  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.
