A new experiment by chemists from Stanford University and the University of Edinburgh has finally proven what beer lovers have long suspected: when beer is poured into a glass, the bubbles sometimes go down instead of up.
''Bubbles are lighter than beer, so they're supposed to rise upward,'' said Richard Zare, a professor at Stanford. ''But countless drinkers have claimed that the bubbles actually go down the side of the glass. Could they be right, or would that defy the laws of physics?''
In 1999 Australian researchers announced that they had created a computer model showing that it was theoretically possible for beer bubbles to flow downward. The Australians based their simulation on the motion of bubbles in a glass of Guinness draught - a popular Irish brew that contains both nitrogen and carbon dioxide gas.
But Zare and former Stanford postdoctoral fellow Andrew Alexander were sceptical of the virtual Guinness model and decided to put it to the test by analysing several litres of the liquid brew.
They used a camera that takes 750 frames a second and recorded som video clips of what was happening in the beer glass, their findings confirmed the Australian study.
''The answer turns out to be really very simple,'' continued Zare. ''It's based on the idea of what goes up has to come down. In this case, the bubbles go up more easily in the centre of the beer glass than on the sides because of drag from the walls. As they go up, they raise the beer, and the beer has to spill back, and it does. It runs down the sides of the glass carrying the bubbles - particularly little bubbles - with it, downward,'' said Zare.
The phenomenon also occurred in other beers that did not contain nitrogen. ''The bubbles are small enough to be pushed down by the liquid,'' added Alexander. ''We've shown you can do this with any liquid, really - water with a fizzing tablet in it, for example.''
That bubbles continue to intrigue the scientific community is clear. A recent study published by Belgian scientists in the New Journal of Physics in December revealed for the first time how antibubbles form and move through a liquid.
Antibubbles are the exact opposite of bubbles and move down instead of up. Whereas a bubble is a thin flim of liquid in air and which encloses a pocket of air, an antibubble is a thin film of air made inside a liquid, enclosing a pocket of that liquid. Scientists have known about them for almost a century but why and how they form has been a mystery until now.
Dr Stéphane Dorbolo and colleagues at the university of Liège, in collaboration with the Collège de France, proposed a mechanism which describes how antibubbles are made and also how they move through a liquid.
Antibubbles can be created by pouring a liquid containing a "surfactant" - soapy water for example - onto an identical liquid.
According to the researchers, they form because a thin film of air is sometimes pulled down along with the liquid itself. This air film then separates two liquids with the same composition and so is called an antibubble, since a real bubble is a liquid film separating two regions of air.
Out of curiosity, the researchers also attempted to create antibubbles in Belgium's most famous export - beer. Apparently they thought that this would not be possible because you can not create antibubbles (or bubbles) in pure water, alcohol or oil.
But they found that you can make antibubbles in beer because beer contains protein which makes it a surfactant just like dishwashing liquid.
"We have come up with a good model describing how they[antibubbles] form and move and have also learnt more about the type of liquids you can create them in," said lead researcher Dr Stéphane Dorbolo.