Floating crystals

How a soap bubble freezes

Ice crystals on a freezing soap bubble Farzad Ahmadi and Christian Kingett
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Soap bubbles fascinate with their delicate beauty and iridescent sheen. This snapshot, however, captures such a bubble in an unusual situation - as it freezes. In frosty surroundings, fine crystals form on the thin skin of the soap bubble, which gradually merge with each other.

When water droplets or puddles freeze, this usually happens from a crystallization nucleus: starting from an interface or tiny particles in the water, ice crystals are formed, to which gradually more and more water molecules are deposited. But how is this with soap bubbles? Can these fragile structures ever freeze as a whole? The physics behind freezing bubbles has recently been scrutinized by US researchers.

It showed that cooling a bubble at room temperature by depositing it on a frosty surface does not leave the bubble intact. First of all, a front of ice crystals migrates from below down the bladder wall. But then the whole thing stops and the still liquid part bursts.

But this is different if you bring the soap bubble completely into a climate chamber with freezing temperatures. Then you can watch a fascinating spectacle - similar to the flurry of a "snow globe". Tiny ice crystals seem to float on the shimmering surface of the bubble. They grow up and eventually combine to form a closed, gossamer ice crust.

This photograph shows a ten-microliter small bubble in the early stages of this freezing process. The small crystals are thereby moved by the so-called Marangoni current - a flow of places of low surface tension to places of high tension. This flow distributes the crystals on the thin bubble skin and at the same time prevents the bursting of the bubble, as the physicists discovered. (Nature Communications, 2019; doi: 10.1038 / s41467-019-10021-6)

Source: Nature

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