Last week we explained why beer violently foams when the top of the bottle is tapped. According to a study on the phenomenon, upon impact, a shock wave travels down the glass, and then transfers to the liquid inside as an expansion wave. From there, the wave bounces back and forth, causing a rapid series of expanding and collapsing bubbles. It's a messy prank, but learning why and how these cavitation bubbles form could prove beneficial to engineers looking to reduce wear to underwater equipment.
But something bothered me. Why didn't both bottles foam up? I reached out to study author Javier Rodriguez-Rodriguez for answers. My guess was that the shock propagated differently through the bottom of bottle than through the top. Turns out I was only close. Here's what Javier had to say:
"The main reason is that, in order to make bubbles collapse and breakup, it is far more efficient to first expand them and then compress, rather than the other way around. And this is the big difference between impactor and impacted!
In the impactor, the liquid first "feels" the compression and then the expansion, which is not that efficient in terms of breaking bubbles.
Of course, waves rebound in all the cases, but energy is lost after each rebound, so the expansion that follows the compression is smaller in intensity..."
So while the hitter's beer experiences the same expansion and compression forces, the order in which they occur after the initial shock makes all the difference. Use this knowledge responsibly.