Active motility and wetting cooperatively regulate liquid-liquid phase separation

Abstract

Liquid--liquid phase separation in aqueous two-phase systems is widespread in physical and biological settings, yet how motile agents regulate its morphology and kinetics remains poorly understood. Combining experiments on \textit{Pseudomonas aeruginosa} in a dextran--polyethylene glycol mixture with hydrodynamic simulations, we show that interfacial wetting couples bacterial motility to phase-separation dynamics. At high dextran fraction, bacteria-driven rotational flows shear inter-droplet gaps and hinder liquid-bridge formation, thereby suppressing coalescence and coarsening. As the dextran fraction decreases, wetting-driven bridging becomes increasingly important, producing an intermediate chain-like state and, ultimately, branched bacterial clusters in which activity instead accelerates capture and aggregation. A biofilm-associated protein provides an independent wetting component that similarly promotes bacterial clustering. These results reveal a composition-controlled crossover between activity-induced suppression of coalescence and wetting-dominated bridging.