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A ferrofluid is a specific type of liquid which responds to a magnetic field. Ferrofluids are composed of nanoscale magnetic particles suspended in a carrier fluid. The solid particles are generally stabilized with an attached surfactant layer. It must be made perfectly clear that ferrofluids are stable, meaning that the solid particles do not agglomerate and phase separate even in extremely strong magnetic fields. Ron Rosensweig's book Ferrohydrodynamics (1985) is the usual starting reference for learning the details of ferrofluids.
The term magnetorheological fluid refers to liquids similar to ferrofluids that solidify in the presence of a magnetic field. Magnetorheological fluids have micrometre scale magnetic particles that are 1-2 orders of magnitude larger than those of ferrofluids.
Ferrofluids are superparamagnetic. Ferrofluids have very low hysteresis.
The particles are usually iron, magnetite, or cobalt, and are smaller than a magnetic domain, typically 10 nanometers in diameter. The surrounding liquid is typically oil or water (or possibly wax). Surfactants are used to make the suspension more stable, so that the micelle-trapped particles repel each other due to steric hindrance effects.
Ferrofluids form intriguing three-dimensional shapes in the presence of magnetic fields, and patterns of stripes when confined to a thin sheet (as between two plates of glass) due to the individual particles' magnetic fields aligning and repelling each other, but the surface tension forces of the liquid holding them together.
They are used in loudspeakers to sink heat between the voice coil and the magnet assembly, and to passively damp the movement of the cone. They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet.
Using electromagnets and sensors, the fluid's viscosity can be controlled dynamically, allowing for active damping (in car shocks, for instance). This allows hundreds of watts of mechanical power to be controlled with a few watts of electrical power, which is much more efficient than other methods of vibration control, such as piezoelectric crystals.
Ferrofluids are similarly used to form liquid seals around the spinning drive shafts in hard disc drives.
Again, ferrofluids are stable and do not "solidify." This property only occurs in magnetorheological fluids which have much larger particles.
A simple magnetorheological fluid can be home-made out of small magnetic particles mixed with mineral or vegetable oil. Iron filings do not work well; they are too big. Good sources for small magnetic particles are:
This is not very stable, however. The particles will tend to clump and the fluid properties will be lost quickly. Fluids created for professional purposes use emulsifiers to suspend very small oily (octane or kerosene) magnetic particles in water. The particles are very fine; less than a micrometre in diameter.
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