Falling stout bubbles explained
By Jason Palmer
Science and technology reporter, BBC News
Irish mathematicians may have solved the mystery of why bubbles in stout beers such as Guinness sink: it may simply be down to the glass.
Simulations suggest an upward flow at the glass’s centre and a downward flow at its edges in which the liquid carried the bubbles down with it.
But the reasons behind this flow pattern remained a mystery.
“If you imagine your pint is full of bubbles, then the bubbles will start to rise,” Dr Lee said.
But the bubbles in a standard pint glass find themselves in a different environment as they rise straight up.
“Because of the sloping wall of the pint, the bubbles are moving away from the wall, which means you’re getting a much denser region next to the wall,” Dr Lee explained.
“That is going to sink under its own gravity, because it’s less buoyant, and that sinking fluid will pull the bubbles down.”
The bubbles, that is, are “trying” to rise, but the circulation that creates drives fluid down at the wall of the glass.
“You’ll see sinking bubbles not because the bubbles themselves are sinking, but because the fluid is and it’s pulling them down with it.”
The same flow pattern occurs with other beers such as lagers, but the larger bubbles of carbon dioxide are less subject to that drag.
Calculations show that a glass shaped “upside-down” would exhibit the opposite effect on bubbles
Why do bubbles in Guinness sink?
(Submitted on 23 May 2012)
Stout beers show the counter-intuitive phenomena of sinking bubbles while the beer is settling. Previous research suggests that this phenomena is due the small size of the bubbles in these beers and the presence of a circulatory current, directed downwards near the side of the wall and upwards in the interior of the glass. The mechanism by which such a circulation is established and the conditions under which it will occur has not been clarified. In this paper, we demonstrate using simulations and experiment that the flow in a glass of stout depends on the shape of the glass. If it narrows downwards (as the traditional stout glass, the pint, does), the flow is directed downwards near the wall and upwards in the interior and sinking bubbles will be observed. If the container widens downwards, the flow is opposite to that described above and only rising bubbles will be seen.