A floating droplet

A drop or two of cold cream in hot coffee can go a long way toward improving one’s morning. But what if the two liquids didn’t mix?

MIT scientists have now explained why under certain conditions a droplet of liquid should not coalesce with the liquid surface below – using silicone oil sitting on a warm bath. If the droplet is very cold, and the bath sufficiently hot, then the droplet should “levitate” on the bath’s surface, as a result of the flows induced by the temperature difference.

The team’s results, published in the Journal of Fluid Mechanics, offer a detailed, mathematical understanding of drop coalescence, which can be observed in everday phenomena, from milk poured in coffee to raindrops skittering across puddles, and sprays created in surf zones.