Cocktail science

This weekend I went to a cocktail party (featuring Budweiser Select) at a friends house. It was meant to be fancy dress, but given the mix of classy beer (I would have chosen the champagne of beers) and cocktails, I chose to dress in a short sleeve button up with a little bowtie, classy.

But on to the point of this post. Some of you may be familiar with the hilarity involved in "capping" when you hit the top of a friends freshly opened beer with your empty bottle (to avoid having the fun back-fire on yourself). The beer in your friend's hand foams over onto their hand and floor, causing hilarity and sticky linoleum. Having witnessed just such an event, we began wondering why this actually happens, what makes the beer foam over like that. Fortunately the party was full of scientists, unfortunately most of them were geologists and proved fairly useless in answering our question. The only possible option was to run a a highly controlled test in the backyard.

The first part of the test was to recreate the trick for a girl who had never seen it, but was our best hope for an explanation. Not having seen it before she of course was foolish enough to hold the target bottle...Hilarity all over her hand. We were working on two hypothesis (like I said it was official sciency business) one that it was the pressure created by forcing air into the target bottle, the other that it is the vibrations moving through the target bottle. The pressure idea was put forward by the geologists, the vibration by and architect. To test the pressure theory we tapped the top of the bottle with an open palm, the idea being that the palm would actually create a tighter seal... hence more pressure, but would absorb more of the shock and create less vibration. Unfortunately we did not account for the loss of friction in the now beer covered hand, and the application of downward force resulted in a broken bottle and beer sacrificed to the gods of science and party. At that point, fearing the loss of additional beer, we switched to a non-alcoholic beer that had rightfully been left on the back porch after it was brought to a party two summers ago. The second attempt at the palm test did not result in foamy beer.

The next test was to place the bottle onto a table and hit it with another bottle. The reason it was on a table was to remove the up and down shaking effect caused by being hit downward. On this attempt the beer (if you can call it that) did foam over, proving that it was not caused by shaking. At this point we declared vibration the likely cause and went back into the house, because even when most of the guests are architecture and geology students, you can only run highly scientific experiments in the backyard for so long without risking your party cred.

So now I ask all of you, what do you think causes the foam effect?