The surface of heated milk is a pretty active region. The skin is a film of milk fats, proteins and sugars congealed and dried out, floating on the surface. Below there are vigorous patterns of convection in the hot fluid, and above there is vigorous air convection, with relatively cold, dry air passing over the skin and rippling it.

The skin also contracts according to where it has dried or cooled most, and expands wherever it begins to absorb more moisture than it loses. This in turn depends partly on where it contains a less continuous skin of fat, and more protein. Contraction and expansion of the skin causes it to form strongly-oriented rippling. The motion of the liquid beneath the skin causes some wrinkles, and the air movements above interact with the wrinkles to cause different wrinkles. By gently fanning the surface you can see certain effects when the skin is still very delicate, but once it has become thicker and more leathery (and deliciously chewy) its patterns set and no longer change much.

As for me, I am more of a custard skin man.

Cheers,

Jon