Active motility and wetting cooperatively regulate liquid-liquid phase separation

Abstract: Liquid-liquid phase separation of aqueous two-phase system (ATPS) is fundamental across physical and biological sciences. While well understood for passive systems, how this process is regulated by active agents such as motile bacteria remains largely unexplored. By combining experiments on Pseudomonas aeruginosa in a prototypical dextran-polyethylene glycol ATPS with hydrodynamic simulations, we show that the interplay between bacterial activity and interfacial wetting gives rise to a robust sequence of nonequilibrium morphologies, including self-spinning droplets, elongated droplet chains, branched capillary-like clusters, and highly deformed droplets. We find that activity plays a dual role in coarsening kinetics: it suppresses coarsening through hydrodynamically driven, activity-induced droplet rotation, yet accelerates it when dextran is the minority phase, where wetting-mediated attraction governs bacterial aggregation. These findings reveal a generic physical mechanism through which motility and wetting cooperatively control phase-separation dynamics, offering new physical insight into activity-regulated LLPS and suggesting strategies for engineering ATPS morphology using active agents.