When a droplet falls on a pool, we expect it to coalesce. There are exceptions, like bouncing droplets, but in general a droplet only sticks around for a split second before being engulfed. And yet, from morning coffee (top image) to walks in the woods, we frequently see millimeter-sized droplets sticking around for far longer than it seems like they should. New research offers a clue as to why: it’s thanks to a temperature difference.
When there’s an appreciable temperature difference between the drop and the pool, it causes rotating convective vortices (bottom image) in both the drop and the pool. When the temperature difference is large, the vortices are strong enough that their motion recirculates air inside the tiny gap between the drop and the pool. This supports the weight of the drop and keeps the two liquids separate. But the convection also redistributes heat, and eventually the drop and pool become similar enough in temperature that the circulation dies out, the air gap drains, and the two coalesce. (Image and research credit: M. Geri et al.; via MIT News; submitted by Antony B.)