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Science behind beer prank revealed

Scientists in France and Spain have explained why beer foams so much if the top of the bottle is tapped, likening the reaction to a nuclear explosion.

Researchers at Carlos III University in Spain have attributed the reaction to “cavitation”, apparently a common engineering concern often seen in the erosion of ship propellers where bubbles forming around the propeller blades then collapse quickly due to pressure causing a shock wave. This can lead to cavities forming on the metal.

The scientists explained that when the bottle is bumped from above, a compression wave travels down and rebounds as an expansion wave.

Within 0.1 to 1 millisecond the two waves have broken up the larger “mother” bubbles in the beer and within 1 to 10 milliseconds these have become much smaller “daughter” bubbles.

It takes these smaller bubbles less than a second to expand rapidly, growing in size and forming a foam which rises and overflows the container.

Lead researcher, Javier Rodriguez-Rodriguez, explained: “Buoyancy leads to the formation of plumes full of bubbles, whose shape resembles very much the mushrooms seen after powerful explosions.

“And here is what really makes the formation of foam so explosive: the larger the bubbles get, the faster they rise, and the other way around.”

Although it may appear fatuous to study the effects of what is usually a common prank, Rodriguez-Rodriguez explained that the findings of the study can be applied to other situations and help explain the movement of gas in the natural world.

He referenced in particular the sudden release of trapped carbon dioxide from Lake Nyos in Cameroon in 1986.

Located in the north-west of the west African country, the lake is one of only three lakes in the world known to expel large amounts of gas (which is created by a pocket of magma beneath the lake).

On 21 August 1986, a limnic eruption (possibly caused by a landslide), led to well over 100,000 tonnes of CO2 – some sources estimate over 1.6 million tons – being released from the lake.

Rushing to the surface at over 62 miles per hour, the gas erupted in a wall of water 25 metres high and spread out over an area of some 25km, suffocating over 1,200 people and 3,500 livestock.

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