Ferrofluid Recipe?

June 6, 2011 5:21 PM

I have oddles of iron oxide and I am not afraid to use it... so, I would like to do so and make ferrofluid. Problem is, all instructions I have been able to find in my search thus far have provided directions in which I am obtaining my magnetic particles from an alternate source. Can any of you Mefites help me come up with a recipe for ferrofluid success?
posted by caveat empress to science & nature (3 answers total)

What sort of iron oxide do you have? Remember that iron can form several stable oxides; the one you want for ferrofluids is specifically magnetite (FeO-Fe₂O₃, aka Fe₃O₄).

The thing about ferrofluids is that the 'particle' size is insanely small, and bound to a surfectant - something that's just not possible to do with straight granulated iron, even if it is ground magnetite. You really need to synthesise it to get the small particle size; the easiest way is probably to start with ferric chloride (FeCl₃), reducing some to ferrous chloride (FeCl₂), mixing it that with more FeCl₃ to form Fe₃O₄ (and, in the presence of water, HCl?), precipitating it out, and mixing it with your surfectant.

If you want to go that route, this looks like a reasonable procedure.

Disclaimer: I'm an ecologist, not a chemist, and I've never done this - but I've investigated doing it a few times. I also have plenty of ferric chloride left over from my electronics days…
posted by Pinback at 5:59 PM on June 6, 2011

What have you got so far? I looked into making ferrofluid, as every shop which had it listed quite a dear price.

Pinback is quite right, the magnetite particles must be small. How small? Well, ten nanometers appears to be the right range for them. Why that small? The particles must remain suspended in the fluid (it is a colloid), so that is small to begin with. You also do not want them attracting one another due to the magnetic interaction. And then you have to overcome van der Waals forces. These dominate in the short range because the forces involved have high exponents on the distance. They would also like to shove the magnetite together, but by adding a surfactant, van deer Waals forces become repulsive.

The actual ferrofluid as "particle" is a bit more complex. Once the surfactant, like tetramethylammonium hydroxide, coats the magnetite particle, the hydroxide anions are drawn to the magnetite, then the teramethylammonium cations hang around those. Magnetic center, negatively charged hydroxide layer outside of that, positively charged teramethylammonium layer on the very outside.

It would be rather difficult to grind magnetite down to ten nanometers or less. Apparently, by the time your magnetite particles hit 100 nanometers, they start having more than one magnetic domain. You're getting into magnetorheological fluids at that point, which behave differently than ferrofluids. You might be able to make those, they are valid up to nearly a micrometer in size. They are not stable and will want to settle out, although you can slow this with an emulsifier. MHRs are more goopy and dense and can be quite resistive, enough to be used as a brake.

If you really want to have fun with iron oxide and your particles are too large, make thermite instead!
posted by adipocere at 6:30 PM on June 6, 2011

To give you an idea of the particle size, I had a chance to play with about an ounce of ferrofluid last year. LOADS of fun. At one point, I tried putting a magnet in a ziplock bag and dipping it in the fluid. Not even at a seam, just pouched up in the continuous side of the bag. More fun, fun, fun for a while, sculpting it and teasing it between two magnets, etc...

Then for the cleanup. Open the bag, take the magnet out, and... Discover the magnet covered in a good thick layer of black grime. The bag had not a single tear, no stressed areas, not even abrasions... The iron nanoparticles simply went right through the bag.

So you need it that finely ground. Pinback has it right - You need to grow them from scratch, not grind them from something bigger.
posted by pla at 7:10 PM on June 6, 2011 [2 favorites]