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; they represent a flow of fluid into the space behind the obstacle, and this inflow begins only when the general flow is fast enough to produce a lowered pressure there.’
Because a coffee cup is round and the south end of the cup is a stark endpoint to the prevailing direction of flow, two strong eddies form in it when you pour. If you pour exactly in line with the north-south axis, you will form two equal eddies, one on each side of the cup. Milk will circulate on both sides equally and will flow all the way back to a position behind the spout of the pitcher. At this point it will re-enter the main flow, just as circulating water in a river eddy reenters the primary flow of the river upstream of the obstruction. The spout behaves like an obstacle in a river.
Asymmetry in latte art designs can be explained by uneven eddies in the cup. If you position the pitcher closer to the east side of the cup, the eddy flow on the west side will dominate, receiving a higher quantity of the recirculating milk. This result can be seen particularly in the petals at the base of a latte art tulip, when one side wraps around farther than the other side.