Why does beer overflow when you tap one bottle on top of another?

Depending on your point of view (or how drunk you are), tapping another person's beer bottle so that it overflows is either incredibly obnoxious or a funny prank. But only recently have physicists explained exactly why that motion causes the beer to foam so rapidly.

You're sitting with a friend (preferably outside), drinking a couple of beers out of bottles. When you aren't paying attention, your friend taps the mouth of your bottle with their bottle, causing foam to shoot up from the mouth of your bottle and spill everywhere. Your friend has a good laugh as you either rush to drink the foam or end up a bit wet.

Members of the Fluid Mechanics Group at Carlos III University of Madrid were at a tavern when someone played this boozy joke and they began to throw out hypotheses as to what caused this overflow. When they weren't satisfied by any of their own explanations, Javier Rodriguez-Rodiguez , Almudena Casado-Chacon, and Daniel Fuster decided to put the trick to the test. They performed two experiments: First, they filled commercial beer bottles with deionized water at a hydrophone was placed at various depths to measure pressure wave propagation in the liquid when brass weights were dropped on the mouths of the bottles from various heights. Then the bottles were filled with beer and impacted with the same brass weights; the evolution of the bubbles following each impact was recorded with a high-speed camera.

What they observed was that the impact of a weight against the mouth of a bottle triggers a compression wave, which hits the bottom of the bottle and bounces off as an expansion wave. Then the expansion wave hits the free surface of the liquid and bounces back as a compression wave, and so on and so on until the waves are damped out. Because the free surface is relatively close to the bottom of the bottle, we get a train of expansion and compression waves, driving the rapid cavitation of the air bubbles in the beer.

The air bubbles collapse due to the train of expansion-compression waves, forming clouds of much smaller daughter bubbles. These daughter bubbles have a larger surface to volume ratio than their parent bubbles, and therefore expand much more quickly. This turns the liquid to foam and results in buoyant clusters of expanding air bubbles that rise to the surface—and swiftly make their way to the mouth of the beer bottle.

So the next time you tap a friend's beer and watch it overflow, tell them that you're not being a jerk; you're just appreciating fluid mechanics in action.

Image credit: Rikard Fröberg.

Why does a beer bottle foam up after a sudden impact on its mouth? [arXiv via PhysOrg]