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See this short video.
Of special note is the fact that this does not occur when a beer bottle is hit from below or on the sides.
What mechanism causes this? Is it a property of beer, or of glass beer bottles, or something else?
Tapping the top causes compression waves started through the air in the opening (which is why it only works from the top.) The compression waves bounce at the bottom and become expansion waves. The compression and expansion causes agitation which foams up the beer.
The layman's summary from Scientists discover why beer bottles overflow after a sudden impact:
A lot happens in the short period of time between tap and torrent. The moment some jerk clocks your bottle's mouth, a compression wave travels down through the glass. When the wave hits bottom, it's reflected as an expansion wave that travels through the beer. These waves keep bouncing back and forth, with the compression waves breaking up the CO2 bubbles in your beer into thousands of incredibly tiny microbubbles, and the expansion waves causing those microbubbles to violently expand into skyrocketing plumes.
The result? Millions and millions of expanding CO2 bubbles turn your beer into foam shooting out of your bottle. With any luck, it spills all over your jerk friend's pants.
The abstract from the published article:
A sudden vertical impact on the mouth of a beer bottle generates a compression wave that propagates through the glass towards the bottom. When this wave reaches the base of the bottle, it is transmitted to the liquid as an expansion wave that travels to free surface, where it bounces back as a compression wave. This train of expansion-compression waves drives the forced cavitation of existing air pockets, leading to their violent collapse. A cloud of very small daughter bubbles are generated upon these collapses, that expand much faster than their mothers due to their smaller size. These rapidly growing bubble clusters effectively act as buoyancy sources, what leads to the formation of bubble-laden plumes whose void fraction increases quickly by several orders of magnitude, eventually turning most of the liquid into foam.
The process is a bit different in freely moving bottles when compared to bottles on a solid surface.
Bottles that can't move only foam a little bit, where the hitting bottle foams as much as the hit bottle.
Bottles that can move (even 1 mm is enough) move down faster than the beer. This creates a lower pressure inside the bottle. Beer can hold a lot less CO2 at lower pressure so any bubble that can grow, will grow and new ones will form at an increased rate. The top bottle will have an equal but opposite pressure change, but increased pressure just increases the amount of CO2 that the beer can hold so no extra foam will be produced here.
Fun sidenote: If you hit hard enough the pressure in the hit bottle gets low enough to create a small and very short lived vacuum at the bottom of the bottle, the beer rushes towards the vacuum with such power that the bottom of the bottle breaks off. You can also do this with your hand instead of with another bottle. How to Break a Beer Bottle With Your Bare Hands.
