Focus: Video—Liquid Drop Bursts into Thousands of Pieces

Physics 10, 19
A drop of water-alcohol mixture on a layer of oil was caught on video bursting into thousands of tiny droplets.
L. Keiser et al., Phys. Rev. Lett. (2017)
Droplet explosion. Three frames from the video (see below) show a puddle of a water-alcohol mixture shrinking as it emits a stream of droplets in all directions.

A new video of a drop of water-alcohol mixture exploding into tiny droplets on a layer of oil looks more like Hollywood digital effects than a typical physics experiment. Still, the physicists who created it say it demonstrates the combined effects of evaporation, surface tension, and fluid flow in an experiment that could provide a new way of producing many droplets in a short time. The technique could be useful for dispersing pollutants.

In the experiment by Etienne Reyssat of the Institute of Industrial Physics and Chemistry (ESPCI Paris) and his colleagues, the ratio of alcohol to water in the mixture determines the surface tensions at the mixture-air and mixture-oil interfaces. These two surface tensions in turn determine whether the mixture will spread out across the oil surface or bead up like water on a waxed car.

L. Keiser et al., Phys. Rev. Lett. (2017)
A drop of water-alcohol mixture on a layer of sunflower oil first spreads and then contracts as it emits droplets. Blue dye improves the fluid’s visibility. (Frame is about 10 cm across.)

At first, if the initial alcohol fraction is above 0.35, the mixture spreads, but after a few seconds of alcohol evaporation, the surface tensions increase, causing the spreading to reverse. The most significant evaporation occurs at the puddle’s shallow edges, so the surface tension at the edges becomes larger than at the center. This surface-tension difference leads to an outward fluid flow that results in fluid bursting out of the puddle and producing thousands of droplets. The team can adjust the droplet diameter from a few micrometers to a fraction of a millimeter by varying the alcohol-to-water ratio, and they can rapidly produce as many as ten million droplets.

This research is published in Physical Review Letters.

–David Ehrenstein

David Ehrenstein is the Focus Editor for Physics.

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Soft MatterFluid Dynamics

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