Beer Tapping Frat Party Prank Explained: The Physics Behind Overflowing Foam

Researchers have explained the physics behind the beer tapping prank. A compression wave starts at the bottom of the beer bottle leading to the foam overflow.  avier Rodriguez-Rodriguez et al.

The beer tapping trick is broken down by researchers from Carlos III University, located in Spain, and University Pierre et Marie Marie Curie, Institut Jean le Rond d’Alembert, located in France. The research, presented at the American Physical Society’s 66th Annual Division of Fluid Dynamics Meeting, entitled “Why does a beer bottle foam up after a sudden impact on its mouth?”

For the researchers it’s all about cavitation, the formation of bubbles after an impact. In this case, larger bubbles form in beer after a sudden impact, the bottom of a friend’s bottle with the mouth of the bottle you are holding, and soon break up. The breaking up process is due to “back and forth movement of compression and expansion wave,” which cause the bubbles to form and then collapse. The wave starts at the bottom of the bottle and works its way to the top.

The overflow is caused by larger bubbles, dubbed “mother” bubbles, collapsing to form smaller bubbles, “daughter” bubbles. These daughter bubbles expand at a much faster rate than mother bubbles which causes the foam to rise past the point of no return, to the chagrin of the victim but to the delight of the prankster. The smaller bubbles are more carbonic, drawing in more carbon dioxide that’s already present in the beer.

Lead researcher Javier Rodriguez-Rodriguez, from Carlos III University, said in a statement, “Buoyancy leads to the formation of plumes full of bubbles, whose shape resembles very much the mushrooms seen after powerful explosions.” The researchers note that the findings could help explain other engineering problems and potential natural disasters, referring to the 1986 Lake Nyos, located in Cameroon, disaster.

Lake Nyos is an exploding lake, due to a carbon dioxide leak beneath the lake, due to volcanic activity. The CO2 dissolves in water to become carbonic acid and could be a threat to the surrounding villages. A huge outgassing of CO2 in 1986 led to the suffocation deaths of village residents and animals, with the estimated human death toll at 1,700. Scientists are currently investigating ways to degas Lake Nyos to prevent another disaster.