Composition-dependent shape changes of self-propelled droplets in a phase-separating system

Chemical potential gradient in a multicomponent fluid system undergoing phase separation acts as a driving force for transporting a fluid. We have experimentally shown the self-propelled motion of the droplet undergoing phase separation and its shape changes using the aqueous two-phase system. The droplet behavior depended on the composition of the continuous phase. For higher concentrations of the continuous phase than the equilibrium concentration, a droplet moved unidirectionally even in a homogeneous concentration field with a constant radius of the droplet. However, for slightly lower concentrations, a droplet shrunk in proportion to time while moving. For much lower concentrations, the shape of the droplet periodically changed from a sphere to a bullet shape, and then to a parachute shape. The droplet has deformability like a biological cell. The self-propelled motion is due to the coupling between mass and momentum transfer via Korteweg force which arises from minimizing the free energy of the system. For investigating the composition-dependent shape changes, we carried out the visualization experiments of concentration distributions inside and outside the droplets.