University of Limerick mathematicians Eugene Benilov, Cathal Cummins and William Lee have answered the difficult question of why bubbles in stout beers sink. Along with his research team, Professor Benilov concluded that the downward flow responsible for the sinking bubbles was not due to bubble density or composition, but the shape of the glass.

Previous experiments and simulations showed a circulation of the liquid within the glass carried the bubbles downwards. But the reasons behind this flow pattern remained a mystery. The team's recent study on the Arxiv server reports simulations and experiments showing the shape of the glass was responsible.

Often, a stout beer will contain nitrogen as well as the carbon dioxide that is present in other forms of beer. Nitrogen is less likely to dissolve in liquid, which results in the formation of smaller and longer-lasting bubbles than those composed of carbon dioxide.

The real mystery that baffled physicists and mathematicians was the downward motion of the bubbles. Discovering the truth behind this question was not easy. Only recently was it proved they actually sink rather than being the result of an optical illusion. Now the University of Limerick's Eugene Benilov, Cathal Cummins and William Lee have discovered the answer to the problem - and have devised a simple test that can be carried out by consumers as well.

Speaking about his previous research in this area, Dr Lee told BBC News. “One of the things we found was it's actually very easy to see bubbles forming in stout beer rather than in, say, champagne where the bubble formation process is much more violent." To solve the challenging problem of sinking bubbles Dr Lee joined a team led by fluid dynamics expert Professor Benilov. The team showed that the relative density of bubbles and the surrounding liquid could explain the strange activity. Guinness settling in a cylindrical glass held at an angle shows both falling and rising bubbles in the same glass.

"If you imagine your pint is full of bubbles, then the bubbles will start to rise. "Because of the sloping wall of the pint, the bubbles are moving away from the wall, which means you're getting a much denser region next to the wall," Dr Lee explained. "That is going to sink under its own gravity, because it's less buoyant, and that sinking fluid will pull the bubbles down."

The bubbles, that is, are "trying" to rise, but the circulation drives fluid down at the wall of the glass. "You'll see sinking bubbles not because the bubbles themselves are sinking, but because the fluid is and it's pulling them down with it."

Mr Cummins used a simulated pint and "anti-pint" (the upside-down version of a pint glass) to show the effect at work. In the pint glass, the bubbles fall, but in the anti-pint, the bubbles rise as expected.

For further information go to http://www3.ul.ie/wlee/