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When two separate masses of the same fluid are brought gently into contact, they are expected to fully merge into a single larger mass to minimize surface energy. However, when a stationary drop coalesces with an underlying reservoir of identical fluid, merging does not always proceed to completion1. Occasionally, a drop in the process of merging apparently defies surface tension by ‘pinching off’ before total coalescence occurs, leaving behind a smaller daughter droplet1, 2, 3. Moreover, this process can repeat itself for subsequent generations of daughter droplets, resulting in a cascade of self-similar events2. Such partial coalescence behaviour has implications for the dynamics of a variety of systems, including the droplets in clouds4, ocean mist and airborne salt particles5, emulsions6, and the generation of vortices near an interface7, 8. Although it was first observed almost half a century ago, little is known about its precise mechanism. Here, we combine high-speed video imaging with numerical simulations to determine the conditions under which partial coalescence occurs, and to reveal a dynamic pinch-off mechanism. This mechanism is critically dependent on the ability of capillary waves to vertically stretch the drop by focusing energy on its summit.

Partial coalescence of drops at liquid interfaces