Monomolecular blades and wires

[GTOM]

A common thrope in SF (Endymion, Battle Angel Alita, even Three Body problem mentioned it), something that is so sharp that it can easily cut through tank armor. Could it possibly work in reality without some sort of stabile superheavy element?

 

[DEvens]

Not yet, but there are hints.

The usual buzz phrase is "mono filament." The idea is a cable made of a single strand of atoms, or possibly molecules. For example, it has been contemplated that "bucky balls" could be lined up and made into a tube of carbon atoms.

https://en.wikipedia.org/wiki/Carbon_nanotube

This would be a few 10's of Angstroms across. This is far narrower than the edge on a steel blade could be maintained. So it means the force required to pull it through a material being cut would be far smaller.

Now there are lots of problems. It isn't clear that such nanotubes can be made arbitrarily long. It isn't clear they will stay strong over that length. It isn't clear how to manipulate them. For example, it might be necessary to "braid" them in some fashion, possibly with a bonding agent. That would mean the cord was not as strong as first indicated, since braided cables are usually weaker, especially if they are made of short segments of fiber. And the bonding agent would be the limitation. And it would mean the cable was thicker. Think of yarn as compared to a single thread of sheep's wool.

Also, cutting something with such a thread has some interesting features. Some materials will separate quite nicely if cut this way. But some materials, often metals, will close over the cut and re-join. It may leave a flaw that is weaker, but might not separate. So, in the scene you reference in "The Three Body Problem" there is a large steel structure affected. (Trying not to give too many spoilers.) This might not have gone the way depicted. The steel structure might have re-welded itself. That is, the cut might have been narrow enough, and dislocated the metal sufficiently little, that it closed over the hole. It might have left a weak point but not split the structure. This is due to the same process that happens in vacuum welding.

https://en.wikipedia.org/wiki/Vacuum_cementing

Because the hole is so small, very little air can get in. So the ultra clean surfaces left by the thread might just re-join. Especially if there was significant pressure pushing them together. As there was in the steel structure in "The Three Body Problem."

 

[GTOM]

Not yet, but there are hints.

The usual buzz phrase is "mono filament." The idea is a cable made of a single strand of atoms, or possibly molecules. For example, it has been contemplated that "bucky balls" could be lined up and made into a tube of carbon atoms.

https://en.wikipedia.org/wiki/Carbon_nanotube

This would be a few 10's of Angstroms across. This is far narrower than the edge on a steel blade could be maintained. So it means the force required to pull it through a material being cut would be far smaller.

Now there are lots of problems. It isn't clear that such nanotubes can be made arbitrarily long. It isn't clear they will stay strong over that length. It isn't clear how to manipulated them. For example, it might be necessary to "braid" them in some fashion, possibly with a bonding agent. That would mean the cord was not as strong as first indicated, since braided cables are usually weaker, especially if they are made of short segments of fiber. And the bonding agent would be the limitation. And it would mean the cable was thicker. Thing of yarn as compared to a single thread of sheep's wool.

Also, cutting something with such a thread has some interesting features. Some materials will separate quite nicely if cut this way. But some materials, often metals, will close over the cut and re-join. It may leave a flaw that is weaker, but might not separate. So, in the scene you reference in "The Three Body Problem" there is a large steel structure affected. (Trying not to give too many spoilers.) This might not have gone the way depicted. The steel structure might have re-welded itself. That is, the cut might have been narrow enough, and dislocated the metal sufficiently little, that it closed over the hole. It might have left a weak point but not split the structure. This is due to the same process that happens in vacuum welding.

https://en.wikipedia.org/wiki/Vacuum_cementing

Because the hole is so small, very little air can get in. So the ultra clean surfaces left by the thread might just re-join. Especially if there was significant pressure pushing them together. As there was in the steel structure in "The Three Body Problem."

Although a blade can make a thicker cut, if only the edge is mono.

 

[Rive]

Could it possibly work in reality ... ?

No.
The first problem is, that the 'monomolecular' edge must cut up atomic bonds ~ at the same strength that keeps the edge itself together. This can be mitigated by thickening the 'edge' with adding more atoms, but that leads to the second problem: since the edge is not exactly two dimensional, there is always diagonal relocation present in the material being cut. This relocation will always consume energy - even if you cut butter, you have to supply this energy somehow (most likely through the mechanical structure of the 'knife'.

'Realistically' you have to add some energy field or two-dimensional technobabble to make it 'work'.

 

[pinball1970]

A common thrope in SF (Endymion, Battle Angel Alita, even Three Body problem mentioned it), something that is so sharp that it can easily cut through tank armor. Could it possibly work in reality without some sort of stabile superheavy element?

Reminds of William Gibson Johnny Mnemonic
"spool and socket molded from one of the Ono-Sendai diamond analogs. Then they'd carefullywound the spool with three meters of monomolecular filament."

This was used to kill Ralfi

 

[hmmm27]

Monomolecular wire is prevalent enough in SF you can probably slip it in without explanation.

Or, how about an ionic blade that sucks the electrons out of the surface ? The atoms - being then mostly protons and neutrons - then would naturally repel. Works best on non-conducting material.

 

[stefan r]

Although a blade can make a thicker cut, if only the edge is mono.

That only means the edge penetrates the steel. The corner of a diamond is often a single atom thick. Diamond grains can certainly scratch steel. So can a bunch of other stuff. Alumina will scratch hardened steel. Quarts will scratch normal steel. Tooth enamel can scratch iron. This is why grinding wheels work.

 

[stefan r]

Monomolecular wire is prevalent enough in SF you can probably slip it in without explanation.

Or, how about an ionic blade that sucks the electrons out of the surface ? The atoms - being then mostly protons and neutrons - then would naturally repel. Works best on non-conducting material.

An anti-matter beam would be a bit overkill. Does a plasma cutting torch count as an ionic blade?

 

[DaveC426913]

I too have always kinda wondered about this issue.

Can a super-hard, super-sharp blade cut through things "like butter"?
For me, the obvious example of Wolverine's adamantium claws.
Sure, they won't dull, and they will cut into steel, but surely not without a tremendous force applied.
Yes, Wolvie is also very strong, so that supplies a greater force, but is it nearly enough?

Really what I'm asking about is - assuming an arbitrarily sharp, thin and strong blade - how much is the required force reduced by?

 

[GTOM]

Not yet, but there are hints.

The usual buzz phrase is "mono filament." The idea is a cable made of a single strand of atoms, or possibly molecules. For example, it has been contemplated that "bucky balls" could be lined up and made into a tube of carbon atoms.

https://en.wikipedia.org/wiki/Carbon_nanotube

This would be a few 10's of Angstroms across. This is far narrower than the edge on a steel blade could be maintained. So it means the force required to pull it through a material being cut would be far smaller.

Now there are lots of problems. It isn't clear that such nanotubes can be made arbitrarily long. It isn't clear they will stay strong over that length. It isn't clear how to manipulate them. For example, it might be necessary to "braid" them in some fashion, possibly with a bonding agent. That would mean the cord was not as strong as first indicated, since braided cables are usually weaker, especially if they are made of short segments of fiber. And the bonding agent would be the limitation. And it would mean the cable was thicker. Think of yarn as compared to a single thread of sheep's wool.

Also, cutting something with such a thread has some interesting features. Some materials will separate quite nicely if cut this way. But some materials, often metals, will close over the cut and re-join. It may leave a flaw that is weaker, but might not separate. So, in the scene you reference in "The Three Body Problem" there is a large steel structure affected. (Trying not to give too many spoilers.) This might not have gone the way depicted. The steel structure might have re-welded itself. That is, the cut might have been narrow enough, and dislocated the metal sufficiently little, that it closed over the hole. It might have left a weak point but not split the structure. This is due to the same process that happens in vacuum welding.

https://en.wikipedia.org/wiki/Vacuum_cementing

Because the hole is so small, very little air can get in. So the ultra clean surfaces left by the thread might just re-join. Especially if there was significant pressure pushing them together. As there was in the steel structure in "The Three Body Problem."

What kind of materials could be really cut like this? Flesh?

 

[DEvens]

What kind of materials could be really cut like this? Flesh?

Basically anything that is held together by tension, but that does not have the kind of structure that steel has.

So yes, flesh is likely to be cut very effectively. Probably most biological substances. Bone might possibly have some ability to close over such a cut, but it would not be perfect. The bone would certainly be much weaker along the cut line. And it will depend very strongly on the speed the mono filament moves through.

Most non-crystalline non-metallic substances would very likely separate very readily under such a cut. Many crystals probably separate quite readily as well, though a very few will be able to close over the cut. The cut will probably produce massive flaws in crystals even if they can manage to close over.

There's an old "physics trick" of pulling a thread through a block of ice. If you do it correctly, the thread can pass through and the ice will close over it. So ice is a very special kind of solid. However, you can't go quickly, even if it does not break the thread.

 

[snorkack]

Really what I'm asking about is - assuming an arbitrarily sharp, thin and strong blade - how much is the required force reduced by?

I can figure what it is reduced to.
The cohesive energies of solids are in the same order of magnitude as the cohesive energies of liquids. Because while freezing does increase the binding energy of atoms, the latent heat of melting is normally several times smaller than the latent heat of evaporation.
Therefore, the force needed to part a solid should be slightly but not much bigger than the force needed to part the same substance when molten (surface tension).
Now, hard transition metals do have a bit higher surface tension than common molecular substances.
While the surface tension of water is about 70 mN/m, that of quicksilver is 450 mN/m.
The surface tension of molten iron turns to be about 2000 mN/m when not under influence of surfactants.

Supposing surface energy of cold solid steel were, say, 2500 mN/m, then a perfect blade needs to create 2 surfaces, so the blade faces resistance of 5 N/m.

Typical sword blades have thickness of 6...10 mm.
Meaning that a perfect blade needs 30...50 mN force - in other units, 3...5 g force - to cut in half an opposing sword blade.

 

[stefan r]

Typical sword blades have thickness of 6...10 mm.
Meaning that a perfect blade needs 30...50 mN force - in other units, 3...5 g force - to cut in half an opposing sword blade.

Using the 0.71 nm diameter of a C₆₀ you have a cross section of 0.4 square nanometers, or 4 x 10^-19 m². If it has tensile strength 6.3 gigaPascal it should take 2.5 nano Newtons force to snap the fiber.

 

[GTOM]

Using the 0.71 nm diameter of a C₆₀ you have a cross section of 0.4 square nanometers, or 4 x 10^-19 m². If it has tensile strength 6.3 gigaPascal it should take 2.5 nano Newtons force to snap the fiber.

So much lesser than cut the sword. Do we know anything about actual tensile strength of carbon nanotubes?

 

[stefan r]

So much lesser than cut the sword. Do we know anything about actual tensile strength of carbon nanotubes?

I took the number 6.3 gigapascals from the optimistic high number on a wikipedia page. Actually 63 gigapascals (sorry). So 25 nano Newtons to snap one.

The diameter of nanotubes varies depending on the crystal orientation. They can be resist more force if thicker and/or multi-walled but that would also make them displace more material as they cut something.