Irish Mathematicians Solve The Guinness Sinking Bubble Problem Bubbles sink in Guinness because of the peculiar geometry of pint glasses, say a dedicated group of researchers at the University of Limerick

Over the last ten years or so, physicists have begun to pick this problem apart. Most recently they've shown that it is not the bubbles that sink but the liquid, which circulates in a way that is downwards near the glass walls and upwards in the interior. As long as the downward flow of the liquid is faster than the upward motion of the bubbles, they will appear to sink.
source: http://www.technologyreview.com/blog...