Puzzle of Irish stout Guinness' bubbles solved

Researchers at the University of Limerick in Ireland found that after one pours a glass of the famous Irish stout, the white bubbles settle downward. Since bubbles are lighter than beer, one might think this defies the laws of gravity.

"In one's everyday life, one rarely comes across such a counterintuitive phenomenon, challenging equally the imagination of a university professor as well as that of Bill, John and Harry from the local pub," said Eugene Benilov, an applied mathematician who led the research.

The solution to this puzzle lies in how the beer flows in the glass. The beer flows downward near the walls of the glass, dragging the tiny bubbles along with it, and then upward in the interior. This circulatory pattern eventually leads to a creamy white head of foam resting on top of the black brew, the researchers said.

The question, then, is why the beer flows this way in the first place. But computer models and lab experiments carried out by the researchers revealed that the answer lies in the geometry of the pint glasses in which stouts are typically enjoyed, LiveScience reported.

Normally, with a perfectly straight cylindrical glass all the bubbles in the beer would rise together from below. But pint glasses are typically narrower at the bottom and wider at the top. There is more space under the wide flat middle of the glass than under its angled walls, which means more bubbles rise from the middle than the sides, the researchers said.

This higher density of bubbles in the middle of the pint glass leads to a kind of fountain of beer there, with a strong upward rush of bubbles from the middle that ultimately results in the brew flowing downward along its sides and then back up, they added.

Such research, the researchers said, might not only solve a mystery of beer. Understanding these kinds of bubbly flows could help control how bubbles flow in champagne glasses. "We will probably look into potential industrial applications of our results," Benilov added.