Physics: How soap bubbles freeze

Insights into how soap bubbles freeze and what causes the so-called ‘snow-globe effect’ that can be produced are revealed in a Nature Communications paper this week.

As soap bubbles freeze under certain conditions, a number of growing ice crystals can be seen to swirl around in an effect visually reminiscent of a snow globe. However, the underlying physics of this phenomenon and how soap bubbles freeze has not been studied.

Jonathan Boreyko and colleagues investigated the heat transfer processes that govern the dynamics of freezing soap bubbles. The authors placed bubbles on a chilled, icy surface under different ambient temperatures and filmed the freezing process. They observed that two different freezing mechanisms take place depending on the temperature.

When the surroundings were at the same temperature as the bubble, freezing started from the bottom of the bubble and produced a Marangoni flow (a liquid flow from areas of low surface tension to areas of high surface tension) causing ice crystals to detach from the freezing front and swirl around the bubble like flakes in a snow globe. The bubble then completely froze as the crystals grew and interlocked together. However, when the surroundings were at room temperature, the freeze front slowly propagated upwards and eventually stopped midway up the bubble, owing to poor conduction. The partially frozen bubble then remained in equilibrium before deflating, and the liquid dome collapsed.

The authors suggest the findings will help contribute to a better understanding of heat transfer phenomena.

doi:10.1038/s41467-019-10021-6

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