"Bubbles are lighter than beer, so they're supposed to rise upward," said Richard N. Zare, the Marguerite Blake Wilbur Professor in Natural Sciences 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?"

This frothy question reached a head in 1999 after 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 J. Alexander were skeptical of the virtual Guinness model and decided to put it to the test by analyzing several liters of the liquid brew.

"Indeed, Andy and I first disbelieved this and wondered if the people had had maybe too much Guinness to drink," Zare recalled. "We tried our own experiments, which were fun but inconclusive. So Andy got hold of a camera that takes 750 frames a second and recorded some rather gorgeous video clips of what was happening."

Bottoms up, bubbles down
A careful analysis of the video confirmed the Australian team's findings: Beer bubbles can and do sink to the bottom of a glass. Why does this happen?

"The answer turns out to be really very simple," Zare explained. "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 center 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. After a while it stops, but it's really quite dramatic and it's easy to demonstrate."

The phenomenon also occurred in other beers that did not contain nitrogen, said Alexander, now a professor at the University of Edinburgh in Scotland. "The bubbles are small enough to be pushed down by the liquid," he said. "We've shown you can do this with any liquid, really - water with a fizzing tablet in it, for example."

Confirmation of the sinking-bubble phenomenon has relevance beyond settling barroom bets, according to the researchers.

"There's a certain aspect of bubbles that always make you think it's kids' play and relaxation, but it's serious stuff, too," Zare said, pointing to ongoing research on fluidized beds - the mixing of solid particles with liquids and gases - which have important industrial and engineering applications.

"It's just paying attention to the world around you and trying to figure out why things happen the way they do," Alexander added. "In that case, anyone that goes into a pub and orders a pint of Guinness is a scientist."

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