrating flow-dependent boundary behavior and proposing an active analogue to classical interface conditions.

Findings

Interfaces behave as diffuse zones with flow-dependent properties

Interfacial velocity couples to swarm speed and curvature

Active interface dynamics extend classical theories to active matter

Abstract

Propagating interfaces are ubiquitous in nature, underlying instabilities and pattern formation in biology and material science. Physical principles governing interface growth are well understood in passive settings; however, our understanding of interfaces in active systems is still in its infancy. Here, we study the evolution of an active-passive interface using a model active matter system, bacterial swarms. We use ultra-violet light exposure to create compact domains of passive bacteria within Serratia marcescens swarms, thereby creating interfaces separating motile and immotile cells. Post-exposure, the boundary re-shapes and erodes due to self-emergent collective flows. We demonstrate that the active-passive boundary acts as a diffuse interface with mechanical properties set by the flow. Intriguingly, interfacial velocity couples to local swarm speed and interface curvature, suggesting that an active analogue to classic Gibbs-Thomson-Stefan conditions controls boundary propagation. Our results generalize interface theories to mixing and segregation in active systems with collective flows.