Eddies
Have you ever stood by a fast-flowing river and witnessed an unexpected reversal in some of its direction of flow? In white-water rapids, this usually occurs after the main current passes a large boulder in the water. The main body of the water hurtles downstream, but some of the water recirculates, only to reenter the flow behind the obstacle. This recirculation is called an eddy flow. In situations of high flow rate, the high liquid content and slow drainage rates of steamed whole milk make it subject to similar forces.
The Encyclopaedia Britannica offers this explanation of eddies: ‘In the lee of an obstacle, eddies form only when the flow around the obstacle reaches a critical velocity;