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Cold case solved? Study probes riddle of sinking beer bubbles

May 24, 2012
Special to World Science  

One rid­dle of hu­man ex­ist­ence has been a cold case for years—but we can fi­nally put this one away, ac­cord­ing to three sci­ent­ists. They claim to have solved the puz­zle of why bub­bles in dark beer sink rath­er than rise, as com­mon sense, and a cur­so­ry grasp of phys­ics, sug­gest they should.

Their an­swer in a nut­shell: para­doxic­ally, bub­bles in dark beer fall be­cause they’re try­ing to go up. But in try­ing, they cre­ate cur­rents that en­a­ble some of them rise only at the ex­pense of oth­er, more clearly vis­i­ble ones, which in­stead drop. The shape of the glass, mean­while, plays a key role, said the in­ves­ti­ga­tors, who stud­ied per­haps the best-known brand of stout, Guin­ness.

(Image © Sarah J. Moon) 


“The sink­ing bub­bles of Guin­ness and oth­er stout beers have in­trigued beer drink­ing phys­i­cists and their stu­dents for some time,” wrote Eu­gene Be­nilov, Cathal Cum­mins and Wil­liam Lee of the Uni­vers­ity of Lim­er­ick in Ire­land, re­port­ing their find­ings.

“We com­plete the ex­plana­t­ion” of the phe­nom­e­non, they wrote, though as they ac­knowl­edged, they did not be­gin the ex­plana­t­ion.

Four years ago, Youxue Zhang and Zhengjiu Xu of the Uni­vers­ity of Mich­i­gan de­clared that the much smaller bub­ble sizes char­ac­ter­is­tic of dark beers is a key clue in the mys­tery. Bub­bles want to go up be­cause, be­ing lit­tle balls of gas, they’re light­er than the sur­round­ing liq­uid. But the up­ward drive is weak­er if the bub­ble is smaller. If the liq­uid hap­pens to be flow­ing the op­pos­ite way, all it takes is for the liq­uid speed to ex­ceed the bub­ble speed—and the bub­ble will be forced to go with the flow.

“Be­cause of their small size, the bub­bles in Guin­ness beer rise slowly and hence can be en­trained by down­ward flow if the down­ward flow ve­lo­city ex­ceeds the small ve­lo­city of ris­ing bub­bles,” Zhang and Xu wrote, re­port­ing their work in the Feb. 2008 is­sue of the jour­nal Ele­ments.

But why would there be a down­ward flow? Be­n­ilov and col­leagues be­lieve they have un­rav­eled that one.

When a fizzy drink is poured, many bub­bles form when the liq­uid hits the bot­tom of the glass. If we as­sume for sim­pli­city’s sa­ke that they form un­iformly all over the bot­tom, then they would al­so rise in a un­iform col­umn through­out the drink, Be­nilov and col­leagues ex­plain in their pa­per, posted on arXiv.org, an on­line phys­ics re­search data­base.

But one fac­tor, pri­mar­i­ly, dis­rupts this un­iform­ity, they say: they shape of the glass. The stand­ard pint glass typ­ic­ally used for Guin­ness in bars is—like many glass­es—nar­row at the bot­tom, wid­er near the top. Since bub­bles from the bot­tom rise ap­prox­i­mately straight up­ward, then as the glass widens, the ar­ea near the walls finds it­self with a short­age of bub­bles com­pared to the mid­dle of the glass.

This thicker con­centra­t­ion of bub­bles in the cen­tral ax­is of the glass has great con­se­quenc­es for the out­ly­ing bub­bles near the edges, they claim: these fringe el­e­ments are pushed down­ward in or­der to al­low their more main­stream breth­ren to reach the top. 

The key to under­stand­ing why this hap­pens is that “whichever way the bub­bles move, they ex­ert a dra­g force on the sur­round­ing liq­uid”—they car­ry the li­quid with them to some de­gree, they ex­plained. But of course, the whole sop­ping mass of beer can’t simp­ly lift it­self out of the glass just thanks to a lift of­fered by its bub­bles. So if some bub­bles do man­age to push the liq­uid up­ward in their lit­tle area of the re­cep­ta­cle, that liq­uid must fall back down in anoth­er area. 

Thus a cur­rent arises, Be­nilov and col­leagues argue: beer in the cen­tral col­umn goes up, be­cause there are more bub­bles there. Beer near the sides goes down, be­cause there are few­er bub­bles there. The lit­tle, out­ly­ing orbs suf­fer the con­se­quenc­es as their up­ward strug­gle is more than coun­ter­bal­anced by the down­ward speed of the liq­uid. These bub­bles, being near the edge, are the ones we see most clearly, es­pe­cially in a freshly poured glass.

This same log­ic dic­tates that if the glass is nar­rower at the top than the bot­tom, the bub­bles near the edges should flow up­ward in­stead of down­ward, Be­nilov and col­leagues said—which is ex­actly what hap­pens. They de­signed an “anti -pint” glass that had the form of a stand­ard pint glass turned upside-down. Lo and be­hold, the bub­bles near the edge rose.

The beer ex­pe­ri­ments are more than fun and games, Be­nilov and col­leagues said; there are in­dus­trial uses to under­stand­ing how bubbles behave. Zhang and Zu wrote in their 2008 pa­per that the “fizzics” of bub­bles, as they dubbed it, is al­so rel­e­vant to un­der­stand­ing ex­plo­sive vol­ca­no erup­tion­s—as well as lake erup­tions such as a 1986 dis­as­ter at Lake Nyos Cam­e­roon. In that trag­e­dy, 1,700 peo­ple suf­fo­cat­ed af­ter more than a mil­lion tons of car­bon di­ox­ide burst out of the lake.


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One riddle of human existence has been a cold case for years—but we can finally put this one away, according to three physicists. They claim to have solved the puzzle of why bubbles in dark beer sink rather than rise, as common sense, and at least a cursory understanding of physics, suggest they should. Their answer in a nutshell: paradoxically, bubbles in dark beer fall because they’re trying to go up. But in trying, they create currents that enable some of them rise only at the expense of other, more clearly visible ones, which instead drop. The shape of the glass, meanwhile, plays a key role, said the investigators, who studied perhaps the best-known brand of stout, Guiness. “The sinking bubbles of Guinness and other stout beers have intrigued beer drinking physicists and their students for some time,” wrote Eugene Benilov, Cathal Cummins and William Lee of the University of Limerick in Ireland, reporting their findings. “We complete the explanation” of the phenomenon, they wrote, though as they acknowledged, the explanation didn’t begin with them. Four years ago, Youxue Zhang and Zhengjiu Xu of the University of Michigan declared that the much smaller bubble sizes characteristic of dark beers is a key part of the equation. Bubbles want to go up because, being little balls of gas, they’re lighter than the surrounding liquid. But the upward drive is reduced if the bubble is smaller. If the liquid happens to be flowing downward, all it takes is for the liquid speed to exceed the bubble speed—and the bubble will sink as well. “Because of their small size, the bubbles in Guinness beer rise slowly and hence can be entrained by downward flow if the downward flow velocity exceeds the small velocity of rising bubbles,” Zhang and Xu wrote, reporting their But why would there be a downward flow? Benilov and colleagues believe they have unraveled that one. When a fizzy drink is poured, many bubbles form when the liquid hits the bottom of the glass. If we assume for simplicity’s sake that they form uniformly all over the bottom, then they would also rise in a uniform column throughout the drink, Benilov and colleagues explain in their paper, which is posted on Arxiv, an online physics research database. But factor, primarily, disrupts this uniformity, they say: they shape of the glass. The standard pint glass typically used for Guinness in bars is—like many glasses—narrow at the bottom, wider near the top. Since bubbles from the bottom rise approximately straight upward, then as the glass widens, the area near the walls finds itself with a shortage of bubbles compared to the middle of the glass. This thicker concentration of bubbles in the central axis of the glass has great consequences for the outlying bubbles near the edges, they claim: these fringe elements are pushed downward in order to allow their mainstream colleagues to head for the top. This is because “whichever way the bubbles move, they exert a drag force on the surrounding liquid,” carrying it with them to some degree, they explained. Of course, the whole sopping mass of liquid can’t just lift itself out of the glass just because of a lift offered by its bubbles. So if some bubbles do manage to push the liquid upward in their little part of the receptacle, that liquid must fall back down in another. Thus a current arises, Benilov and colleagues said: beer in the central column goes up, because there are more bubbles there. Beer near the sides goes down, because there are fewer bubbles there. The little, outlying orbs suffer the consequences as their upward struggle is more than counterbalanced by the downward speed of the liquid. These bubbles, of course, are the ones we see, especially in a freshly poured glass. This same logic dictates that if the glass is narrower at the top than the bottom, the bubbles near the edges should flow upward instead of downward, Benilov and colleagues said—which is exactly what happens. They designed an “anti-pint” glass that had the form of a standard pint glass turned upside-down. Lo and behold, the bubbles near the edge rose. The beer experiments are more than fun and games, Benilov and colleagues said. As Zhang and Zu wrote in their 2008 paper, the “Fizzics” of bubbles, as they dubbed it, is also relevant to understanding explosive volcano eruptions—as well as lake eruptions such as a 1986 disaster at Lake Nyos Cameroon. In that tragedy, 1,700 people suffocated after more than a million tons of carbon dioxide burst out of the lake.