Avoiding Coalescence

Droplets hitting a liquid surface don’t always coalesce. Above you can see a tiny droplet bounce and skate along the surface of a larger, vibrating drop. The smaller droplet doesn’t coalesce because a tiny layer of air sits between it and the vibrating drop. To actually contact and coalesce, the droplet has to sit still long enough for that air layer to get squeezed out. Instead, the vibration of the larger drop bounces it upwards, refreshing the air layer and scooting the droplet along until it falls off the vibrating drop. (Image credit: C. Kalelkar and S. Phansalkar, source)

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