Paper detail

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

Liquid-liquid phase separation is important across biology, physics, and materials science. Although usually studied at equilibrium, active components - such as motor proteins, enzymes, and synthetic microswimmers - are increasingly recognized as key players in reshaping phase separation dynamics. Here, we encapsulate motile bacteria inside phase-separated aqueous droplets to investigate how internal activity alters interfacial behavior. By varying bacterial density, we control the active stress at the droplet interface. At low activity, we observe scale-dependent interfacial fluctuations that propagate as waves. In this low Reynolds number regime, these waves arise from an effective inertial response, generated when active bacterial stresses balance passive viscous damping of the interface. At higher activity, droplets deform strongly - exceeding the Plateau-Rayleigh instability threshold - and even form cell-sized filaments - a morphology without a passive counterpart. Enhanced droplet motility and accelerated coarsening accompany these shape changes. Our work shows how active stresses can reshape the morphology and dynamics of multiphase systems, offering new insight into the physics of internally driven phase-separated fluids.