When paper gets wet, why does it dry wrinkly instead of flat?

Question

When paper gets wet, why does it dry wrinkly instead of flat?

In: Physics

Answer 1

The fibers in the paper swell when they absorb water, but the fibers are not all the same size, so the swelling is not uniform. This means the paper deforms because the surface area of some fibers has changed relative to other connected fibers that did not change as much (or did not change at all).