If I’m holding a beer, and my buddy clicks the top of it with his own bottle, why does mine foam over?
I think it’s because the underwater pressure suddenly decreased, which made lots of tiny bubbles appear, then
each bubble filled with CO2 gas and grew fairly large. (Make that “underbeer” pressure.)
If you whack the top of the bottle, the whole bottle moves down, but the beer doesn’t respond instantly. The beer lags behind as the bottle moves down. This should create a partial vacuum near the bottom of the beer, and if the pressure is low enough, it can trigger the formation of a cloud of incredibly tiny bubbles.
Note that beer-foaming happens when something creates lots of tiny bubbles. If you put some salt or some melted ice-cream in the bottom of a mug and then pour in the beer, you’ll get foam all over. Salt carries air on it’s surface, and ice-cream is already made of foam in the first place. The millions of tiny bubbles rapidly grow because CO2 inflates them. (The CO2 wants to leave the beer, but it can’t, it needs a pre-existing bubble.)
I’ve managed to blow the bottom out of jars of sauce in a similar way. BEFORE you open a glass jar of spagetti sauce, hole it over the sink and give the top a sharp whack with the heel of your hand. Hear a “click” noise? If not, whack it even harder. That “click” is from a vacuum-pocket at the bottom of the sauce opening up and then slamming shut again. The sauce lags behind when the jar moves down, then it catches up again and slams into the jar bottom. If you hit the top hard enough, the “click” will be so violent that the water-hammer effect will knock the glass bottom cleanly off the jar. Well, maybe not so clean when the spagetti sauce comes spewing out.
It helps if the jar is under partial vacuum already. That’s why you need to perform the trick before opening it. Oh, and glass bottles of orange or tomato juice do this too. Whack the top hard, and you can make the bottom fall off and spew the contents all over your shoes!
WOW - this will be a great trick to teach the kids! Can’t wait to try this at home! 