Marangoni Bursting; Evaporation-induced emulsification of drops

The Marangoni-effect; the generation of a flow at a fluid-fluid interface due to gradients in surface tension, is an interfacial-phenomenon that has seen rich research since it’s discovery, with the historical example being the generation of ‘tears of wine’ visible on the interior wall of a glass of wine. As alcohol has a lower surface-tension than water, a Marangoni-flow (flow from a region of low surface-tension to a region of high surface-tension) is generated from regions rich in alcohol to regions rich in water. Alcohol evaporates faster than water, therefore given a geometry whereby evaporation is more efficient in one area over another allows for Marangoni-flows to be established. For a glass of wine, evaporation is most efficient at the interior wall and least efficient in the centre of the glass, hence a radial flow outwards from the centre is generated, forcing the wine up the glass until break-up occurs due to instabilities brought on by gravity, and the wine falls back down.

The following video by researchers at PSL Research University and Université libre de Bruxelles describes a fascinating example of the Marangoni-effect as alcohol-water droplets ‘burst’ into a myriad of smaller droplets when placed onto a bath of oil, and show that changing the ratio of alcohol and water affects the bursting process.

A pure water droplet floats upon the oil – never wetting the surface, whereas a drop of alcohol will wet the oil, spread, and then evaporate shortly after. When a droplet composed of a mixture of both alcohol and water is deposited, the titular ‘Marangoni-Bursting’ occurs and satellite droplets are ejected radially, shrinking the centre of the drop until it has itself reached the order of size similar to its neighbours. Upon contact, the droplet is alcohol-rich and spreads across the surface of the oil. The alcohol begins to evaporate and does so most efficiently at the outer edge of the droplet, the resulting radially-outward Marangoni-flow from the centre creates a visible rim. The break-up of this rim sends satellite droplets flying outwards from the mother droplet.

In the supplemental work, researches formed analytical scaling laws for the maximum spread of the droplet and a time-scale for the experiment depending on the initial alcohol concentration and viscosity of the underlying bath. There is a strong dependence on the size of the satellite droplets with the concentration of the alcohol used in the droplet-mixture; the higher the alcohol concentration, the smaller the satellite droplets. Perhaps the most fascinating part of the experiment; the mechanisms by which the satellite droplets are formed and their motion away from the mother drop, are left as open questions.