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.

Several have said that there are convections in the beer, and I think this is correct.  As for the cause of the convections, yes the convections from the pouring could be a cause of this. 

Another cause of convection is the rapid upward movement of huge numbers of bubbles, each carrying some liquid along for the ride.  So somewhere else in the glass, there must be liquid flowing downward.  If the cross sectional area of the down moving regions is fairly small, then the velocity of beer moving downward would be fast enough to carry some bubbles downward.

In addition, I think there may be a density gradient, temporarily, due to an uneven distribution of bubbles in a somewhat viscous liquid.  A deeper region of low density, with many bubbles, moves upward en mass, each bubble carrying some liquid along with it.  Compared to this, a region of beer with just a few bubbles has more density and will fall.

Also, as bubbles rise, they increase in size.  In water, the volume of a bubble 33 feet deep will double when it reaches the surface.  As a bubble rises from the bottom of a glass, it might increase in volume by 2 or 3 percent, displacing some fluid and creating more convection.

Posted by Larry on 2005-01-12 21:14:34