How does cutting through material work on an atomic scale?

pedicabdriver said:
I understand the whole concept of pressure being focused on a very small area on a small scale, but how does cutting through stuff happen on a molecular level? Are the bonds between atoms being broken? My head hurts thinking about it. :confused:
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When I first read this I thought..Dang must be some good stuff.:D

I'll tell you, this is a Great forum, but most of you all already know that. Friends would ask why I would want to talk about knives, how much can you really talk about...umm were talking about cutting atoms ( not really ). But defiantly deep thinking. Cool thread, I love science and am pretty good with most things but I have no answer for this one. Caught me off guard. But I will defiantly read your responses to expand my knowledge. Thanks.

I'll go with Cynic2701 responce..well said, I like laymen terms.
 
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Cynic2701 said:
Imagine one of those indoor parks that kids play in. Specifically, think of the small pool filled with small plastic balls.

At the molecular level things behave "kind of" (emphasis on "kind of") like this. The balls settle into a regular shape and volume due to gravity. The same with molecules and atoms--they are always moving toward a "position" of stability.

Now, imagine coating those play balls with something sticky--maybe honey or molasses. This would act somewhat like the inter-molecular forces that keep molecules together.

A giant wedge made up of similar play balls, but held together with superglue, is pushed into the pool of play balls. The balls held together by honey slide apart, pushed out of the way by the "harder" superglued wedge of balls.

This is something like what cutting at the molecular level would be like.
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I like it.

To be more realistic you simply have to think larger. Much larger. If atoms are the size of those balls I don't even want to go on record as to how wide I think the edge of the blade would be, but suffice it to say that it would be a lot wider than any kid's ball pool I have ever seen.
 
I would be willing to bet that with long polymer molecules you are indeed breaking covalent bonds, not just separating molecules that are weakly attracted through electrostatic forces, London forces, van der Waal forces, etc. Synthetic polymers are often made up of large molecules.
 
If I recall correctly, the radius of an iron atom is about 140 picometers (+ or - about 5 picometers). Vanadium is about 135. I'm not sure what size the carbides formed would be, but I'd guess somewhere in the neighborhood of 350 picometers.

The grit size of whatever you use to sharpen will equate approximately to the size of the thinnest possible leading edge you can produce.

I'd be willing to guess that the edge on a sharp knife (perhaps hair-popping sharp) is around 3 microns wide. Hair whittling is probably around or below 1 micron in size.

For comparison, there are 1 million picometers in a micron.
 
Ok, so I'm on board with the separating clusters of molecules or even splitting a molecule here and there while separating them but how do you equate hardness levels into this mix? i.e. not being able to cut through a piece of granite with a knife. Is it just that the electromagnetic bond is too strong and it's not the mineral crystals (silica, horneblende micas, feldspar) just being harder than the steel?
 
The Rockwell hardness scale is a measurement of the amount of force a given substance can resist from a focused load. I believe it is a 120 degree conical piece of diamond. The higher the hardness, the correspondingly larger amount of force needed to leave an indentation.

Lets return to the plastic ball model. That superglued plastic ball wedge was held together only by a little bit of glue; say, one or two drops at each point of contact. It's certainly held together very well - in comparison to the honeyed plastic balls - but lets imagine another set of plastic balls.

These plastic balls are smaller, you can pack more of them into the same volume, as compared to the regular plastic balls. Lets call them marbles just for the ease of size relation.

Coat these marbles with even more superglue, a liberal amount of superglue for each contact point. This mass of balls is going to be held together much better than the superglued plastic balls.

Now, using the superglued plastic ball wedge (SGPBW) try and and "cut" through this mass of superglued marbles (MSGM).

It's just not going to happen. The "bonds" between the plastic balls are going to be much weaker than the "bonds" between the marbles.

Say, though, we decided to drop both the SGPBW and MSGM. The marbles are likely going to break apart from the impact. The SGPBW probably won't, since it is a little more elastic on impact. This is a model for toughness vs. hardness.

The stronger the "bond" the more energy is dumped directly into the substance. The weaker the "bonds", the less energy is translated into the system as a whole--the energy breaks up some of the bonds, instead of being translated across the entire substance. For example, would you rather punch a punching bag filled with marbles that had been glued together, or a punching bag filled with marbles coated with honey?
 
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Reeek said:
Ok, so I'm on board with the separating clusters of molecules or even splitting a molecule here and there while separating them but how do you equate hardness levels into this mix? i.e. not being able to cut through a piece of granite with a knife. Is it just that the electromagnetic bond is too strong and it's not the mineral crystals (silica, horneblende micas, feldspar) just being harder than the steel?
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I think the point is, 'hardness' (& strength, toughness, abrasion resistance, etc.) of a material is directly related to the relative strength of the electromagnetic bonds between the atoms/molecules of that particular material. The stronger the bond, the harder/tougher/stronger the material.

At least that's how I understand it. :p
 
I think I should re-work the analogy a bit to make it better. I'm revising my second post in this thread.

If you replace the superglue on the plastic balls with Elmer's glue, the difference in "bond" strength becomes more apparent.

The Elmer's glued plastic ball wedge will "cut" through the honeyed plastic balls fairly easily.

Lets add another plastic ball substance to be cut. This time use superglue on the plastic balls.

Now, using the Elmer's glued plastic ball wedge (EGPBW) try and and "cut" through this mass of superglued plastic balls (SGPB).

It's just not going to happen. The "bonds" between the EGPBW (which is Elmer's) are going to be much weaker than the "bonds" between the SGPB (superglue)
 
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