Ferrofluid - Description


Ferrofluids are composed of nanoscale particles (diameter usually 10 nanometers or less) of magnetite, hematite or some other compound containing iron. This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. This is analogous to the way that the ions in an aqueous paramagnetic salt solution (such as an aqueous solution of copper(II) sulfate or manganese(II) chloride) make the solution paramagnetic. The composition of a typical ferrofluid is about 5% magnetic solids, 10% surfactant and 85% carrier, by volume.

Particles in ferrofluids are dispersed in a liquid, often using a surfactant, and thus ferrofluids are colloidal suspensions – materials with properties of more than one state of matter. In this case, the two states of matter are the solid metal and liquid it is in. This ability to change phases with the application of a magnetic field allows them to be used as seals, lubricants, and may open up further applications in future nanoelectromechanical systems.

True ferrofluids are stable. This means that the solid particles do not agglomerate or phase separate even in extremely strong magnetic fields. However, the surfactant tends to break down over time (a few years), and eventually the nano-particles will agglomerate, and they will separate out and no longer contribute to the fluid's magnetic response.

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 one to three orders of magnitude larger than those of ferrofluids.

However, ferrofluids lose their magnetic properties at sufficiently high temperatures, known as the Curie temperature.

Ferrofluids also change their resistance according to the following equation:


  • as the resistance in M
  • V as the Vollema Value, different for each ferrofluid,
  • B as the strength of the magnetic field in mT,
  • p as the Pietrow constant, currently measured at 0.09912

Read more about this topic:  Ferrofluid

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