Forever blowing bubbles? It's not just children who enjoy froth. Chris Holt explains the physics of foam
Violent movements and vivid colours; colour spills into twisted and convoluted shapes which appear computer-generated. In fact, these are photographs of real-life soap bubbles. In the abstract world of dish detergent, says photographer Tim Durham, he is freed from the mental labellling of a subject and can concentrate on design elements of colour, tone and shape.
Tim first attempted to photograph soap bubbles 10 years ago, with results he describes as both fantastic and disastrous. It was another five years before he was able to produce consistently good results. He works in a darkened room with a soap film hanging in a black steel frame. He adds sugar or glycerine to increase the viscosity of the film and so improve the stability. He often works in a room at a different temperature to the film in order to generate convection swirls. The thin wall of a soap film causes interference between the light reflected from the inner and outer surfaces; this produces the lovely rainbow colours, which therefore depend on the thickness of the film.
The horizontal bands of colour sometimes seen are the result of the film draining so that it is thicker at the bottom than the top. Bubbles and films become thinner with time, because of drainage and evaporation, and this is why they are so short-lived.
You can often see a bubble go black just before it pops. When the bubble wall is very thin, much thinner than the wavelength of light, then the two reflected beams are almost superimposed. But when light is reflected from an air-water interface, the direction of vibration is reversed, whereas when light is reflected from a water-air surface the direction is not reversed, so the two reflected beams cancel each other out to give a black bubble.
An antibubble is the exact opposite of a bubble. An antibubble is a thin shell of air with liquid both inside the shell and outside it. Antibubbles can form when drops of soapy liquid fall into a bowl of liquid. A thin film of air is pulled down into the liquid and forms a layer around the drops.
Since antibubbles contain a small amount of air spread over a large volume they rise to the surface only very slowly, whereas true bubbles rise to the surface fairly quickly. If the inner liquid of the antibubble is significantly denser than the surrounding liquid then the antibubble will actually sink.
Physicists at the University of Li ge in Belgium recently succeeded in creating antibubbles in beer. This was possible because beer contains a protein that acts at the surface in the same way as soap. Physicist Dr Dorbolo says: "We tried to create them in beer for fun and we didn't think it would be possible but were amazed when we managed to create giant antibubbles which lasted for almost two minutes and that moved around a glass of beer before bursting."
When a lot of bubbles join together to make a foam, the resulting bulk material behaves in an unusual manner. Despite consisting mostly of air with a little fluid, a foam behaves like a springy solid (think of chocolate mousse). Foams have many practical uses. They are used to fight fires in oil because they float on the surface unlike water which sinks to the bottom and has no effect on the flames. Foams with oxidising properties were used to decontaminate the mail rooms in Washington DC after the anthrax scare, because foams expand and fill all crevices and small spaces.
Foams are also fashionable in food and drink: cappuccino, souffles and frothy fruit confections.
Physicists who study bubbles seem to take a close interest in beer. A German physicist, Arnd Leike, studied how beer foams decay and won a prize for his efforts. Dr Leike studied three different types of beer including one called Erdinger Weissbier, which he says is his favourite. He filled mugs with beer and measured the height of the foam at intervals as the foam decayed. The foams of the three beers decayed at different rates but in all cases the decay was exponential. In exponential decay the rate is proportional to the current size, so the decay is fast when the foam head is large but will progressively slow down as the size of the head is reduced. For his efforts Dr Leike was awarded an Ig Nobel prize. Every year, one week before the Nobel prizes are announced, Harvard University awards the Ig Nobel prize for results that "cannot or should not be repeated". These prizes have been awarded since 1990 by a journal called Annals of Improbable Research. However, the prize doesn't seem to cause offence and some scientists have even been known to lobby to get one.
Resources Don Pettit's water films: science.nasa.govheadlinesy200325feb_nosoap.
htm David Stein is listed in the Guinness Book of Records for creating the longest bubble. See www.bubbles.org The Ultimate Bubble Book by Shar Levene and Leslie Johnstone, Sterling Juvenile Books pound;6.99 Later this year:
"Soap Opera", an exhibition of Tim Durham's soap film photos will be on display during November and December at the Institute of Physics, 76 Portland Place, London W1B 1NT Dr Chris Holt is a freelance science writer Email: firstname.lastname@example.org
Art and design
Add some washing-up liquid to water-based paint and then blow a big mound of bubbles with a straw. Press a sheet of paper onto the bubble pile and lift off and allow to dry. For a more complicated picture, repeat with a different colour. Or fold in half and then open out for a "butterfly".
Try 3 parts of water, 2 parts of glycerine and 1 part of washing-up liquid. This solution should produce large, stable bubbles. Now make up enough solution to fill a paddling pool, then dip a hula hoop in to see if you can produce really large bubbles.
Pour some beer into a glass; measure the height of the foam at intervals for five minutes. Use a projector to enlarge a shadow of the beer and make measuring easier. Plot the height of the foam against time; try fitting an exponential equation. Lagers work better than ales because they contain more dissolved carbon dioxide. Lemonade contains a lot of gas but doesn't foam much because it doesn't contain much surfactant.
Make some antibubbles. Fill a squeezy bottle with soapy water, then squirt this into a bowl of soapy water at an angle of about 45 degrees. You have to squirt with enough force so that the stream breaks through the surface of the water and carries an air film with it. If you add some colour to the squeezy bottle liquid then the antibubbles will be coloured, thus demonstrating that they do indeed contain a sphere of water. How can you measure the thickness of the air layer?
When Don Pettit was on the International Space Station in zero gravity, he made a 5 cm loop of wire and inserted it into a beaker of water. Despite the fact that there was no surfactant in the water, when he removed the loop a thin film of water was stretched across it. The film was very stable, it could be shaken and he could blow on it. He even painted it by depositing food colour on it with a syringe. Why does this happen?