© Etienne REYSSAT / PMMH / CNRS Images



Eclatement d’une goutte d’eau colorée et d’alcool sur un bain d’huile

A drop of coloured water and alcohol poured onto an oil bath, bursting into a myriad of droplets. The addition of alcohol to the drop forms an emulsion, i.e. a suspension of droplets in another liquid. On the periphery of the drop, where the edges are thinner than the domed centre, the alcohol is exhausted first by evaporation, causing a surface energy gradient. A centrifugal flow, facilitated by the oil bath, then drives the mixture at the centre of the drop, which has a greater concentration of alcohol, towards the outside. This liquid accumulates at the periphery and forms a bead which ultimately becomes unstable, continuously releasing myriads of droplets. A better understanding of this process could be applied to the treatment of polluted water. Indeed, some substances are easier to clean when they are dispersed. Conversely, keeping materials cohesive after evaporation of solvent would help to stabilise films of complex solutions.

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