Bottoms up, bubbles down

An experiment finally has proven what beer lovers long have suspected: When the alcoholic beverage is poured into a glass, the bubbles sometimes go down instead of up, confirm chemists from Stanford (Calif.) University and the University of Edinburgh, Scotland.

"Bubbles are lighter than beer, so they're supposed to rise upward," explains Richard N. Zare, the Marguerite Black Wilbur Professor in Natural Sciences at Stanford, "But countless drinkers have claimed that the bubbles actually go down the side of the glass."

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.

However, Zare and former Stanford postdoctoral fellow Andrew J. Alexander--now a professor at Edinburgh--were skeptical of the virtual Guinness model and decided to put it to the test by analyzing several liters. "Andy got a hold of a camera that takes 750 frames a second and recorded some rather gorgeous video clips of what was happening," notes Zare. A careful analysis of the video confirmed the Australian team's findings: Beer bubbles can and do sink to the bottom of a glass--but why?

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

This also occurred in other beers that did not contain nitrogen, indicates Alexander. "The bubbles are small enough to be pushed down by the liquid. 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. "There's a certain aspect of bubbles that always makes you think it's kids' play and relaxation, but it's serious stuff, too," Zare claims, 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 adds. "In that case, anyone that goes into a pub and orders a pint of Guinness is a scientist."

COPYRIGHT 2004 Society for the Advancement of Education
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