Active hole generation in a liquid droplet dissolving into a binary solvent.

In liquid-liquid dissolution, the critical point of phase separation is determined by the temperature. When the solvent consists of multi-components, in contrast, the mole fractions in the solvent also take on the role of control parameter. In this study an ionic liquid dissolves into a binary solvent composed of ethanol and water. It is found in this system that, near the critical point, a hole is spontaneously created in the droplet of the ionic liquid. The creation of the hole is initiated by a mutual interaction between the concentrations of the ionic liquid and the binary solvent via their affinity. A spatial inhomogeneity of the interfacial tension is induced through an amplification of fluctuation in the concentration due to an instability mechanism, and causes the Marangoni effect to create the hole. The hole moves inside the droplet and consequently leads to the motion of the droplet. The present system provides not only a new type of dissolution process but also a peculiar example of active matter realized in a liquid droplet.