Scientists have figured out how flying insects fly. They've created real-life lightsabers. But they're at their best when they're tackling the mysteries of beer. Now, thanks to a research team studying fluid mechanics, we finally know why bottled beer foams over after a tap on the mouth — with slow-mo footage and everything.
A lot happens in the short period of time between tap and torrent. The moment some jerk clocks your bottle's mouth, a compression wave travels down through the glass. When the wave hits bottom, it's reflected as an expansion wave that travels through the beer. These waves keep bouncing back and forth, with the compression waves breaking up the CO2 bubbles in your beer into thousands of incredibly tiny microbubbles, and the expansion waves causing those microbubbles to violently expand into skyrocketing plumes.
The result? Millions and millions of expanding CO2 bubbles turn your beer into foam shooting out of your bottle. With any luck, it spills all over your jerk friend's pants. [Francis WorldInsideOut via Smithsonian]