Having been out of school for many years, some beer-related phenomena have slipped into the recesses of my memory, but I was reminded this week of one in particular: strike the top of a beer bottle, typically with the bottom of another bottle, and the first bottle will lose all its dissolved gasses in a matter of seconds, producing a sudden outpouring of foam. The effect is remarkably specific in that nothing happens to the bottle doing the striking and the tap has to take place on the top of the neck—tapping a bottle's sides just won't do. But why?

Researchers at Spain's Carlos III University and France's Université Pierre et Marie Curie have figured out what's going on. In a presentation at the meeting of the American Physical Society, they described how the initial tap sets off a compression wave that gets transmitted to the bottom of the bottle. Once there, the force is transferred to the beer, and a series of compression and expansion waves bounce between the top and bottom of the bottle. This process accounts for the delay between the tap and the foam explosion.

The waves fragment the bubbles already present in the beer, resulting in many smaller bubbles. The increased surface area of these smaller bubbles allows them to rapidly grow by drawing in more of the gas dissolved in the beer. The net result is a rapidly growing foam that forms a plume "whose shape resembles very much the mushrooms seen after powerful explosions," said Javier Rodriguez-Rodriguez, lead researcher on the project. The plume's buoyancy ensures that it shoots quickly to the surface of the beer and expands out rapidly once it reaches the air.

So there you have it—physics. This bit of enlightenment was brought to you by the Spanish Ministries of Science and of Economy and Competitiveness, which funded the research team.