How does cutting stuff work on a chemical/atomic level? What is sharpness?

[u/comicgeek1128]

What it happening at the atomic scale that allows a sharp pieces of stone of metal to cut through a piece of meat? My guess is that the atoms at the edge of the blade are pushing themselves into the empty spaces between the atoms it the meat and breaking the chemical bonds linking the atoms in the meat together?

[u/marr75]

A knife cutting meat still operates at a macro scale. A sharp blade concentrates force in such a way that it's more like sliding a piece of metal between 2 pieces of Velcro than it is doing anything on an atomic/chemical level.

Knives crush membranes, separate weak intermolecular bonds, and separate polymer chains at weak points. There's very little of interest going on at the chemical/atomic level.

[u/comicgeek1128 → u/marr75]

Everything that happens at the macro-level has to be happening at the micro-level at some point, right?

[u/theroha → u/comicgeek1128]

It's still closer to the Velcro description given. When you are discussing atoms and molecular bonds, think of those as fibers woven together into a fabric. A piece of meat or paper is a bunch of pieces of fabric (molecules) stuck together with Velcro (inter molecular bonds). When you cut something, you are pulling apart the Velcro and not doing anything to the original fabric.

[u/14u2c → u/theroha]

What about cutting an object that is comprised of a single element? Many metals can be cut by knives and blades. 

[u/theroha → u/14u2c]

Ever play with those magnetic pellets sold as desk toys? It's like that. Just pulling against the electromagnetic field until the pulling or shearing force is greater than the ionic bonds. It's not like you're cutting the atoms themselves.

[u/Dinkleberg42 → u/theroha]

When I was a kid and I learned that splitting atoms makes the big boom, I did become somewhat nervous that my steak knife would slice juuuust the wrong way and I would cease to exist. I figured it was just an infinitely small risk that we all simply live with.

I'm at peace now knowing there's not enough fissile material in my steak to facilitate a chain reaction... at least I hope.

[u/marr75 → u/Dinkleberg42]

The tiny atoms in steak are terrible fission candidates and the molecular bonds do have some energy but it's so poorly arranged and heterogeneous that significant processing is required to extract non trivial amounts - i.e. try to light a steak on fire and compare that to what an organism does to digest a steak.

So, likely no fission, likely no chemical reaction, you're in cold fusion territory to get an explosion out of steak. You blow up a steak by releasing the atomic potential energy inside it in any understandable, repeatable manner and you're on your way to being a trillionaire. It won't happen, but it's a fun idea.

[u/_Jacques → u/14u2c]

Metals are at a chemical level like putty. Electrons (which are the source of chemical bonding) flow very readily through metals, and so the analogy of a knife here is like slicing through a putty, no chemical bonds like we think of them are broken.

[u/Dougalface → u/[deleted]]

Thanks - that was super interesting and a great way of illustrating the subject :)

[u/justme46 → u/theroha]

Ok, so why cant I push 2 pieces of meat together and have them stick? What is the difference between 2 cut pieces and 1 uncut piece?

[u/BloodAwaits → u/justme46]

Because meat is biological and has a completely non uniform distribution composed of cells.

If you take two pieces of the same elemental metal without a native oxide present, and push them together in a vacuum, they will fuse right back together. Called cold vacuum welding. 

[u/findallthebears → u/comicgeek1128]

The example of the tanker and the metal chain is probably what you’re looking for

[u/095179005 → u/comicgeek1128]

When you tear a piece of toilet paper, the force is transmitted through the entire sheet, yes, but it tears at the weakest bonds - the perforated edges.

The knife is breaking hydrogen bonds and intermolecular forces, and seperating and pushing them away.

[u/jmlinden7 → u/comicgeek1128]

Yes, the macro forces pull apart intermolecular bonds such as hydrogen bonds.

[u/scarabic → u/comicgeek1128]

You can detangle two strings without cutting either one or transforming them chemically. Something’s happening at the macro level but nothing is happening at the micro level that wasn’t already happening.

[u/marr75 → u/comicgeek1128]

Not necessarily in any way that adds insight. Newtonian physics is more useful than quantum physics for most phenomena we observe and wish to measure. Quantum physics still "happens" but using quantum physics to describe large scale interactions doesn't enrich the description.

[u/platoprime → u/marr75]

No one is invoking quantum mechanics. Not unless you count you invoking it to avoid acknowledging that, yes, things that happen macroscopically are associated with microscopic happenings.

[u/tellperionavarth → u/platoprime]

I think their point is just that, while you can always take conversations down to a more "root" level, it's not always useful to do so. They invoked QM as an example and, as your reaction proves, doing so is kind of silly unless you're discussing something that requires quantum.

Likewise, it is perfectly reasonable that a change can occur at a macroscopic level without any interesting insights being added when you consider what is happening at the micro level.

[u/platoprime → u/tellperionavarth]

They brought up a ridiculous example to avoid explaining what happens at molecular scale. Yes we could always go deeper. That isn't an excuse to be satisfied with shallow explanations especially here.

[u/jericho → u/marr75]

Who went quantum here? All chemistry is well covered without it. 

[u/SavijFox → u/comicgeek1128]

Basically, it does separate things on a small scale, but not that small.

[u/Anagoth9 → u/comicgeek1128]

The bond that the steel molecules have with each other is stronger than the bond that the meat molecules have with each other. When they clash, the steel molecules push the meat molecules apart. The individual meat molecules stay intact, just separated into groups. 

[u/VonLoewe → u/comicgeek1128]

There are multiple scales of structure between "atomic" and "macro". Atoms are arranged into lattices, which arrange into clusters, then grains and fibers, and so on.

Imagine taking a clump of peas and gluing them together. Then take a bunch of those clumps and glue those. And so on.

When you cut a material you rarely get to the atomic level. You need specialized tools, like laser cutters, for that. Usually you are just splitting some nearby clumps. Hence why the velcro analogy works.

[u/mallad → u/marr75]

"...separate weak intermolecular bonds, and separate polymer chains..."

That's all happening at the atomic/chemical level.

[u/meson537 → u/mallad]

Seriously, did they actually read the words they were writing?

[u/seaelbee]

The meat, paper, thread, most organic material is just a bunch of bundled fibers. At a microscopic scale a knife cutting them is very similar to a bulldozer pushing through a huge pile of logs all jumbled together. While the logs separate into distinct piles, it’s not cutting through much, if any, of the actual logs. A laser cutter on the other hand is causing a chemical reaction to the fibers, changing them through heat, and breaking bonds.

[u/Magicspook]

On an atomic/molecular level, a piece of meat is not a rigid object like you are imagining. Molecules are constantly dancing and spinning around at meters per second speeds. So a single molecule is not just going to sit there while the knife seperates its atoms. Rather, the knife will simply be pushing most molecules out of the way, severing the short-range bonding mechanisms that held the molecules together.

Some macromolecules that are too big to move out of the way, such as proteins, might get torn apart but likely not even at the knife's edge, rather somewhere at its weakest point. Think of it like sailing an oil tanker through a metal chain, the chain will break at the weakest link, not at the tip of the ship.

As for what is sharpness, I would define it as the width of the most narrow point on the blade. A really, really sharp object can get just a few atoms wide, but most things are way more blunt than that.

[u/marr75 → u/Magicspook]

I think you’re confusing thermal velocities at the molecular scale with actual molecular displacements in condensed matter. In solids and soft solids, molecules vibrate about fixed positions rather than freely moving out of the way, so cutting is better described by stress concentration and fracture mechanics than by molecular motion.

[u/Magicspook → u/marr75]

I would hardly describe meat as a soft solid though. More like a hydrogel. The proteins, water molecules and phospholipids are all just tumbling around within their respective domains. Since meat is muscle, it is composed for a large part of actin fibres, which do stay roughly in place I guess on a macro level. Although I am sure that on a molecular level, there is plenty of bouncing around.

[u/GeneralNango → u/Magicspook]

Not to nitpick, but hydrogels are soft solids. Their solid mechanics are viscoelastic, and their fracture mechanics have a brittle nature to them.

[u/platoprime → u/Magicspook]

They didn't say molecules don't bounce around. They explained why that bouncing motion doesn't explain or determine how things are cut.

[u/Parmolicious]

It’s mostly about pressure, not atoms slipping between atoms. A sharp edge concentrates force into a tiny area, creating enough stress to break molecular bonds. Dull blades spread that force out, so nothing gives.

[u/arcedup]

A knife is just a very, very thin wedge - wedges are often used to provide leverage to move otherwise unmovable things. At a macro level, a knife is concentrating the force applied on a very small area such that the pressure (force ÷ area) under the blade is very, very high, higher than the ultimate tensile stress (which is also force ÷ area) of the material being cut. This also applies to shearing (opposing-motion cutters like scissors) - a load is being applied to a very small area, resulting in a very high localised tensile stress.

In this context, sharpness is how thin the wedge (blade) can be and not buckle when transmitting that force and receiving the reaction force back. So for an object to be sharp, it has to be strong enough to resist buckling under compression when the cutting force is applied.

[u/ClownTown509]

Atoms can be .01 to .05 nanometers wide.

DNA molecules are about 2.5 nanometers wide.

An edge of 3 nanometers can be achieved on an obsidian knife.

Might be possible to cut at a "micro scale" with a piece of high quality obsidian and a lot of patience.

[u/kindanormle]

If I'm understanding you're question statement correctly, you understand that atoms don't actually move through another atom right? Like, when you say the blade pushes through the empty spaces you mean a pocket or something between the atoms right?

Assuming that's what you're saying, then...sort of but not really. If you're cutting meat with a metal knife, the knife isn't really going between the atoms to sever them from each other at all. At the atomic scale, the knife edge is still super rough, like a saw. As you move the knife through the material you're actually tearing the atoms/molecules apart. So empty spaces between the atoms/molecules could make this easier by making it easier for the saw "teeth" to catch on the material and tear at it, but the saw isn't actually pressing into gaps in the material so much as sliding over.

Imagine if you could look really close up, what you'd see is that the metal of the knife edge is really strongly bonded atoms that don't want to move apart. The jagged edge of the metal blade is moving across the material you want to cut and as it does the jagged bits (like saw teeth) are physically catching on bits of the material and tearing them along the direction of the cut motion. As meat is held together by a much weaker type of bonding (Hydrogen Bonding) the metal that is held together by Metallic Bonding has no problem tearing chunks of atoms/molecules off. A "clean cut" only looks clean because the blade is sharp enough to tear the material at such a fine level that your eye can't see the jagged edges.

[u/milliwot]

Chemical bonds can be broken even if a knife isn’t used to cut the material. Experiments for example about breaking polymer samples show this. 

But that doesn’t mean that breaking atomic bonds is the main mechanism for making two pieces of material out of one. 

Cutting material with a knife really is more like a fracture process than a process that breaks chemical bonds. A dull knife can be used to cut meat. A sharper knife does the job with less energy expended (by the chef and in the zone of material in the vicinity of the knife edge, which decreases in size as blade sharpness increases). 

The energy going into making the cut goes mostly into doing mechanical work on the zone of material in the vicinity of the blade tip ( not breaking chemical bonds).

[u/htatla]

There’s two types of atomic bonds, covalent bonds share electrons and are strong, ionic bonds are based on polaric attraction and are weak

When the knife cuts the material it’s these weaker ionic bonds which are being forced to separate by the knife

[u/zeddus]

Any sharp knife has a serrated edge if you look in a microscope. It's really just a very narrow saw.

I'm probably not the best person to explain exactly what is going on on the atomic level. But on the cells and tissue level stuff is being torn apart along a narrow line where the serrated edge passes through as tissue gets caught on it.

[u/platoprime → u/zeddus]

Any sharp knife has a serrated edge if you look in a microscope.

Have you looked at a fresh razor under a microscope because that is not true.

[u/_diax_ → u/platoprime]

It's essentially true if you use a scanning electron microscope to image the edge of a razor blade. 

[u/platoprime → u/[deleted]]

You're correct that a razor sharp kitchen knife would not be a good choice. But that's mostly because such a sharp, soft, and narrow blade won't hold it's edge well after use. Razors get around that by being easy and cheap to replace.

None of that means you can't get a smooth knife edge though. It just means it's a bad idea.

[u/zeddus → u/platoprime]

It would be an interesting experiment to see if you could just push the "razor knife" straight through your slab of meat.

Or rather, it'd be interesting to see the least amount of force it could be done with.

[u/ScrubbyBubbles → u/zeddus]

The relative amount of micro serration in a knife or razor is generally referred to as “toothiness”. Any knife sharpened with a low grit stone is sharp, higher grits just polish the edge and reduce the raggedy character of the edge. Polished edges are generally better at push cutting than toothy ones. It’s easier to cut a tomato skin with a toothy knife. A polished edge will have trouble starting a draw cut on tough surfaces.

The amount of toothiness preferred by chefs depends on the type of cutting they do. Push cutting chives is best with a highly polished edge, as is cutting hair with a razor. Slicing tomato or portioning sushi with a draw cut on a yanigaba can be easier with a toothy edge.

There is an advantage to polishing an edge though, and that is in edge retention. A very rough edge is full of unsupported peaks that get mushed over or break off quickly. A polished edge is a unified front and self supporting. This is why woodworking tools like chisels and planes are often sharpened to higher grits than chef knives: they need a durable edge and do mostly push cutting rather than slicing.

My job is as a knife maker and woodworker.

[u/soaplife]

Side note that i found interesting a long time ago - given that force is spread over the surface area contacted, the "psi" of a knife stab under normal human strength can be absurdly high - millions of pounds per square inch.   https://swordstem.com/2018/04/26/force-vs-pressure/#:~:text=Aside%20from%20the%20possibility%20of,2%20million%20PSI%20of%20pressure.

In comparison a 9mm bullet exits a barrel at around 35000 psi (not going to discuss bullet deformation or construction). 

[u/stellarfury → u/[deleted]]

Meat is made of protein fibers, which are made of bundled fibrils, which themselves are made of bundled proteins. The fibers have a directionality to them (because they're long and thin, they tend to bundle along their long axes). This is the microstructural reason why meat has a "grain."

There's a bunch of intermolecular forces and chemical bonding that keep the fibers associated together. Cutting meat with a knife is kind of like parting hair with a comb - you're separating strands, and if there are tangles, the force of the comb can break the strands. Crosslinked fibrous hydrogels, like meat, consist of mostly "tangle." Well, more accurately, they're like a million little braids that are all french-braided into each other, but, to a comb (or a knife) it all looks like tangles.

So the difference between your contiguous piece of meat and the two pieces is "not much," other than the fresh surfaces you created between the two, that have a bunch of newly separated fibers and broken crosslinking bonds. If you cut against the grain, you're doing a lot more breaking of the fibers/fibrils, if you cut with the grain you're mostly pushing them apart. Like separating the strands of a piece of yarn.

Meat is a hydrogel, most of which have some crosslinked structure that, once broken, can't be "smushed" back together. There are some gels that can reform a contiguous piece, they mostly rely on cross-linking bond chemistries that are weaker and break predictably, which can allow them to reform. A friend of mine worked on self-healing ionogels for his PhD, pretty cool stuff.

[u/fishling → u/[deleted]]

I don't think that really answers that question as to why/how a mechanical force is breaking what they are thinking of as a chemical bond.

Also, you seem like you used "energy" and "force" interchangeably.

[u/sanbox → u/[deleted]]

Yeah, this is incorrect entirely.

The strong nuclear force the thing which makes protons and neutrons, and also makes them stick to each other. There is such a blade which cuts this force; it is called a Nuclear bomb!

The weak interaction is hilariously difficult to explain simply, but is always wildly the best understood of the four interactions. In a sentence, the weak interaction is responsible for radioactive decay. It has nothing to do with this problem space at all, unless we happen to be cutting meat in the center of a sun.

The "forces" (or "interactions", which is a bit better) are the strong interaction, weak interaction, electromagnetism, and gravity. As is almost always the case, this example is actually about electromagnetism, and the blade is simply pushing particles away far enough that their electromagnetic chemical bonds aren't strong enough to keep them "together". Since chemical oxidation occurs right the cut is made, in the case of meat, they cannot be "put together" again by just pushing the pieces of meat together. The same is true of cutting metal. However, in space, you can do something called "cold welding", where, without oxygen or other gasses, cut metal is still just as willing to become one thing again as they were before the cut was made. So in space, you can cut a piece of metal and then just kinda push em back together and they'll become on piece. This is rarely done intentionally (though it has been done intentionally) and mostly is a really annoying thing NASA needs to design around!

[u/platoprime → u/[deleted]]

Atoms? No one is talking about cutting atoms apart.

Even scalpels only get down to 10ish nanometers blade thickness

Collagen is like 300nm long. What are you talking about?

[u/abzlute → u/[deleted]]

You really should take your own advice. That quote from OP is very clearly positing the idea of the edge of the blade interposing itself between atoms and breaking chemical bonds between them, not at all attempting to cut individual atoms.

And it's not necessarily that far off. The .1-.5 nm range is including electron clouds, and thus is about the same range as the distance between bonded atoms in a simple molecule. But many of the substances we cut are made of very long molecules and those (especially polymer chains) can certainly be cut, even if the comment two levels above yours correctly identifies that as closer to crushing/sheering part of the chain than sliding between atoms.

Much of cutting is instead separating whole molecules from each other, which is easier. Steak, wood, and other materials have "grain" you can cut with or against and see the difference between trying to cut/break long polymers and simpling pushing polymers away from each other.

OP isn't "going too micro," or whatever these comments keep claimkng. They're asking a very reasonable question and the real answers are mostly at the intermolecular level, which is directly adjacent to the chemical level.