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

Think about what paper is. It’s basically wood mush mixed with plastic and pressed into a flat shape.

Now we get it wet. The wood mush will absorb lots of water, whereas the plastic parts in it won’t. So when it’s left on a table to dry, the parts of it with more plastic will dry quicker than the parts that are more wood mush.

Now, the water molecules in the paper take up space. When these evaporate, that space is either left empty (the paper is damaged or torn) or it is filled with wood mush. In the second scenario, this leads to some parts of the paper stretching, and some parts constricting.
This is what causes the warp in the paper once it dries.