Ring of bright air
While I was watching beluga whales at Vancouver aquarium I noticed that one of them was blowing air rings. These appeared as annular bubbles that exhibited no obvious buoyancy and could be propelled to the bottom of the pool. Some of the whales would blow one horizontally then swim over to it and suck it back in. Have there been any studies of these intriguing air rings? How fast can these bubbles be projected, does this vary between animals and to what depth? And why do they produce this shape: does it have any beneficial purpose?
鈥� Unlike soap bubbles in air, air bubbles under water are maintained by the pressure of the surrounding water rather than tension. The beautiful bubble rings blown by dolphins are actually toroidal vortices that they create by sending a localised gush of water into an otherwise still region, which they then inject with air. This air moves to the centre of the vortex because that is where the pressure is lowest.
Stir water rapidly in a cylindrical glass with a smooth rod (to avoid too much turbulence) and you may see the centre of the vortex sharply depressed, where the pressure is lowest. If you stir faster, bubbles may detach and move down the core of the vortex, illustrating the behaviour mentioned above.
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Alex Vallat of Cambridge, UK, tells us that blowing air rings is not difficult to do. Puff your cheeks out with your lips pursed. Then, with your throat closed, make a P sound with the lips and use the stored air to blow out quickly. The tube that forms the bubble rotates around its core, like a smoke bubble, but the ring itself does not rotate around its centre like a steering wheel.
Steve Gisselbrecht
Boston, Massachusetts, US
鈥� A ring-shaped bubble is an example of a vortex ring, which is similar to a columnar or tornado-like vortex, but bent round into a circle. Other examples include smoke rings. They are formed when a flow through a circular opening is forced back on itself.
There is also a second class of bubble ring. These consist of water vapour in water, and form when a vortex ring generates low enough pressure to cause vapour to form around the ring of the vortex, giving it a similar appearance to an air ring underwater, though filled with vapour. This type of 鈥渃avitating鈥� vortex is used by some under water weapons systems.
Steve Backshall of Wooburn Green, Buckinghamshire, UK, has a different approach. Try sitting on the bottom in relatively still water, he says. Then rock backwards so your mouth is facing upwards, place your tongue firmly on your upper lip, then forcefully expel air before briefly sucking back in and closing your lips. Accomplished ring blowers can create mesmerising, expanding doughnuts of shimmering air that gyrate towards the surface. The photo shows biologist Ron Liedich鈥檚 efforts under water at Palau in the Pacific Ocean.
David Hambling
London, UK
鈥� Cavitation vortices can be seen spinning off the tip of ships鈥� propellers, and they form when the liquid flows so rapidly that its pressure drops and it turns into vapour 鈥� in essence the water boils under water.
There is a another, even more exotic way, to create such bubbles. Reader David Williamson of London points out it is possible to create 鈥渁ntibubbles鈥�. An antibubble is a bubble in reverse. Just as the soap bubbles that children blow are a thin skin of liquid in air, an antibubble consists of a thin skin of air with water inside and out.
Their properties have been explored by S. Dorbolo, H. Caps and N. Vandewalle of the Institute of Physics at the University of Li猫ge in Belgium, and described in a paper published in 2003 in New Journal of Physics (DOI: 10.1088/1367-2630/5/1/161). See also New 杏吧原创, 10 January, p 17. You can find out how to make antibubbles at . 鈥� Ed
Dolphins blow toroidal rings for fun and then play with them under water. Reader Alistair Eberst of the University of Abertay Dundee in Scotland drew our attention to where you can see pictures similar to the one above and much more.
This week鈥檚 question
Stained plastic window
In my mother鈥檚 kitchen, one pane of the window was replaced by perspex so that an extractor fan could be installed. Fifteen years later the fan still works but the perspex 鈥� as clear as the surrounding glass when it was cut 鈥� is now an opaque yellowy orange. Why did it change clarity and colour?
Rick Eraho
Cleckheaton, West Yorkshire, UK