Falling Stout bubbles explained

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.

Now a study on the Arxiv server reports simulations and experiments showing the standard glass' shape is responsible. Many stout beers contain nitrogen as well as the carbon dioxide that is present in all beers.Because nitrogen is less likely to dissolve in liquid, that results in smaller and longer-lasting bubbles.

But it is the sinking bubble that has confounded physicists and mathematicians alike for decades.Like many such "fluid dynamics" problems, getting to the heart of the matter is no easy task; only recently was it proved they actually sinkrather than being the result of an optical illusion.

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. 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.

The same flow pattern occurs with other beers such as lagers, but the larger bubbles of carbon dioxide are less subject to that drag.