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Ferrofluid

    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 is important to state that true 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 (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field Magnetorheological fluids have micrometre scale magnetic particles that are 1-3 orders of magnitude larger than those of ferrofluids
    The big difference between MRF's and FF's is how they react on a magnetic field An MRF has large particles Those particles form chains when a magnetic field is applied This increases the MRF's viscosity and can even solidify it, if the pressure applied to it is not large enough to break the chainsA FF on the contrary does not form chains The random movement of the particles is larger than the force pulling them together Their viscosity doesn't change but they like to stay in high magnetic fields The magnetorheological effect starts above a particle size of 10 nanometers
    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

    Applications

    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
    Ferrofluids are similarly used to form liquid seals around the spinning drive shafts in hard disks
    Again ferrofluids are stable and do not "solidify" This property only occurs in magnetorheological fluids which have much larger particles
    Using electromagnets and sensors the viscosity of magnetorheological fluids 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

    Home-made magnetorheological fluid

    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:
    • magnetic inspection powder from welding shops
    • particles from burned steel wool (after mortar and pestle)
    • particles scraped from the surface of video tapes
    • particles "mined" from sand with a plastic bag and a magnet (see external links)

    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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