Ever wonder why tapping the top of a beer bottle unleashes an eruption of foamy bubbles down your shirt and onto the floor? Now, scientists have the answer.
It’s all about compression waves and cavitation.
When you hit the top of the glass bottle, that sudden force creates a compression wave, similar to what you see if you tap one end of a Slinky toy and the distance between the coils increases and decreases. When this compression wave reaches the bottom of the beer bottle, the wave is pushed up through the liquid as an expansion wave.
While compression waves increase the pressure, density, and temperature of the beer, expansion waves reverse the process, reducing pressure, density, and temperature. When an expansion wave reaches the surface of the beer at the top of the bottle, it turns around and becomes a compression wave again. A “train” of expansion and compression waves then travels back and forth between the top and bottom of the bottle, causing the beer to undergo cavitation, or bubble formation.
Cavitation happens when a liquid forms small voids, or liquid-free areas, we call bubbles. It generally occurs when a liquid is subjected to rapid changes in pressure. Understanding cavitation is important in engineering such things as ship propellers because the rapid formation and collapse of bubbles places stress on the metal, resulting in metal fatigue.
When beer undergoes cavitation, large bubbles are created that quickly collapse. Collapse of the large bubbles creates many smaller bubbles that expand rapidly, sloshing over the mouth of the bottle and onto the floor in a big beery mess.
“Buoyancy leads to the formation of plumes full of bubbles, whose shape resembles very much the mushrooms seen after powerful explosions,” Javier Rodriguez-Rodriguz, a fluid mechanics researcher at Carlos III University in Madrid, Spain, explained Sunday (Nov. 24) at the annual meeting of the American Physical Society’s fluid dynamics division in Pittsburgh, Pennsylvania. “And that is really what makes beer so explosive: the larger the bubbles get, the faster they rise, and the other way around,”
Investigating beer bubble explosions has a wider application than just understanding party mishaps. Rodriguez-Rodriguez and his colleagues studied beer in order to better understand larger-scale gassy eruptions, such as occurred in 1986 at Lake Nyos in Cameroon. One of three known exploding lakes, Lake Nyos sent an enormous cloud of carbon dioxide into the air that tragically killed some 1,700 people and 3,500 livestock in nearby villages. Caused by volcanic activity that dissolved CO2 in the water, it was the first known mass asphyxiation caused by a natural disaster.