An article in Physics Central, a site of the American Physical Society, explains the fluid mechanics of why beer in a bottle becomes a mass of foam when the bottle is given a vertical tap on the mouth.
The article says:
The act is colloquially referred to as “beer tapping”: Someone hits a beer bottle on the head, often with the bottom of their own bottle, and within seconds the victim of the prank is left with a small amount of flat beer and a bottle dripping with foamy bubbles of carbon dioxide.Engineering students and others who consume beer might find the physics behind this noteworthy, even enlightening, thus making those that drink beer more knowledgeable about their habits and diversions. Such research also shows that physicists take their work seriously, not only unraveling mysteries of the cosmos, but also everyday phenomenon that takes place regularly at the corner bar or pub. Cheers.
Javier Rodríguez-Rodríguez, assistant professor at the Fluid Mechanics Group of Carlos III University of Madrid and lead author of an abstract about the research, and his colleagues were at a bar discussing the process behind this phenomenon when they realized they did not fully understand it. And according to their unsuccessful search for a solution online and through scientific databases, neither did anyone else.
Through experimentation and computational simulations, they determined that the process starts with a series of waves and ends up fizzing all over the place.
The initial tap to the bottle sends a shock wave through the glass to the bottle’s bottom. The energy from the wave transfers to the beer inside sending a second shock wave up toward the beer’s surface. It then bounces back again and keeps moving back and forth through the liquid until its energy dissipates.
You can read the rest of the article at [Physics Central].