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The Physics is Clear on Foamy Beer

A team of three international scientists has explained the physics behind why beer in a bottle transforms into an overflowing mass of foam when the bottle receives a vertical tap on the mouth, as shown in the video. They will present their work and its applications outside of the bottle at the 66th Annual Meeting of the APS Division of Fluid Dynamics.

The act is colloquially referred to as “beer tapping”: Someone hits a beer bottle on the head, often with the bottom of their own bottle, and within seconds the victim of the prank is left with a small amount of flat beer and a bottle dripping with foamy bubbles of carbon dioxide.

Javier Rodríguez-Rodríguez, assistant professor at the Fluid Mechanics Group of Carlos III University of Madrid and lead author of an abstract about the research, and his colleagues were at a bar discussing the process behind this phenomenon when they realized they did not fully understand it. And according to their unsuccessful search for a solution online and through scientific databases, neither did anyone else.

Through experimentation and computational simulations, they determined that the process starts with a series of waves and ends up fizzing all over the place.

The initial tap to the bottle sends a shock wave through the glass to the bottle’s bottom. The energy from the wave transfers to the beer inside sending a second shock wave up toward the beer’s surface. It then bounces back again and keeps moving back and forth through the liquid until its energy dissipates.

The wave motion and changing pressure knocks free tiny gas pockets trapped inside microscopic imperfections in the glass bottle. The pockets implode and create clouds of smaller bubbles from the dissolved carbon dioxide in the beer. Though these bubbles can quickly grow up to three times larger than their original size, that alone is not what makes the bottle overflow with foam.

Individual plumes of bubbles float up through the beer, building up speed as they get bigger, until they burst out of the bottle’s top in a shower of foam. All of this happens within a matter of a few seconds and once the first tap is initiated is difficult to prevent.

From the time of the tap to the first implosion of a single bubble, about one millisecond passes – too short for human reaction to kick in. However, you can keep the bubbles from growing and creating plumes by quickly plugging the bottle with your thumb until the carbon dioxide has a chance to dissolve back into the liquid – much like re-capping a soda bottle if it starts to spew everywhere upon opening. Although, that may take longer than you’re willing to wait.

The team’s work is one piece of a larger ongoing research topic on carbonated beverages. In 2011, William Lee at the University of Limerick in Ireland, along with two colleagues, published a study on the “Bubble nucleation in stout beers”. And in 2005, Gérard Liger-Belair and two colleagues at the Laboratory of Oenology and Applied Chemistry in France published a study investigating bubble behavior in a glass of champagne.

“We have observed such kind of uncontrolled bubbling production in sparkling wine and champagne. Uncorking the bottle may sometimes lead to such kind of phenomena,” said Liger-Belair who was not involved with Rodríguez's study.

As Rodríguez and his colleagues delved deeper into the problem, they realized that the process that makes beer bottles foam might also have applications outside of bubbly beverages.

“Recently, we have started working with an oil company,” Rodríguez said. “The dynamics of bubbles of a soluble gas, say carbon dioxide, in response to large amplitude pressure waves is of interest for the oil industry.”

Another application might explain the physics behind underwater mud volcanoes.

“There are natural phenomena called mud volcanoes in which an underwater volcano erupts, but instead of lava, it produces large quantities of mud. We have reasons to think that a mechanism very similar to the one occurring in beer bottles is behind these volcanoes,” Rodríguez said.


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