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# Energetically favorable knife (very bad idea)

Specialized knife whose surface is designed to have high affinity for the material being cut
 (+2) [vote for, against]

One of the factors in the resistance you feel when you cut something with a knife is that you're increasing the surface area of what you're cutting, and that requires energy input to increase the material's surface energy (because an object's total surface energy is proportional to its total surface area, and that added surface energy has to come from somewhere).

To reduce or eliminate this energy input and make cutting easier, it seems like a good idea to make a knife with a surface that has high affinity for the material being cut*, which will let its edge move between particles of the material without needing to increase their surface energy (as much, anyway) because they have (nearly) as much attraction to the knife's material as they do to other particles of their own material. Therefore, the energy it takes to insert the blade between them is reduced, because they accept the blade almost as if it was the same as them.

*which does mean each knife can only cut one specific material, but that's OK in a lot of applications

-----

I realized about a minute after coming up with this that it's a really bad idea. It can't result in a more effective knife, and it might actually result in an overall less effective knife (even only considering cutting the one material it's made for). This is not to say that this kind of knife couldn't be modified in a way that solves its main problem, and such a modified knife might be better than a regular knife in certain ways, but it still couldn't be better in terms of the fundamental problem. Also, even without any such modification, one of these energetically favorable knives might be useful for purposes other than cutting.

Explaining why it can't work (or why it can, and thereby proving me wrong about that), and figuring out what I'm referring to as the "fundamental problem", will be an exercise for the class. (Is there a category for ideas that are bad in an educational way?)

N/A [2020-02-29]

 — notexactly, Feb 29 2020

maxel, no no, not quite a tiny little door moving gubbin https://en.wikipedi...Programmable_magnet
A more specific material control is always desired. [wjt, Mar 01 2020]

VA-111 "Shkval" ("Squall") https://en.wikipedi.../wiki/VA-111_Shkval
Impressive, but not outstandingly safe, even to the launch vehicle. [8th of 7, Mar 04 2020]

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 // I realized about a minute after coming up with this that it's a really bad idea //

[marked-for-tagline]
 — whatrock, Feb 29 2020

Suppose your knife could vibrate really really fast. Would this help the edge get between the molecules?
 — whatrock, Feb 29 2020

 My fraction of a cent is that the side of the blade will stick to the material designed for. I would suggest having the side of the blade material phobic and the cutting edge oscillating between phobic and phillic to give energy to bonds of material so blade pressure can cut/break more easily.

^vibration could be an helping dimension. More energy to the grindstone.
 — wjt, Feb 29 2020

 I can see one case where this would work, and that would be in the case of a blade designed to "cut" liquid. I can imagine a thin blade being drawn into the liquid by wetting. For instance, take a droplet of water sitting on a waxy surface, and now nudge a tiny sliver of a hydrophilic metal foil into contact with the droplet. Wetting would pull the foil (blade) into the liquid.

 Now you could repeat the process, but this time using an aqueous gel instead of water. If the gel were weak enough, it might still pull the foil (blade) into itself by wetting, and yet the gel could be considered a solid, maybe.

Whether this effect would ever be significant to help with the cutting of "real" solids, I don't know.
 — MaxwellBuchanan, Mar 01 2020

A company is designing the patterns of magnetic fields entering and exiting a magnet's surface as sort of magnetic field pixels. This, in my extrapolating mind, is half the technology needed for this idea. Admittedly, the resolution as a way to go.
 — wjt, Mar 01 2020

// He took his vorpal sword in hand;
Long time the manxome foe he sought—
So rested he by the Tumtum tree
And stood awhile in thought. //
 — sninctown, Mar 01 2020

 //a blade designed to "cut" liquid//

A keel? A hydrofoil? An unusually dangerous surfboard?
 — pertinax, Mar 01 2020

 // My fraction of a cent is that the side of the blade will stick to the material designed for. //

 That's what the main problem is. Not the "fundamental" one, but the main one.

 // I would suggest having the side of the blade material phobic and the cutting edge oscillating between phobic and phillic to give energy to bonds of material so blade pressure can cut/break more easily. //

 That's a potential way of modifying this kind of knife to make it work. It's still subject to the "fundamental problem", which (hint!) cannot be solved (but can be satisfied in different ways).

 // I can imagine a thin blade being drawn into the liquid by wetting. //

Yes. The same can happen with a solid, but then what? The knife and the material are stuck together; you need to supply the surface energy to separate them, unless you want them stuck together. That is a way of getting around the "fundamental problem", though, if that outcome is acceptable.
 — notexactly, Mar 04 2020

You mean something along the lines of "You'll take my knife away when you pry it from my cold, dead fingers ?"
 — 8th of 7, Mar 04 2020

 //The knife and the material are stuck together; you need to supply the surface energy to separate them, unless you want them stuck together.//

 What if the "blade" was actually two half-blades touching one another? Then you can easily separate the two halves of the cut object by separating the two halves of the blade.

Should be a disposable blade, I suppose.
 — pocmloc, Mar 04 2020

 Why not make the tip spercavitating, like the VA-111 ?

A superthin layer of gas along the side of the blade, reducing friction, would make a difference.
 — 8th of 7, Mar 04 2020

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