Wednesday, May 30, 2012

Why DO Guinness bubbles sink?

Apparently, the Irish have been thinking about this problem for a while, and a team of Irish mathematicians have discovered the answer. Via PhysOrg:
that the sinking bubbles result from the shape of a pint glass, which narrows downwards and causes a circulation pattern that drives both fluid and bubbles downwards at the wall of the glass. So it’s not just the bubbles themselves that are sinking (in fact, they're still trying to rise), but the entire fluid is sinking and pulling the bubbles down with it.

...[S]tout beers such as Guinness foam due to a combination of carbon dioxide and nitrogen bubbles, while other beers foam due only to carbon dioxide bubbles. The nitrogen results in a less bitter taste, a creamy long-lasting head, and smaller bubbles that sink while the beer is settling. ... During the past decade, experiments have shown that the phenomenon of sinking bubbles is real and not an optical illusion, and simulations have demonstrated the existence of a downward flow near the wall of the glass and an implied upward flow in the middle. But this is the first time that researchers have shown that the mechanism of this circulation pattern depends on the shape of the glass.
How did they test this idea? Well, they made a computer program that would create an "anti-pint glass"; a glass that is shaped like an upside-down Guinness pint (i.e., it looks like a pint glass turned upside-down, but with what would normally be the lip being closed, and the base open):
To analyze the effect of different glass shapes, the mathematicians modeled Guinness beer containing randomly distributed bubbles in both a pint glass and an anti-pint glass (i.e., an upside-down pint). An elongated swirling vortex forms in both glasses, but in the anti-pint glass the vortex rotates in the opposite direction, causing an upward flow of fluid and bubbles near the wall of the glass.

The researchers explain that the difference arises from the way the sloping glass walls affect the surrounding bubble density. Once a drink is poured, bubbles start to rise. In the typical pint glass, the bubbles move away from the upward and outward sloping wall as they rise, resulting in a much denser region of fluid next to the wall, with fewer bubbles. Because this region is less buoyant, it sinks under its own gravity. Although the nearby bubbles are still trying to rise, the velocity of the downward flow exceeds the upward velocity of the bubbles, so the bubbles that are close enough to the wall get pulled down by the surrounding liquid.

The opposite effect happens in an anti-pint glass, where bubbles tend to clump more near the oppositely sloped wall as they rise. The increase in bubbles results in a less dense region next to the wall, and fluid near the wall moves upwards.
And so now you know: it's the shape of the glass; if your glass was curving inward, you'd have bubbles rising along the outside of the glass (I guess it would be something to try if drinking out of wine glasses).

If you want to (try) and read the paper, here it is:
  • E. S. Benilov, et al. "Why do bubbles in Guinness sink?" arXiv:1205.5233v1 [physics.flu-dyn]

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