Why Beer Foams Over When Tapped

Beer Tapping Prank Explained

Beer Tapping

Apparently it’s not all work and no play for scientists.  They like to party too, as evidenced by a newly-released scientific study in which a group of researchers studied the workings behind an age-old party trick called beer tapping.

The way someone performs the prank is they secretly tap the top of someone’s beer bottle just after they open it.  Then, they step back and laugh uproariously as the beer foams over the top of the bottle onto their unsuspecting victim.

Clearly, the team of researchers – from the Carlos III University in Madrid, Spain and the Insitut Jean le Rond d’Alembert in France – must have fallen prey to beer tapping a few times; and, being scientists, they decided to learn about the physics behind this effect.

The answer, they say, lies in a phenomenon called cavitation.

Cavitation is a concept which is relevant in a variety of engineering concerns, such as when ship propellers undergo erosion.  It is also what causes bubbles to form in a liquid, like your favorite brew, after a sudden impact.

More specifically, cavitation is the formation of gas bubbles in a liquid in an area where the pressure of the liquid drops below its vapor pressure.  Cavitation is generally divided into two types:  inertial (transient) or noninertial cavitation.  In inertial cavitation, a bubble in the liquid quickly collapses, creating a shock wave.  Noninertial cavitation occurs when a bubble in a fluid is forced to oscillate in either size or shape due to some sort of energy being applied to it.

When the opening of a beer bottle is struck, it causes a back and forth motion of compression and expansion waves, the scientists say, which causes bubbles to appear and then quickly burst.  When larger bubbles collapse, they break down into very small bubbles, which tend to expand and grow faster than the larger bubbles.  The relatively quicker expansion of the smaller bubbles is what gives the foam its buoyancy.

This buoyancy then causes plumes of bubbles to be formed, they say, creating forms very similar to that of mushroom clouds seen in explosions.  Also, says the team, the bigger the bubbles get, the more carbonic acid gas they entrain and the faster the plume rises.

Javier Rodriquez-Rodriquez, the lead author of the study, who works for Carlos III University, says that he believes that their work is the first providing an analysis of the phenomenon.

Learning about what creates the foamover effect isn’t just useful knowledge for college frat boys, however; it has real-world implications in engineering systems as well as in natural phenomena like what occurred in the Lake Nyos disaster in Cameroon.    On August 21, 1986, perhaps because of a landslide, the lake suddenly put out a large cloud of carbon dioxide gas, suffocating 1,700 people and 3,500 livestock in nearby areas.  Understanding this phenomenon better will give scientists insight into what occurs in events such as this.

The findings from their study will be presented at the annual meeting of the American Physical Society (APS) Division of Fluid Dynamics.

To see a slow motion video of the beer tapping phenomenon, complete with the scientific explanation for what is happening, check out the video below:

By Nancy Schimelpfening

The Physics of Beer Tapping

The Lake Nyos Disaster


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