When I use liquid glue to stick one sheet of paper on the other then why it's surface becomes zig-zag/deformed (I couldn't find very good word for it).
First I thought that it's because of the air between two sheets but it happens even if I remove all the air between the two surfaces.
Sorry if it's very silly question as I couldn't find it anywhere on the internet.
This is not the very best picture but it is very similar to it
In the process of its manufacture, paper gets stretched anisotropically (more stretch in some directions than others) while at the same time the fibers that make up the sheet of paper get aligned with their long axies parallel to the direction the sheet had while moving through all the rollers, dryers, etc. in the paper factory. Then, after all that took place, the paper is baked to set up the glue that was mixed into the fibers to add strength to the sheet.
But that glue is soluble in water, and when the sheet gets wet it is allowed to relax and unstretch (more in some directions than in others) as well as swell up as the cellulose fibers rehydrate themselves.
This makes a wet sheet of paper do a variety of unexpected things which make it get corrugations, bulges, creases, buckling, and so on as the fibers are allowed to relieve their internal stresses. By the time the paper has dried out, it has "forgotten" its flatness.
