Scientists hit on universal theory of bubbles
Thursday 10 June 2010
Bubbles don't just disappear when they pop but deflate in a rapid cascade of ever-smaller "daughter" bubbles, scientists reported on Wednesday.
The physics behind this bursting effect seems to hold true whether the liquid is as thin as water or as thick as heavy oil, suggesting that the researchers have found a universal theory of how bubbles behave when they break.
A host of practical applications could follow in areas ranging from health care to climate to glass manufacturing, according to the study published in the British journal Nature.
It may also prove valuable for controlling industrial processes in which bubble formation can be detrimental.
There was an element of serendipity in the discovery, according to lead researcher James Bird, a graduate student at the Harvard School of Engineering and Applied Sciences.
Bird and one of the study's co-authors were working late one night investigating ways to spread bubbles on different surfaces when they noticed the rings that form when one bursts.
"After that, any time I was just walking around during a rainy day I'd look at the bubbles popping on puddles," Bird said.
"When I went swimming in the ocean I'd watch the bubbles on the surface... And I soon realised that it was everywhere."
In order to minimise surface area, he explained, a bubble forms an almost perfect hemisphere when it is in contact with a solid or liquid surface. When it pops, a two step process unfolds, creating a ring of smaller bubbles.
Until now, the exactly how that happened was not understood.
In the first step, the forces acting on the bubble cause the film to fold in on itself as it retracts, trapping a pocket of air in the shape of a donut.
In the second step, surface tension breaks this donut - called a torus - of air into a ring of smaller bubbles in the same way that surface tension transforms a thin stream of water flowing from a faucet into individual droplets, he explained.
The cascade effect is very short-lived - too short to be seen with the naked eye.
The researchers filmed the collapse with high-speed cameras, and then used the video to construct a mathematical model to test and replicate their experimental hypothesis.
Bird said in a statement that he was anxious to study similar popping effects in more exotic materials such as molten glass, lava and mud.
"What I love about this study is that the overall effect can be seen by anyone in their kitchen," he said.
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