Stuck in the lab waiting for your gels to run? Well, head down to your local pub, order a Guinness, and stare at the glass as the creamy brew settles. Do the bubbles move up or down? Legions of Guinness drinkers would attest that they move downward. Physicists, however, say that's impossible: Everyone knows the higher buoyancy of gas bubbles must drive them up through the liquid.
Well, it turns out everyone's right. Clive Fletcher and his colleagues at the University of New South Wales in Sydney, Australia, recently attacked the problem with fluid dynamics software. They found that some bubbles do in fact move downward.
The key, says Fletcher, is in the size of the bubbles and the viscosity of Guinness--like blood, it's thicker than water. Data provided by the Arthur Guinness & Sons company in Ireland indicate that bubbles in newly poured beer range in diameter from about 0.05 to 1 millimeter. Initially, they all rise, dragging the viscous liquid with them. Those bubbles in contact with the walls of the glass, however, are slowed on their ascent by surface tension. The result, according to Fletcher's model: Liquid that rises in the center of the glass eventually gets pushed toward the edges and descends, sweeping the smallest and least buoyant wall-hugging bubbles with it.
"Even with something simple like this, there is actually a lot of complex physics going on," says Keith Hanna, a chemical engineer in the Evanston, Illinois, office of Fluent, a fluid dynamics software company. He notes that the effect--seen in tapered pint glasses--probably wouldn't work in straight ones, because they wouldn't encourage the same liquid currents to form. Guinness aficionados will have to debate on their own which kind of glass serves the tastier pint.