If you look closely at a glass of freshly poured Guiness (or any beer--but it's easier to see in a dark ale), you'll notice that some of the bubbles are actually falling instead of rising. Explain how this is possible.

(In reply to maybe by alin)

You thought the convection theory sounded too random?  Maybe this will help.

first due to viscosity of the beer and the drag on the glass, any currents caused by pouring should stop in short time. 

The initial tendency of any bubble in a denser liquid is going to be to rise.  However, bubbles in the center of the glass have less drag than the bubbles on the edge (think laminar flow).  So the bubbles in the middle move up creating the upward current of bubbles and beer. (the drag on the bubbles will cause the beer to rise with the bubbles)  Obviously all the beer can't go up so the downward current we observe is formed on the side of the glass. 

Also the bubbles in Guinnes do not appear to get as big as the ones in soda.  Maybe that is because they are nitrogen.  Anyway the smaller bubbles would get pushed down easier than larger bubbles.  (I'm not sure if Stoke's Law can be applied directly to gasses but it would be basically the same idea.) 

In other words, the bubbles would all be rising relative to the beer but the bubbles on the edge are caught in a current that is traveling down faster than the bubbles can rise.

The bubbles all eventually do end up at the top.

 

Posted by matt on 2005-01-23 03:37:46