Interfacial bacteria-driven rotation of free-floating active droplets

Active droplets provide a platform for converting microscopic propulsion into macroscopic flow. Previous realizations of bacteria-driven droplet rotation typically rely on nearby solid boundaries, external fields, or dense active nematics. Here, we report autonomous rotation of droplets in a three-dimensional aqueous two-phase system driven by interfacial population of \textit{Pseudomonas aeruginosa}. Combining confocal microscopy with fluid particle dynamics simulations, we show that amphiphilic bacteria accumulate at the dextran--polyethylene glycol interface and preferentially move and orient along the local tangent. Interfacial localization partitions the force-free swimming dipole between the two liquid phases, allowing either phase to experience a finite active torque while the combined system remains torque balanced. The simulated interfacial population is more spatially heterogeneous than in the experiments, emphasizing that both the bacterial distribution and its signed orientational organization can influence the instantaneous torque. Over the accessible size range, the macroscopic angular velocity exhibits an inverse-radius ($R^{-1}$) dependence. We also formulate a theoretical model to rationalize this scaling law, revealing how the macroscopic rotation is governed by collective interfacial swimming and surface coverage. These results identify a fluid-interface-mediated route for converting swimmer activity into macroscopic rotational flow and establish a strategy for harnessing interfacial active matter to regulate droplet dynamics.