With St, Patricks day comming up i would like to pose to you out there the connundrum of why the bubbles in a pint of GUINNESS will float downwards while the cacade is taking place ? thanx
I wasn’t aware of this phenomenon, but I saw the other day that someone has figured it out. Seems that the larger bubbles rising in the center of the glass create a toroid-shaped circulation pattern, so that the current at the outer side of the glass is down. This carries the smaller bubbles, whose weight differential to surface area ratio is small, down with it.
Also, Guiness is so opaque that you can’t see the bubbles rising in the middle, you just see the little bubbles around the sides.
I don’t know about CurtC reply, but I spent many fun filled hours watching the bubbles and I came to the conclusion that the bubles are actually moving up. It appeared to me that the bubble free liquid between the bubbles moves downward creating the optical illusion that the bubbles were sinking…
Could’ve beent the fun filled hours talking though 
It has indeed been figured out: http://slashdot.org/science/00/01/11/2156213.shtml
unfortunately, the yahoo link in that story doesn’t appear to work anymore. But CurtC’s explanation is the same one I remember.