Researchers at Carlos III University in Madrid, Spain, in collaboration with researchers at the Universite Pierre et Marie Curie, Institut Jean le Rond d'Alembert, France have explained the physics behind the long-standing party trick 'beer tapping.'

Cavitation is why bottled beer bubbles when tapped. Javier Rodriguez-Rodriguez, the lead researcher from Carlos III University, explains that cavitation is a mechanism through which bubbles appear in a liquid after an impact.

After a sudden collision against a bottle's mouth, consecutive movements of compression and expansion waves form bubbles that collapse quickly. A close look at the beer bottle revealed  that due to cavitation larger 'mother' bubbles give rise to very small carbonic gas 'daughter bubbles', which grow and expand much quickly than the larger mother bubbles they split from.

The swift expansion of these daughter bubbles gives the foam buoyancy.

"Buoyancy leads to the formation of plumes full of bubbles, whose shape resembles very much the mushrooms seen after powerful explosions," Rodriguez-Rodriguez said in a press release. "And here is what really makes the formation of foam so explosive: the larger the bubbles get, the faster they rise, and the other way around." He adds that this is because fast-moving bubbles entrain more carbonic gas.

This is the first quantitative analysis of beer bottle foam.

"We wanted to explain the extremely high efficiency of the degasification process that occurs in a beer bottle within the first few seconds after the impact," Rodriguez said.

The finding was presented at the annual meeting of the American Physical Society (APS) Division of Fluid Dynamics.