Milk that has been heated and left to stand often develops a film that can be lifted with a spoon or chopstick. This is not simply fat rising to the top. Heat unfolds some whey proteins and allows them to interact with other proteins. At the same time, water evaporates from the surface, concentrating proteins and fat at the boundary between air and liquid until they join into a film.
The skin forms at the top because two conditions meet there: water escapes most readily, while proteins can collect at the interface. The film becomes more noticeable as the milk cools. If it is removed while the milk remains warm and continues to evaporate, another skin may form.
Milk skin should be distinguished from two other changes. In non-homogenised milk, larger fat globules rise during standing and produce a cream layer. When acid or microbial activity curdles milk, changing acidity destabilises casein. The film on heated milk is produced mainly by heating and concentration at the surface; it does not by itself mean that the milk has spoiled.
Milk skin is therefore a visible piece of food chemistry and physics: heating changes proteins, evaporation changes the local concentration, and the surface fixes those changes into a film.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5355583/
https://www.dairy.com.au/you-ask-we-answer/what-is-the-cause-of-lumps-in-milk-when-its-not-curdled
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