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I notice when I buy aluminium sheets (the kind used for wrapping food) they come in straight, smooth rolls. Once I use them though, they become wrinkled, and they are impossible to un-wrinkle. Image of a wrinkled sheet:

enter image description here

Why is this the case? Only thing I can think of is the Second Law of Thermodynamics, but I can't see why that would be applicable. It's still the same sheet of aluminium, after all, and it's not like it's getting mixed with something else (like when making coffee). Furthermore, the manufacturers are able to make it unwrinkled, and it doesn't become wrinkled until it's perturbed. I'm tagging the question with thermodynamics anyway, because I don't know what else it could be.

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    $\begingroup$ Rhetorical question: Why do plates dropped on the floor break easily but are hard to un-break? $\endgroup$ – David Hammen Sep 13 '20 at 10:44
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When the Aluminium sheet has been deformed beyond the elastic limit the bonds between the Aluminium atoms have been broken/deformed irreversibly.
This is called plastic deformation. So a small amount of deformation (elastic) will allow the Aluminium sheet to revert to its original condition when the deforming force is removed but large deformations (plastic) do not.

Aluminium sheet is produced by passing heated ingots of aluminium through rollers numerous times and then processing them as necessary to obtain the required properties.
The rolling process is irreversible but controlled to produce a smooth surface.

Aluminium is ductile ie has ability to be permanently deformed without fracturing and you will notice this property when you crease Aluminium sheet.

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  • $\begingroup$ Why would aluminium undergo plastic deformation though? It is a metal right - so it should have metallic bonds, which if I'm not mistaken are non-directional. $\endgroup$ – Allure Sep 13 '20 at 9:29
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    $\begingroup$ @Allure Whem Aluminium is rolled headrest neighbour atoms change. $\endgroup$ – Farcher Sep 15 '20 at 12:18

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