In general, some materials are called smart when they can change their properties under various external stimuli.
Magnetorheological Fluids (MRF) are liquids also called colloidal suspensions that change their rheological properties in the presence of external magnetic fields. When they are exposed to a magnetic field, they can quickly solidify, while after removing the external magnetic field, they quickly return to their original suspension state becoming liquid.
The secret of such behaviour lies in their chemical composition. They are based on tiny, nano or micro meter sized, magnetic particles suspended in the oil or other viscous liquid. These particles create chain-like structures, which are responsible for property changes.
Without the magnetic field, all nanoparticles are randomly dispersed in the liquid, while in the presence of the magnetic field, they form tiny chains that giving the liquid a rigid structure.
These nanoparticles are made from magnetic compounds (ferromagnetic powder like carbonyl Iron powder, Cobalt, or Nickel) and non-magnetic compounds (like water, silicone oil, mineral oil, or other liquids having large viscosity, e.g., glycol).
In the beginnings it was used in the damper of car. Presently, they can be found in devices like the damper (or shock absorber), brake, clutch, mount, prosthetic leg, and lower-limb exoskeleton.
These smart fluids are also widely used in buildings and bridges to counteract wind gusts in windy regions or places where earthquakes occur.
Recently, they have been applied even in rehabilitation equipment to stabilize injured joints like ankles, elbows, knees, or other joints.