Collective bacterial motion drives interfacial waves and shape dynamics in phase-separated droplets

Abstract: Liquid--liquid phase separation governs a broad spectrum of phenomena in biology, physics, and materials science. While traditionally explored in equilibrium contexts, growing evidence underscores the transformative role of active components, such as motor proteins, enzymatic catalysts, and synthetic microswimmers, in modulating the dynamics of phase-separated systems. Here, we encapsulate motile bacteria within phase-separated aqueous droplets to examine how internal activity shapes interfacial behavior. By tuning bacterial density, we control the magnitude of active stresses at the droplet interface. At low activity, we observe scale-dependent interfacial fluctuations featuring propagative wave modes.Remarkably, despite the low Reynolds number regime, these waves arise from an effective inertial response that emerges when bacterial active stresses balance viscous dissipation. At high activity, droplets exhibit pronounced deformation that exceeds the Rayleigh-Plateau instability threshold, accompanied by enhanced motility that accelerates coarsening. Our findings demonstrate how active stresses can reshape morphology and dynamics in multiphase systems, offering new insights into the physics of internally driven phase-separated fluids.