Like a water-jet rail gun?

Actually I didn't think of that. I was thinking more of a magnetic nozzle. But yeah if you can steer a fluid with embedded magnets you should be able to accelerate a fluid with embedded magnets, right? :) or maybe not. I have no idea.

I don't think this would work because the particles are free to move within the water stream. Say you used something like a magnetic pinch ( https://en.wikipedia.org/wiki/Pinch_(plasma_physics) ) to focus the stream... my hunch is that all you'd achieve is to compress the iron particles with little or no compression of the water since the iron is just floating in the stream.

It's a good idea though.

That would be something of an advantage, no? The metal particles would be mixed with water still, pulling along some of the stream. The tighter the "beam" of metal particles, the better - presumably that allows a more precise cut.

No, because you also need to increase the pressure of the water for it to cut. The nozzle increases the pressure of the water by constricting a constant flow into a smaller area, which increases the kinetic energy of the water and the abrasive particles.

A magnetic pinch would (I expect, I only really covered them a bit in a plasma physics course so I'm not an expert) basically pull all the suspended iron particles out of suspension and compress them into a thin rod, without actually compressing the water very much. My hunch based on semi-informed knowledge is that it just wouldn't do much to actually cut anything, but I could be wrong.

The other problem is that magnetic fields also produce a lot of heat in a conductor. The metal particles that clump up would probably sinter together or even melt. On top of that, it'd take a lot of power to run... pinches aren't super efficient.

Rail guns work by passing a current through the projectile, which would boil your abrasive carrying fluid, unfortunately.

Perhaps he meant Gauss rifle or coilgun. I remember building one when I was younger, I wish I had thought of experimenting with ferrofluid.

Not a bad idea, but ferrous metals aren't very hard. Garnet abrasive is very very hard.

What if we used just enough ferrous metal as a kernel inside a grown crystal of garnet that the metal is sufficient to magnetically levitate the grain of abrasive; perhaps even extremely fine metal dust embedded within the grown crystal's matrix. Then could we pass the resulting metal-enhanced garnet abrasive grains through a Gatling rail gun?

Furthermore, since garnet has specific refractive properties different from other materials, could we pass all ferrous grains through an extremely high-speed discriminating chamber that looks for these properties in each grain, and magnetically directs the garnet grains back to the abrasive holding bin to vastly increase the recycling, while all other ferrous material goes a separate bin (for waste or other recycling purposes)?

I kind of wonder what problem this would be solving? It sounds magnificently cool, but also expensive. In the long run, I have great hope for fiber lasers becoming cheap enough for a home-version. Maybe a home-grade 1kw. Seems more practical than worrying about recycling sand.

I do love the idea of a Rail gun cutter, though - just not sure how it could really help cut things.

I'm thinking in terms of resource-constrained use cases, like on an aircraft carrier or further out, deep space exploration. Until this thread, I had always assumed a laser cutter would be just as good, but didn't realize some of the unique advantages of a waterjet cutter.

Interesting idea. If the water were conductive -- say salt water -- then I don't see why you couldn't steer it with magnets. The trick would be to keep an electrical current running through the jet.

At normal fluid pressures -- the classic example being peeing on an electric fence -- it's hard to get much current flowing since there's so much empty space between droplets. But at thousands of PSI, is that still true?

Water is dipolar (the oxygen atom 'pulls' harder on the electrons it's sharing with the hydrogen atoms, so the middle is negative and the ends are positive). It can be controlled by both electrostatic and magnetic fields, no salt needed.

Electrostatic, sure. You can deflect a stream of water from a faucet by holding a comb near it. But magnetic?

This sort of thing was actually used for water propulsion system:

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

The hardness of the material is critical. Ferrous material sounds like metal, which could just melt/spatter on impact. Probably not make a good cutting medium.