Cold case solved? Study probes riddle of sinking
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.
“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
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,
“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.
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
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.
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