Pub trick --- beer bottle tapping
Jearl Walker www.flyingcircusofphysics.com
January 2015 Here is a common prank: A friend has opened a bottle of beer (or any other carbonated drink). Tap a second bottle onto the mouth of the friend’s bottle. Within a few seconds, a foam of carbon dioxide bubbles gushes from the friend’s bottle. Here is a (noisy) example:
The questions is why. This is not like the situation where you shake a carbonated beverage while you seal off the opening with a finger and then remove your finger. With beer tapping, you barely tap the friend’s bottle. Why does that seemingly gentle gesture create such a dramatic result?
Most beers have a lot of carbon dioxide in solution under high pressure. When the bottle is opened, the pressure suddenly drops to atmospheric pressure, and then there is too much carbon dioxide in solution --- it is a supersaturated solution. So, the carbon dioxide begins to come out of solution by forming bubbles in any tiny crevice along the glass wall or bottom surface. Normally, the bubbles grow until they are large enough to break free and then float up to the top of the liquid. As they move upward, they grow larger as carbon dioxide diffuses (passes) into them from the liquid they pass. This is all rather gradual. However, if you shake up the contents, the bubbles break free more quickly and thus the bubble production is much more rapid, enough to allow gushing from the bottle. Some people enjoy spraying the contents of a shaken bottle over other party guests.
Something different happens with bottle tapping. The tap sends a pressure wave down through the fairly rigid glass wall to the bottom of the bottle. (The stunt does not work very well with the more flexible walls of a beer can.) When the wave reaches the bottom of the bottle, it cause the bottom surface to begin oscillating. The oscillation sends an expansion wave (the pressure in the liquid is slightly reduced) up through the liquid to the top surface. The wave then travels back down through the liquid as a compression wave (the pressure is slightly increased). The wave can then alternate between the two types and the two directions several time.
The waves travels through any existing bubbles that were on their way up. The expansion wave causes them to expand, but then the compression wave causes them to shrink. In fact, the shrinkage can eventually rupture each bubble into many smaller bubbles. These smaller ones may partially deplete the carbon dioxide from the surrounding liquid but as they float upward into fresh liquid with more carbon dioxide, they grow larger and larger. And the larger they become, the faster they are buoyed upward. Thus, they quickly form fast moving plumes, and as the plumes reach the top surface of the liquid, bubbles gush from the bottle, all because of a gentle tap.
Here comes a video demonstration and explanation:
Here is the marvelous slow-motion video posted by Javier Rodriguez-Rodriguez and Almudena Casado-Chacon of Carlos III University of Madrid, and Daniel Fuster of Universite Pierre et Marie Curie in Paris to accompany their paper in Physical Review Lettters (see below).
Here is a video with animation from Scientific American:
Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
···Rodriguez-Rodriguez, J., A. Casado-Chacon, and D. Fuster, “Physics of beer tapping,” Physical Review Letters, 113, article #214501 (5 pages) (21 November 2014)
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