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Beer foam explodes like a “mushroom cloud,” scientists find

Jan. 21, 2014
Courtesy of Uni­ver­si­dad Car­los III de Ma­drid
and World Science staff

Sci­en­tists say they have ex­plained why beer foams up so quickly when a bot­tle gets bumped. The foam be­gins form­ing at the bot­tom and bursts up­ward, not un­like a mush­room cloud, they ex­plain.

The idea for the re­search came cam at a ba­r, ac­cord­ing to the re­search­ers. They no­ticed that an out­pour­ing of foam re­sulted just when some­one bumped the neck of one beer bot­tle against the base of an­oth­er one.

Courtesy of UC3M


“We all be­gan to pro­pose hy­pothe­ses and the­o­ries about the cause… but none of them con­vinced us, so we de­cid­ed to take it to the lab­o­r­a­to­ry,” said Javier Ro­dríguez of the Uni­ver­si­dad Car­los III de Ma­drid, who pre­sented a pre­lim­i­nar­y re­port on the find­ings at the Amer­i­can Phys­ics So­ci­ety’s most re­cent an­nu­al con­fer­ence on flu­id me­chan­ics.

The re­sult, he said, was “con­trolled ex­pe­ri­ments in well-de­fined con­di­tions.”

Three things hap­pen in se­quence af­ter a bot­tle is bumped, they found. First, “com­pres­sion waves” ap­pear—an ar­ea of the liq­uid be­comes com­pressed, and the ar­ea of com­pres­sion starts trav­el­ing, bounc­ing when it hits a wall. These waves cause the bub­bles to burst at the bot­tom of the bot­tle. They break up in­to smaller ones, form­ing foam. Be­cause these weigh less than the liq­uid sur­round­ing them, they move to the sur­face so quickly that the fi­nal re­sult is like an ex­plo­sion.

“Those clouds of foam are very much like the mush­room cloud caused by a nu­clear ex­plo­sion,” Ro­driguez said—in one sec­ond, al­most all of the beer shoots out of the bot­tle.

The foam is there be­cause the liq­uid can­not ab­sorb all the car­bon di­ox­ide gas that is in there, he went on. “Usu­ally, the CO2 es­capes very slow­ly. But the chain of events set off by the b­low to the bot­tle” makes it much faster, Ro­dríguez said.

The re­search­ers came up with a sys­tem for stu­dy­ing the phe­nom­e­non in slow mo­tion. First, they aimed at the base of the bot­tle with a high en­er­gy pulsed la­ser to cause a bub­ble to ap­pear. Then they hit the neck of the bot­tle and recorded ever­ything with a high-speed cam­era that al­lowed them to get over 50000 still pho­tos per sec­ond. This let them de­scribe in de­tail the pro­cess be­hind this type of bub­ble forma­t­ion, called cavita­t­ion. It’s si­m­i­lar to boil­ing but oc­curs when the pres­sure in a liq­uid drops.

The re­search may have real-life use­ful­ness, he added. “One of ap­plica­t­ions is the pre­dic­tion of the quanti­ty of gasses pro­duced by the erup­tion of a vol­cano,” said an­oth­er of the re­search­ers, Dan­iel Fuster, of the In­sti­tute D’Alem­bert in Par­is. 

In 1986, Lake Nyos in Cam­e­roon, which lies on top of a vol­ca­no, re­leased be­tween 100,000 and 300,000 tons of car­bon di­ox­ide in an ex­plo­sion-like phe­nom­e­non, he not­ed. The gas ex­pand­ed about as fast as a car on a high­way, tak­ing 1,700 hu­man lives.

Un­der­stand­ing cavita­t­ion may have applications beyond the bar, the group added: from im­prov­ing the de­sign of boat pro­pellers, which suf­fer ero­sion caused by bub­bles, to im­prov­ing ex­plo­sion-resistant struc­tures, to stu­dy­ing chem­i­cal re­ac­tions caused by bub­ble im­plo­sion. “This is one of the great ad­van­tages of bas­ic re­search,” said Ro­dríguez. “You learn low-cost phys­ics in the lab­o­r­a­to­ry, with sys­tems that are as sim­ple as a bot­tle of beer.”


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Scientists say they have explained why beer foams up so quickly when a bottle gets bumped. The foam begins forming at the bottom and bursts upward, not unlike a mushroom cloud, they explain. The idea for the research came cam at a bar, according to the researchers. They noticed that an outpouring of foam resulted just when someone bumped the neck of one beer bottle against the base of another one. “We all began to propose hypotheses and theories about the cause… but none of them convinced us, so we decided to take it to the laboratory,” said Javier Rodríguez of the Universidad Carlos III de Madrid, who presented a preliminary report on the findings at the American Physics Society’s most recent annual conference on fluid mechanics. The result, he said, was “controlled experiments in well-defined conditions.” Three things happen in sequence after a bottle is bumped, they found. First, “compression waves” appear—an area of the liquid becomes compressed, and the area of compression starts traveling, bouncing when it hits a wall. These waves cause the bubbles to burst at the bottom of the bottle. They break up into smaller ones, forming foam. Because these weigh less than the liquid surrounding them, they move to the surface so quickly that the final result is like an explosion. “Those clouds of foam are very much like the mushroom cloud caused by a nuclear explosion,” Rodriguez said—in one second, almost all of the beer shoots out of the bottle. The foam is there because the liquid cannot absorb all the carbon dioxide gas that manufacturers put in there, he went on. “Usually, the CO2 escapes very slowly. But the chain of events set off by the blow to the bottle” makes it much faster, Rodríguez said. The researchers came up with a system for studying the phenomenon in slow motion. First, they aimed at the base of the bottle with a high energy pulsed laser to cause a bubble to appear. Then they hit the neck of the bottle and recorded everything with a high-speed camera that allowed them to get over 50000 still photos per second. This let them describe in detail the process behind this type of bubble formation, called cavitation. It’s similar to boiling but occurs when the pressure in a liquid drops. The research may have real-life usefulness, he added. “One of applications is the prediction of the quantity of gasses produced by the eruption of a volcano,” said another of the researchers, Daniel Fuster, of the Institute D’Alembert in Paris. In 1986, Lake Nyos in Cameroon, which lies on top of a volcano, released between 100,000 and 300,000 tons of carbon dioxide in an explosion-like phenomenon, he noted. The gas expanded about as fast as a car on a highway, taking 1,700 human lives. Understanding the phenomenon of cavitation may lead to the improvement in the design of boat propellers, which suffer erosion caused by bubbles, to improving explosion-resistant structures, to studying chemical reactions caused by bubble implosion, point out the researchers. “This is one of the great advantages of basic research,” concludes Rodríguez. “You learn low-cost physics in the laboratory, with systems that are as simple as a bottle of beer.”