well now, that explains it. thanx you maths/science.
Why do the bubbles in a glass of stout beer such as Guinness sink while the beer is settling, even though the bubbles are lighter than the surrounding liquid? That’s been a puzzling question until now, as a team of mathematicians from the University of Limerick has shown 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.