How freezing a soap bubble turns it into a ‘snow globe’

Differences in surface tension create currents in soapy films that move ice crystals around

soap bubble

THIN ICE  A patchwork of ice crystals swirls across the surface of a soap bubble as it freezes. The glittery display is caused by a mismatch in surface tension in different regions of the soapy film.

F. Ahmadi, C. Kingett and J. Boreyko

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Frigid air can transform an ordinary soap bubble into a glittery “snow globe.” No shaking required.

When a bubble is placed in a freezer set to –20° Celsius, delicate ice crystals swirl gracefully across the soapy film, gradually growing larger until the bubble freezes solid. The phenomenon can also be observed when blowing soap bubbles outside in wintry weather. Although the process is captured in a number of YouTube videos (see examples here, here and here), it lacked a scientific explanation. Researchers from Virginia Tech in Blacksburg explained the physics behind the miniature snowstorm on November 20, during the American Physical Society Division of Fluid Dynamics meeting in Denver.

As ice crystals form at a bubble’s base, the change from liquid to solid releases heat. This “latent heat,” the heat that is absorbed or released in a phase transition, is trapped in the bottom of the bubble because the orb’s skin is too thin — about a hundredth of a millimeter — for heat to easily conduct.

As a result, the remaining liquid at the bottom of the bubble is warmer, and so has lower surface tension, than liquid at the top. That mismatch sets up currents in the fluid, causing the soapy water to stream from warmer to colder regions, a process known as Marangoni flow. The flow is so strong, says coauthor and engineer Jonathan Boreyko, “it’s ripping off the ice crystals that are growing from the bottom and taking them along for the ride.” Eventually, the ice crystals grow large enough that they lock into place, and the whole bubble freezes.

SHELL FREEZES OVER Watch how differences in temperature across a freezing soap bubble’s surface create currents that send ice crystals swirling.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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