Active motility and wetting cooperatively regulate liquid-liquid phase separation

Abstract

Liquid--liquid phase separation in aqueous two-phase systems is fundamental across physical and biological sciences. While well understood for passive mixtures, how it is regulated by active agents such as motile bacteria remains largely unexplored. By combining experiments on Pseudomonas aeruginosa in a dextran--polyethylene glycol mixture with hydrodynamic simulations, we show that the coupling between bacterial activity and interfacial wetting converts self-propulsion into mechanically effective interfacial stresses, giving rise to a robust sequence of morphologies, including self-spinning droplets, elongated droplet chains, and branched capillary-like clusters. More importantly, it gives activity a dual kinetic role: activity suppresses coarsening in the droplet regime through rotation-induced hydrodynamic repulsion, but accelerates coarsening when dextran is the minority phase, where wetting-mediated attraction drives aggregation. To probe the biological relevance of this mechanism, we further show that a biofilm-associated protein can act as an interfacial ``wetting glue,'' promoting bacterial clustering even in dilute suspensions. Our findings establish activity--wetting coupling, rather than activity or wetting alone, as a mechanism by which active motility regulates pattern morphology and coarsening dynamics, and reveal a physical route to enhancing bacterial aggregation from dilute suspensions.