Active motility and wetting cooperatively regulate liquid-liquid phase separation

TL;DR

This study demonstrates how bacterial motility and wetting interactions collaboratively influence the morphology and dynamics of liquid-liquid phase separation in active biological systems.

Contribution

It reveals a new physical mechanism where activity and wetting jointly regulate phase separation, combining experimental and simulation insights.

Findings

Active bacteria induce diverse nonequilibrium droplet morphologies.

Bacterial activity can both suppress and accelerate coarsening.

Wetting interactions are crucial for bacterial aggregation and phase dynamics.

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.