Scientists unlock mystery of foaming beer

A group of Spanish scientists have discovered why beer froths up when two bottles are struck vertically against each other. The findings could help predict how much gas is released during volcanic eruptions.

Scientists unlock mystery of foaming beer
"The clouds of foam look like the mushroom cloud of a nuclear explosion," says physics professor Javier Rodríguez. File photo: Rikard Fröberg

It all started in a bar: that's how scientists from Madrid's Charles III University (UC3M) have described the beginnings of their year-long study.

The clashing of base of one beer bottle against the neck of a second bottle got researchers talking about why the beer frothed up.

When no one could provide a convincing reason, the team took their research back to the laboratory.

Using lasers, high-speed photography and bottles of the Spanish beer Mahou, the team — together with scientists at France's Jean le Rond D’Alembert Institute — discovered that knocking two bottles together caused a mini-explosion of carbon dioxide.

Usually carbon dioxide is released very slowly but when bottles collide, this happens in a fraction of a second.

This process involves three separate phases. First, the sudden vertical impact on the mouth of a beer bottle causes a compression wave that spreads through the glass towards the bottom. 

When this wave reaches the bottom of the bottle, it rebounds as an expansion wave that travels to free surface, where it bounces back again.

Next this chain reaction leads to the collapse of air pockets, or bubbles, in the beer, and this creates a cloud of tiny 'daughter' bubbles that expand much faster than their 'mothers' due to their smaller size. 

Lastly, those bubbles expand rapidly and race towards the top of the bottle because they are lighter than the beer that surrounds them. This explosive forces results in the froth.

"These clouds of foam look like the mushroom cloud of a nuclear explosion," Javier Rodríguez, a professor of UCSM's Department of Thermal Engineering and Fluids told Spanish science news agency SINC.

But the research isn't just a novelty.

"One of the applications of this project is the prediction of the quantity of gas which is produced after the eruption of a volcano," explained Daniel Fuster from the Jean le Rond D’Alembert Institute.

The study could also help improve the design of ship propellers and anti-shock structures for buildings.

"This is one of the big advantages of basic research. You learn about physics on the cheap with systems as simple as a bottle of beer and later this helps you understand and try and solve large problems," said Rodríguez of the study.

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