“When I read about the evils of drinking,” the comedian Henny Youngman once
said, “I gave up reading.” And why not? Who needs it? If you’re the patient sort
and can watch carefully, you will probably learn a lot more from studying a
beverage than a book, especially when it comes to physics—bubble physics,
that is. If you’re thirsting for an intellectual challenge this Christmas, then
head down to your local lab, order yourself a frothy pint of draught Guinness,
and prepare for a science lesson.
Guinness may look like the runoff from an old peat bog, but after more than
200 years of research by the Guinness Breweries, it really has something of the
quality of a high-performance motor oil. Draw a pint from the tap and the liquid
doesn’t just pour out—it explodes into the glass, filling it with a
churning brown froth. For a few moments the froth swirls through the glass,
driven by its own momentum. Then it begins to settle, and pure dark beer slides
to the bottom.
Defying gravity
It is another full minute before the maelstrom of beer really comes to rest,
but in the meantime, if you raise your glass and take a close look near the
boundary between the froth and the beer, you will see . . . well, that all
depends on your point of view. Some people see bubbles mocking the laws of
physics by plunging downwards into the depths, instead of heading unerringly for
the surface. But others claim that this is an illusion, caused by the peculiar
stripes or waves of dark liquid that course down through the beer like
sidewinding snakes. So what’s going on?
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“Gas breakout” is what the brewers call it, according to Ian McMurrough, a
former researcher at Guinness’s Park Royal Brewery in London. “It’s a topic in
which the people at Guinness have much expertise,” he says. Unfortunately, he
adds, “this falls into the area of trade secrets”, and Guinness isn’t giving any
away. To get some answers, New Scientist has been forced to make
repeated experiments of its own, and mull over the matter at length with
independent experts. As a result, we can now paint a plausible picture.
Some of the theory is fairly simple. Before it is poured, Guinness is held
under pressure to keep gases dissolved in the liquid. At the tap, however, “the
Guinness is forced at great pressure through six tiny holes”, says physicist and
amateur Guinness theorist Matthew Woods of Keele University. This trauma,
coupled with the pressure drop, brings the gas out. So bubbles form—lots
of them.
In other draught beers, the bubbles produced at the tap immediately rise to
form a head, but in Guinness, things are a bit more complicated. Karl Siebert,
of the Department of Food Science and Technology at Cornell University in
Geneva, New York, points out that whereas the gas in lagers and real ales is
carbon dioxide, Guinness has a mixture of carbon dioxide and nitrogen added. The
nitrogen makes Guinness bubbles smaller than the big bubbles of carbon dioxide
in ordinary beers, perhaps helping to give it such a stable head. Being smaller
they are also less buoyant and tend to rise more slowly, which accounts for the
beer’s ability to fill a glass so uniformly with froth.
When the liquid first falls into the glass, it flows about much faster than
the bubbles can rise. So instead of moving on their own, they behave as what
fluid dynamicists call “passive tracers”, and go wherever the liquid takes them.
In a few seconds the glass is full of liquid mixed with a dense cloud of
bubbles. Then, as the bubbles begin to rise slowly, the liquid percolates down
through the spaces between the bubbles and settles out below. This is what is
known as “two-phase flow”, a flow involving both gas and liquid.
In general, this kind of flow is so complicated, says Hein Auracher, an
expert in the area from the Technical University of Berlin, that the motion
cannot be predicted from theory. Still, Auracher suspects that bubbles near the
froth-liquid interface may be carried downwards by a large-scale liquid flow in
the glass. Chemical engineer John Smith of the University of Surrey agrees. “The
buoyancy forces within the bubble cloud will induce a circulation, up in the
centre and down at the walls.” What’s more, he claims that the downward flow at
the walls should be fast enough to drag the bubbles down with it.
So, in principle, bubbles can travel downwards in a glass. But is this what
happens in a pint of Guinness? Not everyone thinks so. One set of diving-bubble
doubters, chemist Richard Zare and his group at Stanford University, kindly
offered to help New Scientist investigate the mystery with some
experiments. “Breaking the Aristotelian tradition,” reports Zare, “several
members of my research group spent Friday afternoon and Friday evening
researching the rise and fall of beer bubbles in Guinness.” According to postdoc
Andrew Alexander, “We concluded that the bubbles aren’t going down, but that
waves of dark liquid which go downwards give this impression.”
Does that settle it?
Denis Weaire, a physicist at Trinity College, Dublin, comes to the same
conclusion: “The bubbles go up, and it is a wave of density fluctuation which
goes down.” As he is an Irish physicist, Weaire’s words must carry some extra
weight. “If anyone says they go down, then I say: where do they go?” Weaire
points out that similar waves appear in other situations. “The flow of fluids
can be unstable in many ways,” he says. “For example, when water runs down a
shallow slope, within a certain range of angles, it forms a beautiful series of
sharp waves.” A similar kind of spontaneous wave formation may be going on as
beer runs down the inside of the glass. “Another case closer to Guinness,”
Weaire adds, “is sedimentation. As mud settles out of a liquid (like bubbles,
but upside down) it often forms `striations’ just like the bubbles do.”
This may account for the waves, but it doesn’t nail down the ultimate
question: do the bubbles descend, or don’t they? “This is a very profound
question,” says Gad Hetsroni, an expert in two-phase flow from the Technion in
Israel, “which cannot be answered without a lot more details.” So it looks as if
you’ll have to decide for yourself this Christmas. Forget those last-minute
gifts, and take Hetsroni’s advice. “Go have another beer.” He adds just one
stipulation: “Make all your observations on the first glass only.”
