Fragmentation instability of a liquid drop falling inside a heavier miscible fluid

We report a new type of drop instability, where the density difference between the drop and the solvent is negative. We show that the drop falls inside the solvent down to a minimum height, then fragmentation takes place and secondary droplets rise up to the surface. We have developed a theoretical model that captures the essential of the phenomenon and predicts the correct scalings for the rise-up time and the minimum height.