Mischievous beer drinkers in bars and pubs across the world have long known that a simple tap on the top of a buddy's open bottle of fizzy lager will cause an explosion of foam and spilled drink. Now researchers reportedly have solved the mystery with unexpected applications, including predicting volcanic activity.

A team of researchers led by Javier Rodríguez, a thermal and fluid dynamics professor at Universidad Carlos III de Madrid (UC3M) solved the problem by using a high energy laser to create a bubble at the bottom of a newly-opened bottle of beer then hit its neck. A high-speed camera, recording at 50,000 stills per second, found that the process had three distinct phases.

A vertical hit causes a shockwave that generates expansion and compression waves. When these reach the bottom of the bottle, the bubbles there burst into smaller bubbles, creating small balls of foam. These weigh much less than the surrounding beer and rise so rapidly to the top of the bottle that the result is similar to an explosion. In less than one second, virtually all the beer can be made to shoot out of the bottle.

This cavitation effect is similar to the effect in a mushroom cloud caused by a nuclear explosion, and occurs in part because there is more carbon dioxide (CO2) in the solution than it can maintain. Usually it would escape slowly, but a knock sets off a chain reaction that causes the gas to erupt.
While this may seem a frivolous piece of research, the researchers say that this research could have important applications. The knowledge could help predict the volume of gases that might erupt as a result of volcanic activity.

Andy Furlong, director of policy and communication at the Institution of Chemical Engineers (IChemE), said: "This research will be of wide interest including to chemical engineers who are the specialists behind the large scale brewing and bottling of famous household beer brands across the world.
Pranksters also beware. Armed with this underpinning knowledge, it may not be too long before a clever chemical engineer finds a solution to keeping the beer in the bottle and removes this trick from the armory of bar-room pranks.