Can You Blow a Doughnut-Shaped Soap Bubble?

The following is an excerpt from Why Are Orangutans Orange? 

Soap on a Hope

Is it possible to blow a toroidal soap bubble (one shaped like a ring doughnut)? And if it is, would it collapse immediately to a sphere? Could its life be prolonged by spinning its surface, as with smoke rings?

Peter Gardner,

Blawith, Cumbria, UK

A soap bubble is the minimum surface which encloses a given volume. If a toroidal bubble were created, it would not provide such a minimum surface and would therefore tend to contract to reduce its surface area until it collapsed into a bubble which would then burst because of the forces created at the disappearing hole in the torus. This situation differs from that in a solid torus such as a bicycle inner tube, because soap bubbles can transfer part of their surface from the inner to the outer part of the torus as they shrink.

A temporary toroidal bubble could perhaps be created by sticking spherical bubbles in a ring and collapsing their shared walls, but the inner ring would undoubtedly degenerate as the number of bubbles decreased.

Soap bubbles are different from smoke rings, which have no surface but are composed of solid particles suspended in air. These are stable because different parts of the body can rotate at different speeds without causing degeneration.

Jerry Humphreys

Bristol, UK

As a mathematician who studies soap bubbles, I knew that a toroidal soap bubble was, under normal circumstances, impossible. The only stable equilibrium shape for a soap bubble is the sphere that most people easily recognise – a torus bubble should not even exist in unstable equilibrium.

So when the famous performer Tom Noddy (known as the Bubble Guy from the US TV show Tonight) told me that he once blew a toroidal bubble, I didn’t actually believe him until he showed me the photographic proof (below). The bubble didn’t last long, but it did exist briefly. Visit www.tomnoddy.com to see some further interesting examples.

Torus bubbles do occur in unstable equilibrium in double soap bubbles: an outer bubble wrapped around another at the centre, as in the diagram below – a copy of a computer simulation created by John M. Sullivan, Professor of Mathematics at the University of Illinois. More of his images are online at http://torus.math.uiuc.edu/jms/images/.

Frank Morgan

Williams College Massachusetts, US

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Illustrated for the first time, with eighty full-color photographs showing the beauty, complexity, and mystery of the world around us, here is the next eagerly awaited volume of science questions and answers from Mick O’Hare and his team at New Scientist. From ripples in glass to “holograms” in ice, the natural world’s wonders are unraveled by the magazine’s knowledgeable readers. Six years since its debut, this magnificent series still rides high in the international bestseller lists, with well over two million copies sold. Popular science has never been more absorbing or more enjoyable.

Mick O’Hare is the production editor of New Scientist. O'Hare's collections of answers to burning science questions include Will We Ever Speak Dolphin?, Why Don't Penguins' Feet Freeze? and Does Anything Eat Wasps?

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(Top image credit: Flickr user Xtream_i)