Self-propulsion of all-aqueous droplets induced by liquid-liquid phase separation

Current approaches for driving droplet self-propulsion utilizes volatile components, limiting their applications in biocompatible systems. Aqueous two-phase systems, comprising only of water with polymers in solution, offer the unique advantage of high biocompatibility. In these all-aqueous droplets, liquid-liquid phase separation can be easily achieved through the passive evaporation of water on a solid substrate. During phase separation, polymer-rich condensates form within the droplet that coarsen over time. In this work, we report a novel phenomenon where all-aqueous droplets undergoing liquid-liquid phase separation through evaporation on a solid substrate exhibits self-propulsion behaviour. The phase-separating all-aqueous multicomponent droplets exhibits pronounced movement, characterized by a distinctive increase in velocity and centimetre-scale displacements, while non-phase separating droplets remained stationary on the substrate. We demonstrate that phase separation and condensate formation within droplets could induce spatial symmetry breaking, driving the spontaneous motion of droplets.