How does a knife cut food? There are metal atoms cutting carbons in some way, but where does the cut occur? Is it ever intramolecular (instinctively I’d say no, but what if it was a very long carbon chain)? On a larger scale, do the iron atoms of a knife squeeze their way between cells, or just shred them in two? And what makes one knife sharper than another, is it the number of atoms of thickness?
What I know:
Yes, sometimes knives cut molecules. This can be seen when you realize that certain long chain polymers can be visible blocks made out of only a few molecules.
The rest would be a WAG, so I will leave it to someone else.
Knives cut by brute force but on a very small scale, and it’s all down to pressure. Pressure is a measure of the force applied to a given area. Blades are sharp because they have a small cross sectional area, allowing you to create very high pressure whilst applying only a modest force. A good blade is one where the edge is very narrow but not so fragile that it is the edge which is damaged by the applied force rather than that which you wish to cut.
Plus, aren’ the bonds holding the metal together stronger that the bonds holding the material to be cut? So when the two meet, the weeker bonds in say, the meat molecules, are broken?
Atoms stay on, but molecular chains are split.
Can you give an example of this?
In most cases the actual metal edge of the knife isn’t splitting molecular bonds in the sense of slicing them cleanly. The knife edge works by generating a huge pressure impact in a small area causing (relatively) enormous shear stresses in the material that crushes and tears the molecular bonds apart from each other. In micro- level physical terms it’s like a giant hand like pushing the keel of an ocean liner down into a sea of ping pong balls that are glued together with rubber cement. At some point the balls are mashed apart, but the keel is not slicing them apart so much as “crushing” them part.
Isn’t ‘slicing’ just a realtive version of ‘crushing’. In other words, we use ‘slice’ to describe a crushing that happens in a very narrow region (narrow being relative to what we percieve as narrow and wide).
Yes, it’s ‘crushing’ …but slicing is crushing at a narrow specific level. You could seperate cheese with a pencil…effectively crushing it apart…and you could do the same with a knife…just that ‘slice’ implies more control and smaller area.
If you take a highly magnified look at a sharp knife blade, you’ll see it’s not smooth. It’s a line of tiny sharp points, like a saw.
That’s not really true AskNott. While it’s frequently stated as ‘fact’ in knife-sharpening circles that a really sharp knife has a saw-edge on a microscopic level, the evidence is lacking.
Golly day! There is truly a circle for everything!!
IIRC during the vulcanization process a quantity of rubber (say, a tire) becomes one big happy molecule as all of the individual molecules form sulfur crosslinks.
http://rabi.phys.virginia.edu/HTW/plastics.html
http://www.infoplease.com/ce6/sci/A0851203.html
Certain solids which are held together by ionic bonds are affected differently by cutting. What the blade does is push a group of ionic molecules/atoms which are ionically bonded to nearby atoms/molecules so that they are no longer experiencing ionic bonds with nearby atoms. As a result the entire ionic structure holding the object together screws up and you end up with a giant crack or the whole object breaks in half. This is why cutting things like solid salt is hard and the salt breaks in half or cracks instead of cutting cleanly like bread would.
This thread is 6 months old. I forgot about that.
It’s not 6 months old anymore!
Once, when I was slicing a quiche with an incredibly sharp knife, I accidentally split an atom. Boy, was there egg on MY face!!!
Yeah, you really have to cut carefully if your knives are sharp - it sucks to start a nuclear reaction without meaning to. It’s one good reason not to slice fissile material in the kitchen.
Well, there goes my best chili recipe.
Well, since this thread has been resurrected…
Cross-linking also occurs in animal tissue (I’m not sure about plant matter); structural proteins contain covalent cystine cross links that provide their structural integrity. I thought covalent bonds were stronger than metallic bonding, but could be wrong on that. Another effect which occurs in living tissue is hydrogen bonding between polar groups. IIRC, muscle fibers in particular are highly elongated cells loaded with alternating actin and myosin molecules hydrogen bonded to each other. Hydrogen bonds are fairly weak and easy to break up since they do not involve actual exchange of valence electrons.
What happens to the atoms involved all those covalent bonds that get broken - do they always recombine on their own side of the cut, or do they sometimes stick to the metal?