Ferrofluids and magnetorheological fluids
Ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier. The particles, which have an average size of about 10 nm, are coated with a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid.
A magnetorheological fluid is a fascinating smart fluid with the ability to switch back and forth from a liquid to a near-solid under the influence of a magnetic field. It is usually used for applications in braking. The term "magnetorheological fluid" comes from a combination of magneto, meaning magnetic, and rheo, the prefix for the study of deformation of matter under applied stress. Magnetorheological fluids are not currently in wide use but are considered a futuristic type of material.
Magnetorheological (MR) fluid is different from a ferrofluid, in which the particles are smaller. MR fluid particles are primarily on the micrometre-scale and are too dense for Brownian Motion to keep them suspended (in the lower density carrier fluid). Ferrofluid particles are primarily nanoparticles that are suspended by Brownian Motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.