what changes in the structure of an object that allows something to permanently bend (i.e folding paper)
In: Physics
Interesting stuff. I love hearing the chemical explanations for things. On the physics end, we quantify the tensile limits to which a material may bend without permanent deformation with a quantity referred to as [Youngs Modulus](https://depts.washington.edu/matseed/mse_resources/Webpage/Biomaterials/young’s_modulus.htm). Similarly, an object’s ability to withstand “shearing” strain without snapping or breaking is quantified via its [Shear Modulus, aka its Modulus of Rigidity](https://en.wikipedia.org/wiki/Shear_modulus)
Civil engineer here. Besides what’s happening on the molecular and cellulose level, there is also something called Young’s Modulus, which is a ratio of the stress exerted on a material (in terms of force, such as Newtons or lb/ft^2 or kips) vs the strain (change in L or A per original dimensions). All solid materials have this characteristic. For paper, it is very, very, very low, so that humans can rip it easily or whatever. When you bend paper slightly, it will go back into place. This is the plasticity index, and it indicates the threshold before which the material will return to its original form. Again, paper’s super weak, so it’s practically nonexistent, but steel works the exact same way just with much stronger molecular and physical bonds. Upon surpassing the plasticity index, the material can no longer return to its original form. It is therefore “deformed” in whichever position it was put into, and that’s considered a fold. This is a very tangential connection but is nonetheless a phenomena that occurs as a result of the various qualities of paper that make it the way it is, and explains from the physical perspective why creases happen
Edit: forgot about strain
And thanks for the silver!!
Let’s use paper from your example to explain this.
Paper is not one solid, contiguous thing on a microscopic level. Paper is really made from layers and layers of interlocking plant fibers. Those fibers are made of layers and layers of interlocking cellulose molecules, which [look like this, more or less](https://upload.wikimedia.org/wikipedia/commons/f/f9/Cellulose-Ibeta-from-xtal-2002-3D-balls.png).
Those cellulose molecules form fibers because sometimes hydrogen (the white balls) on the outer side of one strand of cellulose will bond to an oxygen (the red balls) on a neighboring strand. The fibers form the paper due to the process in which the paper’s made leaving them physically interlocked, and some of the hydrogen bonding between fibers. It’s a very weak bond, which is why paper’s so easy to tear and bend.
~~As to why it stays bent, though. As you bend paper, it requires you put energy into the act, and that energy breaks some of the hydrogen bonds, changing the orientation of some of the fibers within the paper, which then form new hydrogen bonds.~~EDIT: It was pointed out that what I said wasn’t quite correct. Creasing *does* break some of the fibers, and that does add up over time.
So when you’re bending the paper, you’re changing its structure at a microscopic level.
Solid materials are made up of tightly packed molecules, which is the most energy efficient way to be in. If you bend something, this structure is changed to a less energy efficient form.
The molecules are moving within the material, so when you hold it long enough, they will eventually reach the energy efficient state again, but now in the new shape.
The time and force it takes to achieve this differs for each material.
Sorry, I just want to make sure you’re clear on the use of “i.e.”
i.e. = “id est” (Latin), meaning “that is” or “in other words”. So your question reads “what… allows something to permanently bend, specifically paper”
e.g. = “exempli gratia”, meaning “for example”. So your question using “e.g.” instead would read “what… allows something to permanently bend, such as (but not limited to) paper”
It’s a subtle difference, but it changes how specific your question is, which might change how specific the answers you get are.
Another user already explained it using your example of paper. I’ll draw from my experience in mechanical engineering.
Take a bar of plain low carbon steel that’s long enough and narrow enough that anyone can bend it. Apply just a small force enough to bend it a little then let go. It bounces back to its original shape. Apply a larger and larger force and eventually it actually *bends and stays bent*. Why?
Well a ductile material like mild steel has a crystalline structure at the microscopic level whose atoms are arranged in a way that they can deform and shift ever so slightly. Their atomic bonds are still strong enough to pull them back to their original configuration. This is called ***elastic deformation*** and all metals and their alloys (and also non-metals) will have a particular ratio (i.e. Young’s Modulus a.k.a. the modulus of elasticity) of force applied to the amount they can deform or elongate.
Why does it stay bent after so much force? There’s a proportional limit for ductile materials beyond which that material begins to “yield” or permanently deform. What happens here is that once a certain stress (force applied/distributed over an area) is reached the interatomic bonds in the crystalline structure begin to break and reform new bonds in new shapes of crystals. This behavior is called ***plastic deformation*** (also permanent deformation).
This is irreversible unless one were to heat the metal above a certain point to “reset” it. If you were to try bending it back and forth the location at which it bent will begin to harden and eventually break.
This is easily demonstrated by unfolding a paperclip and bending it back and forth.
Hope that helped further your understanding!
You are breaking the structure. You create a compound on the one side and s tear on the other. Making that state it’s new state. And it will now remain folded or be able to reverse.
Probably late to the game, but gonna try a proper maybe… ELI7?
If you zoom in smaller and smaller things are made of billions of tiny atoms that are basically little balls that are stuck together. When 2 atoms are stuck together we say they’re bonded. The sticking is a bit like how magnets stick together – they’re attracted to each other, but you can still pull them apart, breaking the bond. Atoms stick themselves together into large structures, and sometimes these structures make even bigger structures – like how a chocolate bar is made of collections of chocolate that’s bonded to rice crispies etc.. For solid objects in order to stay the same shape, the atoms can’t move around – the bonds stay the same… Unless…
If you push hard enough, just like pulling magnets hard enough, you can break the bonds and start to move the atoms around. If after you stop pushing the atoms, they can’t move back to where they were before, then the material will permanently change shape.
In the specific case of paper, atoms make molecules called proteins, that form weak bonds to other proteins and these form fibres that in turn bond together with weak bonds and that makes paper! Folding paper in half, some fibres will slide over eachother in order to change the shape, but they can’t slide back, so the shape change is permanent.
Elastic deformation i.e. when it springs back is a little more complicated as things like rubber achieve it in a different way to metal for example.
Feel free to ask any questions / query stuff.
E: just to add some credibility to my answer, I have a Masters in Materials Science.
Paper is just many many tiny wood slivers. Now what happens when you bend a big sliver? It breaks but usually stays intact. That’s what’s happening when you bend paper. Just lots and lots of broken slivers.
Every material has what’s called an elastic limit. When you stretch/strain a material past this limit the deformation stops being reversible, a *plastic deformation*. If you look up a stress vs strain graph the linear part at the beginning is the elastic part.
When you stretch something into the plastic zone and let go of it, before a certain point of stress it will shrink back and recover the same amount it would if you held it at the elastic limit, it just wont go all the way back to normal
For paper this limit is probably really low and the cellular thing that happens with folding is explained better in here, you don’t have to actually fold paper to get it to have a permanent deformation though
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