Magnetorheological fluid, or MR fluid, is a type of fluid that becomes more viscous when exposed to a magnetic field. Using an electromagnet to vary the strength of the field, the physical resistance of the fluid can be varied. Commercial MR fluid can be approximated with a mixture of micron-sized iron powder in oil.
Lord Corporation has a detailed recipe for MR fluid. I've repeated it here with additional notes.
- Latex gloves, because it can get messy.
- Paper towels to clean up that mess.
- Something for measuring out volume or weight. A scale is ideal, but measuring spoons will suffice.
- A chopstick or any other non-metallic object for stirring and evenly measuring iron powder.
- A container for the MR fluid, at least 100 ml.
- Multi-purpose oil like 3-in-1 Oil. It shouldn't be an aerosol like WD-40, but in a "drip can".
- White lithium grease like Lubrimatic's. Again, non-aerosol: it will come in a cardboard can or a tube.
- 1-10 micron iron powder. Powders are sometimes rated using meshes, for example <200 mesh meaning 74 microns or less, see this conversion chart. Fine iron powder is sold by Science Lab, Micron Metals, and on eBay. A list of suppliers is available from ThomasNet.
The oil and grease can be found at home improvement or automotive stores. I found mine in the hardware section at Home Depot for about $3 each.
- Oil 55g / 60.7 ml (from experimentation, this might be a little high)
- Grease 5g / 5.26 ml
- Iron 150g / 49.6 ml
- Pour oil and grease into container
- Close and shake container for 5 minutes
- Allow mixture to sit for a few hours
- Mix in iron
- Shake container for a few more minutes
Lord Corporation also sells MR fluid at exorbitant prices.