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As the title says. It is common sense that sharp things cut, but how do they work at the atomical level?

Chris Mueller
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wtoh
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    My guess: to cut something, you need to break the chemical bonds, and therefore bring more energy than the binding energies. If you use a sharp blade, you concentrate the energy you bring on "a few" chemical bonds, and it's easier to break them. – anderstood Sep 05 '14 at 18:15
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    A normal knife doesn't "cut", at all, at the atomic level. It simply puts so much pressure on the material locally, that it breaks or tears. Having said that, the physical explanation for what happens in detail when materials break is complicated and not fully understood, yet, so your question is perfectly valid. Actually, if you wanted to, you could make a career out of it as a solid state physicist or material scientist, because there is great importance in having materials that are harder to break or tear! – CuriousOne Sep 05 '14 at 18:41
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    What CuriousOne said. At the atomic level, you can "break" stuff apart with lasers, magents and chemical reactions, but not with blades. – Thermo's Second Law Sep 05 '14 at 19:54
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    @CuriousOne: not just that, but also stuff that breaks and tears in predictable ways. – Zo the Relativist Sep 05 '14 at 20:59
  • @JerrySchirmer: I agree. There is a world of possibilities out there to modify the surface and bulk of materials in ways that make them behave very different from what we are used to. – CuriousOne Sep 05 '14 at 21:03
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    This is really a duplicate of What happens when we cut objects?, but lemon's answer is so much better than any of the answers to the previous question that I'm reluctant to vote to close. – John Rennie Sep 06 '14 at 10:10
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    @JohnRennie Maybe close the other one as a duplicate of this one, then? Then, anyone who comes across the other question will be directed to the better answer here. – David Richerby Sep 07 '14 at 20:28

3 Answers3

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For organic matter, such as bread and human skin, cutting is a straightforward process because cells/tissues/proteins/etc can be broken apart with relatively little energy. This is because organic matter is much more flexible and the molecules bind through weak intermolecular interactions such as hydrogen bonding and van der Waals forces.

For inorganic matter, however, it's much more complicated. It can be studied experimentally, e.g. via nanoindentation+AFM experiments, but much of the insight we have actually comes from computer simulations.

For instance, here is an image taken from a molecular dynamics study where they cut copper (blue) with different shaped blades (red):

enter image description here

In each case the blade penetrates the right side of the block and is dragged to the left. You can see the atoms amorphise in the immediate vicinity due to the high pressure and then deform around the blade. This is a basic answer to your question.

But there are some more complicated mechanisms at play. For a material to deform it must be able to generate dislocations that can then propagate through the material. Here is a much larger-scale ($10^7$ atoms) molecular dynamics simulation of a blade being dragged (to the left) along the surface of copper. The blue regions show the dislocations:

enter image description here

That blue ring that travels through the bulk along [10-1] is a dislocation loop.

If these dislocations encounter a grain boundary then it takes more energy to move them which makes the material harder. For this reason, many materials (such as metals, which are soft) are intentionally manufactured to be grainy.

There can also be some rather exotic mechanisms involved. Here is an image from a recent Nature paper in which a nano-tip is forced into calcite (a very hard but brittle material):

enter image description here

What's really interesting about it is that, initially, crystal twins form (visible in Stage 1) in order to dissipate the energy - this involves layers of the crystal changing their orientation to accommodate the strain - before cracking and ultimately amorphising.

In short: it's complicated but very interesting!

lemon
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    These are excellent examples of why "cutting" is a complicated process on the atomic level. Thanks for the images, I hadn't seen those yet, but they are very instructive for the level of difficulty of this research. – CuriousOne Sep 05 '14 at 21:04
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    Great answer, especially since it's your first one! – Brandon Enright Sep 05 '14 at 21:33
  • "For organic matter, such as bread and human skin, cutting is a simple process because the energy required to break cells/tissues/proteins/etc apart is much less than that required to break atomic bonds." Could you give an insight of what's happening then? And why does being organic make it different? – anderstood Sep 05 '14 at 22:32
  • @anderstood I've added a sentence but I can't really say much more than that without speculating - sorry. – lemon Sep 05 '14 at 22:49
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    OK thanks. So in the case of organic matter, one actually breaks the binds, which is possible because they are quite weak, is that it? And for inorganic matter it too hard to break so other phenomena appear (dislocations, change of shape, etc.)? (just wondering) – anderstood Sep 05 '14 at 23:05
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    @anderstood That's exactly right. An extra tidbit: if you add organic molecules to an inorganic crystal (to create an organic-inorganic hybrid - a very important class of nanomaterial) then you typically make the material softer because, after all, 'a chain is only as strong as its weakest link'. Although very little is known about the actual atomic mechanisms involved in deforming such hybrid materials. – lemon Sep 05 '14 at 23:23
  • "For organic matter, [...] cutting is a simple process because the energy required to break [...] apart is much less than that required to break atomic bonds." Question: cutting in organic matter is not "breaking of atomic bonds?" – André Chalella Sep 06 '14 at 00:04
  • @AndréNeves You are right - in my head I meant strong chemical bonds. I have corrected this, thanks. – lemon Sep 06 '14 at 01:03
  • @lemon if you make any more edits to this answer, let me suggest consolidating them: don't make an edit just to add or remove a word or two, but save up your changes for a day or so and then make them all at once. – David Z Sep 06 '14 at 14:58
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    Sorry if this is a stupid question: in the first set of photoes (blue material red knife), is the knife going through the plane of the computer screen, moving from right to left, or going vertically down? – user13267 Sep 06 '14 at 22:33
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    @user13267 in each case the incision is made on the right side of the block and the blade is being dragged from right to left. – lemon Sep 06 '14 at 22:58
  • I had the same question as @user13267, so when you follow David's advice and write out your edits, I suggest including that information. – KRyan Sep 07 '14 at 13:47
  • Is this Randall Monroe? :) – RLH Sep 08 '14 at 12:36
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    Holy molly what a great answer. This is why I love this website. – John Alexiou Sep 08 '14 at 13:49
  • You say "For this reason, many materials (such as metals, which are soft) are intentionally manufactured to be grainy." Do you mean this literally - i.e. are the manufacturers aware of the theory - or has is just grown as the best solution empirically? – Nikolaj-K Feb 26 '15 at 08:48
  • @NikolajK A bit of both actually. Originally, a number of processes were found empirically to produce stronger metals and were only later rationalised to be a result of decreased grain size (ancient Japanese swordsmithing provides a wonderful example of this). But now that the mechanisms are understood, a significant portion of materials research focuses specifically on developing new, and refining old, methods for decreasing grain size further. – lemon Feb 26 '15 at 09:42
  • Did you just call hydrogen bonds weak? – Anurag Baundwal Jun 07 '18 at 09:05
  • @AnuragBaundwal Sure. Thermal vibrations at room temperature are enough to break a hydrogen bond on a time-scale no greater than nanoseconds. – lemon Jun 07 '18 at 11:39
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It depends on what's being cut.

When metal is cut, what happens is that, on a small or not so small scale, it shears. That means layers slide over each other. The mechanism by which they slide over each other is that there are imperfections in the crystal structure called dislocations, and the crystal layers can move by making the dislocations move in the other direction.

You can visualize this with a zipper on a jacket. Suppose the zipper is all zipped up, except for a little bulge where N teeth on one side and N+1 teeth on the other side are not locked together, and suppose this bulge can be moved, by locking teeth together at one end while separating them at the other end.

If the bulge is allowed to travel the entire length of the zipper, then teeth that were originally locked together are now locked with the neighboring tooth. That's how layers in a crystal can slide over each other - by the little bulges traveling fast in the other direction.

A way to make a metal (or any crystalline material) hard, and thus resistant to cutting, is to arrange it so it either has no dislocations, or the dislocations it has are "pinned" so they cannot move.

Mike Dunlavey
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A sharp knife is still several molecules thick on the edge; dull blades are even wider. So when you attempt to cut material, it needs to be ripped apart. As explained in other answers, the material either fractures along faults in the lattice, or you separate molecules (as when you cut bread).

The only materials where you might split chemical bonds are vulcanized rubber and polymers. In theory, a mining truck tire is one molecule.

LDC3
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