If you've ever been a victim of "beer tapping," then you'll know how simultaneously annoying and entertaining the party trick can be. Your buddy uses his beer bottle to strike down on the top of yours. A moment later, your beer foams up and flows out of the bottle, leaving you soggy and thirsty. Now, a team of fluid mechanicists has explained how that volcano of suds emerges. The analysis could help explain the odd release of CO2 from lakes and even sound a warning note on plans to pump CO2 from power plants into the ground.
"It's beautiful work," says Devaraj van der Meer, a physicist at the University of Twente in Enschede, Netherlands, who was not involved in the study. "The explanation is very, very convincing."
The study began as you'd expect. "The first time we started thinking about this problem we were in a bar," says lead author Javier Rodríguez-Rodríguez of the Carlos III University of Madrid. How does the tap on the bottle cause the beer to foam? The answer seems obvious. Beer is mostly water supersaturated with CO2. That is, there is more of the gas dissolved in the liquid than would normally stay in solution at room temperature and atmospheric pressure. So the tap causes the CO2 to rush out of solution in a profusion of bubbles.
But that can't be the whole story. The formation of new bubbles, or cavitation, occurs in milliseconds, as does the diffusion of gas out of the liquid and into the bubbles. So if that's all that's happening, then the beer ought to shoot out of the bottle in a tiny fraction of a second, Rodríguez-Rodríguez says. Instead, the beer oozes out of the bottle in a few seconds, so something else must be going on. To find out what it is, Rodríguez-Rodríguez; Almudena Casado-Chacón, a student at Carlos III University; and Daniel Fuster, a fluid mechanicist at the Pierre and Marie Curie University in Paris, trained a high-speed camera on tapped beers.
The eruption has three stages, the researchers report in a paper in press at Physical Review Letters. First, in two-tenths of a millisecond, the bubbles multiply wildly. The tap sends a wave of compression down through the glass of the bottle. That wave reflects off the bottle's bottom and comes back up through the liquid as a wave of expansion, enlarging the bubbles in the beer—the researchers used the Spanish brand Mahou. The wave then reflects off the surface of the beer and travels back through the liquid, this time as a compressional wave. That sudden squeeze breaks each enlarged bubble into many smaller ones—as many as a million, the researchers calculate.
In the second stage, the tiny bubbles grow quickly. Each cloud of a million mini bubbles has a huge total surface area compared with a single bubble containing the same amount of gas. That surface area lets the CO2 diffuse out of the liquid very quickly, and in a few milliseconds the bubble cloud stretches to about 10 times its initial diameter—as the researchers studied by using a pulse of laser light to form a bubble cloud in a more controlled way. At that point, the growth of the cloud slows, as the bubbles have soaked up most of the excess CO2 in the surrounding liquid.
Finally, after that brief pause, the third phase of the process kicks in. After about 100 milliseconds, the buoyant bubble clouds begin to rise through the liquid. They then start to swirl to form "vortex rings" that resemble smoke rings. That swirling draws in more liquid that is overloaded with CO2, in a process called convection. So the bubbles in the cloud begin to grow even faster than before in a self-reinforcing "autocatalytic" process. By the time a second has passed, a full-fledged beer fountain is bubbling up.
"They really appreciated that there are different stages, and they explained them with simple physics," says José Manuel Gordillo Arias de Saavedra, a mechanical engineer at the University of Seville in Spain. Other researchers might have tackled the problem by simply simulating the whole thing on a computer, he says. "Studies like this show that sometimes it's better to think deeply about a problem," Gordillo says.
The work may also have broader implications for geophysical phenomena, Gordillo says. For example, on 21 August 1986, Lake Nyos, a lake on the flank of an inactive volcano in Cameroon, released a massive cloud of carbon dioxide that suffocated 1700 people. That catastrophic release of gas may have resulted from a landslide within the lake. It could have involved the autocatalytic process at work in the beer, Gordillo says.
The physics identified in foaming beer might also affect plans to stash or "sequester" CO2 from coal- and natural gas–burning power plants deep underground to help combat climate change, Rodríguez-Rodríguez says. The beer study raises the possibility that a sudden "transient" phenomenon could upset such an underground reservoir and trigger a massive release of gas, Rodríguez-Rodríguez says. He stresses that he doesn’t know whether that could happen, but he wants to study the issue. "One of the contributions that the paper could have is just to get people look at what could happen when there are transients," he says.
From one experiment on beer, lots of ideas are bubbling up. Perhaps that's worth a toast.
(Video credit: Almudena Casado-Chacón/Carlos III University of Madrid)