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Subject: Cold case solved? Study probes riddle of sinking beer bubbles

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.

 

“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.