Counter-Rotation of Beer Foam

Synopsis

The effects of swirling a glass of water are intuitive; the water rotates with the glass, gyrating at a given frequency in the same direction as the applied force. However, when this same process is done on a liquid which is covered by a thin raft, such has the foam which rests on a glass of beer, the rotation of the raft can be seen to be opposite in direction. This unintuitive result, which can be seen the with naked eye in a plethora of situations, will be discussed in this seminar as well as a simple model proposed to explain the phenomenon.