Why does hitting the top of a beer bottle make the beer foam out like a volcano?

Tapping the top of an open beer bottle produces a beer volcano - why?
10 September 2006


Beer bottles



When we're out in the bars and people want to be funny (or annoying) people use the bottom of their beer bottle to tap the top of someone else's bottle. This makes the victim's beer fuzz up and overflow. Can you tell me the science behind this please?


This is similar to a question we had a few weeks ago when we were asked why it is that when a can of fizzy drink falls out of the dispenser, it doesn't explode when you open it. Dave and I discussed it and thought that when the can falls, it tends to spin. It lands on its side with the liquid spinning in a circle inside the can rather than striking the top of the can. Because the inside of the can is very smooth, the fluid doesn't come into contact with many rough surfaces and so there is nowhere for the gas to nucleate and form tiny bubbles. What could be going on in the beer bottle is that once the bottle is open, there is no pressure inside. The gas dissolved in the beer wants to come out as bobbles. If you smash the top of the bottle with another bottle, it makes a shock wave and I think that that would be sufficient to start some nucleation inside the drink. Once you have one bubble, it provides a site for other bubbles to form on. The whole thing feeds back on itself. As the bottle has a narrow neck, as soon as you have some bubbles it fills up and suddenly doesn't have anywhere to expand to. This pushes everything out in a massive volcano.


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