A Beer Bottle Prank Is A Lesson In Fluid Mechanics

Even if it’s a waste of beer, tapping the mouth of one beer bottle with the bottom of another produces an impressive eruption of foam. The prank is also a window into the physics of carbon dioxide bubbles, be they in a bottle or in a volcanic crater lake.

A decade ago, Javier Rodriguez, a professor of fluid mechanics, and his colleagues from Carlos III University of Madrid were out for a round at the bar when someone pulled the sudsy stunt. The group immediately tried to come up with an explanation for why the beer spurted out so quickly.

“These bubbles, they grow superfast,” he says. “In a few tens of milliseconds, the clouds of foam can multiply by a factor of 10.”

The scientists theorized that the top bottle caused a pressure wave to reverberate through the beer in the bottom bottle—and as it did so, bubbles of CO₂ formed in a process called cavitation, then split apart into smaller bubbles. “All of the gas in a single bubble is now in millions of bubbles,” Rodriguez says. But the group was unable to connect this important first step with the final outcome of beer on the floor.

The next day they started running controlled experiments in the lab to figure it all out. But they didn’t have a camera fast enough to precisely measure what was going on. Years later, in 2012, the researchers finally acquired a high speed camera that revealed how the bubbles behave as a function of time, depicted in the image above.

The team determined that another phase takes place after cavitation—expansion. The smaller bubbles have a greater surface area, allowing more dissolved CO₂ to seep into them, and they grow rapidly. Being lighter than the liquid, the bubbles rise, but they’re not done growing yet. “This is by far the most explosive part of the process,” Rodriguez says. The bubbles move fast as they ascend, absorbing more dissolved CO₂, which makes them grow even bigger and rise even faster.

The result: a sudden, foamy overflow. (Don’t try this at home, since you can easily break the bottles, but watch the prank in action on YouTube.) The team presented their results at last November’s annual meeting of the American Physical Society Division of Fluid Dynamics.

As the researchers told other scientists about their study, they learned that gaseous CO₂ eruptions could happen outside of a beer bottle, sometimes with grim consequences. For instance, the same physics that can cause beer to overflow were also at work in gaseous eruptions of volcanic crater lakes in the 1980s that suffocated thousands of people and animals in the African country of Cameroon, says Bill Evans, a research chemist at the U.S. Geological Survey who has studied these lakes.

“You don’t expect the lakes to explode and kill people,” he says. But at Lake Monoun in 1984 and again at Lake Nyos in 1986, that’s exactly what happened. Evans thinks that a landslide triggered a deadly flood of CO₂ that swept through the surrounding areas.

Scientists and engineers have since wrangled the bubbling process to prevent these lakes from wreaking more havoc by installing pipes that release the gas in a slow, controlled spurt. Now that’s something we can all raise a glass, or bottle, to cheer about.