Guinness

I saw a commercial today for the famous Irish beer, Guinness. This beer has a very unusual fluid property, the bubbles on the top of the beer sink. This is due to the flow of the bubbles from the bottom of the beer to the top of the beer. Due to the shear from the side of the glass, the bubbles on the outside are slowed and eventually begin to sink. In the center of the beer the bubbles are rising, you just can't see them. This creates a parabolic velocity profile and gives off the appearance that the bubbles in Guinness beer sink.