Last updated June 18, 2018 at 10:30 am
Its fans agree that stout – a dark, thick, heavy form of beer – is a wonderful thing, but some have occasionally speculated that it might be miraculous as well.
Bubbles cascade downward through pint of Guinness. Some say it’s a miracle, but physics can explain. Credit: iStock
More observant drinkers will have noticed a peculiar and counterintuitive phenomenon when stout is poured into a classic pint glass designed by the Irish brewery Guinness.
When first poured, the beer forms an immediate and turbulent head of bubbles. At first, it occupies almost all of the glass, but then gradually resolves: the contents separating into a lower layer comprising mostly liquid, and an upper layer made up of froth.
It’s the behaviour of the bubbles left in the lower layer, however, that suggests something is going on in defiance of the laws of physics: they sink. This shouldn’t happen. The bubbles, being gaseous and therefore less dense than the surrounding liquid, should rise.
Yet they don’t.
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The mystery has long intrigued mathematician William Lee of the University of Huddersfield, UK.
Lee is arguably the world’s leading authority on the physics of stout, having previously published papers on the way bubbles form inside Guinness cans, and the dynamics of their foaming behaviours.
Now, after a long series of simulations, backed up by experiments, Lee and a couple of colleagues have finally found the answer.
In a paper published in the American Journal of Physics, the scientists use an approach called computational fluid dynamics to reveal that the cause of the downward trajectory of stout bubbles is the shape of the glass into which it is poured.
The “classic” Guinness pint – wide at the top, then tapering to a waist and narrow base about two-thirds of the way down – is designed to hasten the settling of the freshly poured drink. Lee and his colleagues, however, show that the shape also affects the circulation of fluid when it is poured in, setting up a system wherein the buoyancy of the bubbles is not sufficient to overcome the downwards flow of the settling drink.
Far from being a mere curiosity, the research has the potential to bring about a redesign of pub glassware. Given the proudly proletarian symbolism of Guinness-drinking, however, this seems unlikely.
“People think that the Guinness glass is designed to optimise the settling time,” says Lee.
“But now we have a better understanding of the theory behind it, we might be able to make an even better glass so that it settles faster. Unfortunately, the ideal shape would look like a giant cocktail glass!”
