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Drops driven by surface tension: self-propulsion and radial migration (youtu.be/LOauAXpUetc) Surface-tension gradients set liquid drops in motion. These two simulations show how the motion changes when the drops generate the gradients themselves or move in an imposed external field.

First, seven chemically active drops develop an asymmetric concentration field that drives Marangoni self-propulsion. The colours show the concentration in the surrounding liquid. Next, eight drops migrate towards the centre of an imposed radial surface-tension field.

Here, the colours show the fluid speed. Magenta lines mark the drop interfaces in both simulations. Simulated using Basilisk. Control parameters are shown before each case. The initial build-up in the self-propelling case is accelerated; the displayed time is the simulation time.