Ferrofluid
A ferrofluid is a liquid that is drawn to the magnet's poles. It's a colloidal liquid made up of nanoscale ferromagnetic particles dispersed in a carrier fluid (usually an organic solvent or water). To prevent clumping, each magnetic particle is fully coated with a surfactant.
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| Ferrofluid |
When exposed to intense magnetic fields, large ferromagnetic particles can be pulled out of the homogeneous colloidal mixture, generating a distinct clump of magnetic dust. The magnetic attraction of small nanoparticles is so faint that the Van der Waals force of the surfactant is adequate to avoid magnetic clumping or agglomeration. Ferrofluids are often categorized as "superparamagnets" rather than ferromagnets since they do not retain magnetization in the absence of an externally applied field.
Magnetorheological fluids (MR fluids) are magnetic fluids with bigger particles than ferrofluids. A ferrofluid primarily comprises nanoparticles, whereas an MR fluid primarily contains micrometer-scale particles. In a ferrofluid, particles are suspended by Brownian motion and will not settle under normal conditions, but particles in an MR will settle.