The shaky life of a water drop in an anise oil-rich environment*

We explore the fate of a water droplet (d∼ 4 mm) placed in a host liquid composed of anise oil (anethole) and a small fraction of ethanol. Water is more dense than this mix, so the droplet sinks to the bottom. There, instead of dissolving by diffusion into the bulk, it starts to grow by incorporating ethanol. After a time t∼10 s from the deposition moment, the drop starts to oscillate in a manner that reminds of Leidenfrost drops, but without depinning from the bottom. Later, it becomes apparent that a mircoemulsion of anethole is also developing inside the droplet. As it keeps growing -whilst reducing its density- it deforms due to buoyancy until at t∼100 s part of the droplet detaches and floats away. The water cap left behind repeats the cycle, now at a slower rate. We discuss the shape evolution, growth rate and oscillation frequency of the droplet as well as the flow fields that develop both inside the drop and on the liquid bulk.