Why does a beer overlow when someone hits the top of a beer with the bottom of theirs?

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Why does a beer overlow when someone hits the top of a beer with the bottom of theirs?

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Because it causes the bottle to almost intantly move down, but the beer tends to stay where it was. This causes a considerable decompression between the beer and the bottom of the bottle, favoring the formation (and expansion) of bubbles, which will rise and form the foam that you see.

Basically umit’s the sped up version of shaking the bottle

P.s. sorry for my english

The impact causes a series of alternating expanding and compressing waves to bounce between the bottom of the bottle and the surface of the beer. These waves have an effect on the beer that cause large air bubbles to form. The same waves then break the big bubbles into many tiny bubbles. These tiny bubbles have a higher surface to volume ratio causing them to expand rapidly and that’s where the overflow comes from

If you take a bottle of water and whack the top of it hard, the bottle will move down very quickly. However, it effectively takes a second for the water to realise this and to move down with the bottle. This is called inertia. The result is a small vacuum at the bottom of the bottle. If you do this just right, the water gets pulled down by the vacuum with such force that it blasts through the bottom of the bottle. This is called a water hammer, and is sometimes the reason you head noisy heating pipes tapping.

Now, let’s replace the water with beer (and lessen the force). The beer stays put for a second and a vacuum forms. Except nature abhors a vacuum. So, the carbon dioxide that’s dissolved in the beer very quickly comes out of the solution to fill the vacuum. Except now you have a bottle of beer with a large bubble of CO2 underneath. This gas quickly rises through the beer, stirring up more bubbles as it goes, until the froth erupts from the bottle.