See title. I’m thinking clothes, hair, paper, etc.
“Wet” reduces friction and allows fibers and such to slide past each other. Once the object dries, the fibers cling to each other and stay somewhat in the same shape and you get wrinkles and crinkles.
It’s slightly different for cloth, hair and paper though. Hairs are of course individual fibers so it’s a question of friction between the bits hairs are made up of. And you can crease paper when dry as well, but that involves bending stiff fibers and is not entirely reversible.
Two processes called stress decay/release and felting.
The core internal structure of a hair or wool fibre are alpha-helixes. If force is applied the helixes stretch.
How much the stress is elastic and returns to it’s original position depends on conditions; how much stress, the duration of the stress and conditions like temperature and moisture.
The balance of the stress is released through rearrangement of the fibres and fibre structures.
A wet fibre undergoes more and faster stress release more than a dry fibre, hot water more effective than cold, and steam more effective again. Which is why you wash woollen jumpers in cold water and do not tumble dry them, and why steam setting is used to obtain “permanent press”.
The other process, felting, is based on the external shape. Natural fibres are not smooth, in the diagram you can see the scales. These are run one way; effectively natural fibres are barbed.
You can test this yourself. Pinch a hair fibre in the middle using the index finger and thumb of both hands. Then try to slide each pinch out to the end. It will always go the same way. One pinch will stay in the middle of the strand, the other will slide to the end. The sliding goes in the opposite direction to the growth of the fibre.
So simply rubbing two fibres or merely having them move past each other causes them to interlock. Getting them wet increases the amount of interaction and the amount of stress release.
Given time you can by simply squashing a ball of woollen yarn repeatedly cause the fibres to become interlocked and become felt and eventually if continued to a form a block with the resistance of wood.
Thank you for that excellent explanation penultima thule.