Qs people asked me on science and my replies to them - Part 270

Q: Which knife is the sharpest and why? Is it made of the hardest diamonds available? 

Krishna: Obsidian knives are the sharpest.

 But very sharp blades can be very brittle, and it’s no use to anyone if parts of a knife break off while it’s being used. The hardest material out there is diamond, so logically a diamond knife should be the sharpest type. The difficulty is that diamond crystals tend to ‘cleave’ in what’s called an octahedral fashion, which doesn’t allow for a very sharp blade cross-section.

Glass on the other hand is ‘amorphous’, so it does not have any natural planes to split, or cleave to, making it much more suitable for making sharp edges.

Obsidian is an amorphous element, meaning that when it fractures it does so into impossibly sharp shards. It is extremely hard, but unfortunately brittle. Obsidian is a confusing element. It cannot be considered a mineral, because as a glass it is not crystalline, whereas minerals are. But at the same time, it is classified as a mineraloid, since it has mineral qualities, but they are too variable to be pure mineral. 

A type of naturally occurring glass that has already been in use since the Stone Age as a blade is obsidian. Obsidian is a volcanic glass that is created when magma is pushed to the earth’s surface and is cooled very rapidly. This stops it from forming a crystal structure and also introduces a form of ‘compressive strength’ which makes the material even stronger.

Primitive Obsidian knife

The Mayan Indians are credited with using obsidian blades first 2,500 years ago, although Stone Age spear tips made of obsidian have been found elsewhere in the world. Since obsidian will fracture down to a single atom, it is claimed to have a cutting edge five hundred times sharper than the sharpest steel blade, and under a high magnification microscope an obsidian blade still appears smooth, whereas a steel blade has a saw like edge.

The thinnest blades are three nanometres wide at the edge – 10 times sharper than a razor blade. These are made by flaking a long, thin sliver from a core of obsidian (volcanic glass).

Obsidian knives are quite delicate and tend to be a little brittle, so they’re probably not your best choice for the rough and tumble of a kitchen, particularly where they might strike something hard. Obsidian is not particularly hard (unlike diamond), it’s just capable of being very sharp.

Obsidian knives have however been used in medical procedures earlier where very precise cutting is required and where laser surgery is not available.  It's sharp enough to cut between human cells and the  incisions can heal faster.

Obsidian knives have been tested for use as surgical scalpels but aren’t currently licensed for use on humans, since they could leave glass fragments in the wound. The likelihood of obsidian cracking or chipping under the simplest of tasks is incredibly high.

 Using an obsidian blade to chop food could potentially result in accidental ingestion of shards of glass. Since this glass is so good at fragmenting, you may not even see that it has chipped.

One of the big advantages of smooth cuts  is that wounds from sharp blades heal more quickly than more jagged cuts, and are less prone to infection. However, even the manufacturer of the medical-type obsidian blades recommended that ‘hard objects should be avoided’ and ‘twisting or lateral motions should not be employed’. 

So this is the sharpest option out there. But it’s very brittle, and not suitable for tasks that a regular pocket knife could handle.

Now we have lasers  to use during surgeries. The CO₂ laser is THE ONLY practical soft-tissue surgical laser, which uses the laser beam directly to both cut and coagulate the soft tissues.

The key to understanding how the laser light ablates (cuts, incises, excises) and coagulates is through the absorption of laser light by the soft tissue. The wavelength-dependent Absorption Depth [mm] for the sub-epithelium connective oral soft tissue . It is derived from absorption and reduced scattering coefficients of oral soft tissue dominant chromophores: water, hemoglobin, and oxyhemoglobin.

The cooling efficiency of the tissue irradiated by laser light is largely determined by the tissue’s own thermal conductivity (or thermal diffusivity) to dissipate (or diffuse) the heat away from the irradiated tissue.