ferrofluidI just realized I haven’t yet written about ferrofluid… I was mentioning it to a few folks last night over dinner (no, dinner with me isn’t as geeky as I’d generally like) and they hadn’t heard of it.

Ferrofluid is a thick oil with tiny suspended particles of iron (magnetite) in it. That means that it behaves like a magnetic fluid — it’s attracted to magnetic fields. You can make this fluid jump with a bar magnet. The picture on the left shows a ferrofluid that is on a surface above a bar magnet. The “hedgehog” shape shows the lines of the magnetic field.

Remember when you were in middle school and you sprinkled iron filings on a piece of paper above a bar magnet? You could see the “magnetic field lines” as the filings lined up on either side of the bar magnet, and made spiky things on the ends. Here’s an image of that, to the right.magnet0873.png Well, this is the same sort of thing. The hedgehog spikes just show the same thing as the iron filings, but in three dimensions.
It’s hard to make ferrofluid, though. I know, I’ve tried. You can take corn oil and put iron filings in it, and that’s kind of fun, but the iron filings just sit in the bottom of the jar. Stick a magnet to it and the filings still look spiky, they don’t flow smoothly. I’ve tried taking magnetic toner powder (used for printing magnetic ink on checks) and suspending that in motor oil, but it’s not magnetic enough. The first problem is the oil — it has to be something very thin (30W motor oil is supposed to be thin enough) so that it can flow, but not so thin (like mineral oil) that the magnetite settles out. Also, big particles, like filings, tend to settle out because they’re heavy. So they make tiny (often nano-sized) particles so that their weight can be held up by the oil. But, when you make particles that tiny, they have a tendency to clump together. So you need some sort of “surfactant” — a coating on the particles that keeps them apart. (Soap is a “surfactant” because it keeps grease molecules apart). Plus, you need the surface tension on the fluid to counterbalance the magnetic attraction in just the right amount so the fluid holds together but it responds nicely to the magnetic fields. You can imagine, for instance, if this had a low surface tension (like water) the stuff would just stream onto the magnet and not hold together enough to make these shapes.
So, it’s hard to make good ferrofluid. But once you do, it’s a lot of fun. You can make it defy gravity by bringing a magnet down towards it from above, until it jumps up to the magnet. (Word to the wise, put the magnet inside a test tube or something, or else you’ll *never* get the ferrofluid off). You can make beautiful patterns, as in “Ebb Protrude Flow”, in this YouTube video.

Or this one also beautiful

In practice, ferrofluid is used to dampen high-end speaker systems (I don’t know much about that), and also in space, where you can’t make hydraulics work like they do on earth because there’s no gravity. Instead, the hydraulics can be driven and precisely controlled using magnetic fields.