It seems that some at least of the bubbles in Guinness do indeed go down. The really little ones. And now I have a rational explanation. Probably long after everyone else, but that is the way of things sometimes.
I never believed it, because I did not have a good scientific explanation. I thought it was likely to be an optical illusion. I (quite rightly) refused to accept those “Did you know” circular emails as valid evidence. I wanted a reference. Now I have one, dating from 2000. It makes sense. In fact, I should have figured it out myself.
Fair enough. In a nutshell:
Professor Clive Fletcher and students at the University of New South Wales, Sydney, Australia, used FLUENT Inc. computational fluid dynamics software to show how bubbles can both go up and down in a glass of Guinness. As you might expect, most bubbles do move upwards. The bubbles in the center of the glass, move upwards most quickly and lift liquid with them. The liquid moving up must circulate downward again, and tends to go down the wall of the glass. As it does so, it will tend to drag bubbles down with it. Larger bubbles have sufficient buoyancy to resist the current, and float up. Smaller bubbles are carried by the current to the bottom of the glass, where they join the rising column in the centre.