A ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier. The particles are about 100 Angstroms in size, and are coated with a surfactant to keep the particles from clumping together (even when a magnetic field is applied). By volume, a ferrofluid is about 5% magnetic solids, 10% surfactant, and 85% carrier.
When no external magnetic field is present, the magnetic moments of the particles are random, and the fluid is not magnetic.
When an external magnetic field is applied, the magnetic moments of the particles orient themselves almost instantly to the magnetic field lines of the external source. When the magnetic field is removed, the particles go back to random alignment very quickly.
Ferrofluid is used in many industrial applications. Many kinds of loudspeakers are damped with ferrofluid. If you own a CD or DVD player, the laser head most likely has a drop of ferrofluid in there to damp it. It's used in sensors and switches, and for testing magnetic audio, video and data media (floppy discs, etc.) Some new bridges in windy or earthquake-prone zones use a thicker version of ferrofluid to damp movement. When the wind starts the bridge swaying, large magnets in the structure come closer to each other, and since ferrofluid gets thicker (more dense) in a magnetic field, it stops the bridge motion.
A ferrofluid will always tend to move to the region of highest flux. This can create some VERY interesting patterns when magnets of different shapes are used!
The spikes that stick up from the ferrofluid also show you the field strength at that spot. Lots of very small spikes indicate a stronger field; fewer large spikes indicate a weaker field. You can observe this effect by slowly bringing a magnet closer to the bottom of the dish of ferrofluid.
On the of the interesting properties of ferrofluid is how it gets denser the stronger the magnetic field applied to it is. You can feel this effect with any of the experiments above by wearing latex gloves and sticking your finger into the ferrofluid! The effect is eerie--it gets more firm and less squishy as you get closer to the magnet.
And, another REALLY cool experiment. We wound up a simple coil of 300 turns of #30AWG magnet wire, put it under a dish of ferrfluid, and hooked it to the output of a stereo! The coil should have an impedance of around 8 ohms so you don't ruin your stereo. The effect is amazing! The ferrofluid moves with the music.
CLICK HERE to watch a 10-second video (231K) of the coil under the edge of the ferrofluid, and a small magnet submerged in the ferrofluid, with the coil hooked to the speaker outputs.
Both videos are REALLY interesting to watch!
Ferrofluid is not inherently dangerous or toxic. However, it is NOT for children! It stains skin, clothing and tabletops, and is very difficult to wash off. Wear latex gloves when handling it, and DON'T splash it in your eyes (it would be wise to wear safety goggles too when handling it). Since it is ferociously attracted to large magnets, use caution and think about what you are doing. If you let a strong magnet snap to the bottom of your dish of ferrofluid, you will get a ferrofluid shower (hope you wore those goggles...) Use dishes that are disposable....plastic petri dishes are great.
If you bring a strong magnet anywhere near ferrofliud, the fluid will LEAP right out of the dish or bottle onto the magnet....and you will NEVER be able to remove it from the magnet!!! In our photo shoot, this happened a couple times -- the ferrofluid crept right over the edge of the dish.
Ferrofluid is somewhat volatile -- it evaporates. We chose the type that we have for sale because of its lower volatility -- but after 48 hours of sitting out, the fancy patterns in our petri dish have reduced by more than half in volume. If you keep your bottle and your experiments sealed from air, they will last much longer.
It is possible to make ferrofluid, but it's very difficult to make GOOD ferrofluid. The magnetic particles must be EXACTLY the right size. If they are too big, the particles will come out of suspension when the magnetic field is applied. If they are too small, it won't work at all. Search Google for some ideas -- some we have seen used burnt steel wool for iron oxide. Plain iron filings in vegetable oil with dish soap do NOT work -- we tried it! The best homemade ferrfluid that we've read about was made using iron-containing solutions, and the particles were precipitated out to make them the right size.