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.
The method of operation of a magnetorheological fluid is simple. A magnetorheological fluid is made up of micrometer-sized ferroparticles, particles like iron that respond to a magnetic field, suspended in an oil-based medium. When outside the influence of a magnetic field, the particles float freely, causing the material to behave like any colloidal mixture, such as milk. When a magnetic field is turned on, however, the ferroparticles align in vertical chains along the field's flux lines, restricting the fluid flow and increasing the viscosity up to around that of a weak plastic.
Because the strength of the magnetorheological fluid comes from aligned ferroparticles that only make up a minority of the overall mixture, there are definite limits to how strong it can be, but the significant difference between the "off" and "on" modes makes it appealing for use in a variety of applications where conventional brakes are ineffective. Typically, the magnetorheological fluid is kept between two small plates, only a few millimeters apart, which maximizes the mixture's braking properties. The system must be arranged such that the magnetic flux lines are perpendicular to the direction of motion to be stopped.
For circular motion, one of the best target applications, imagine a rotor with spokes spinning freely in a magnetorheological fluid. When the rotor must be stopped, a series of small magnets produces magnetic flux lines that radiate away from the center rotor, creating ferroparticle chains in the medium like rays pointing away from the sun. These rays catch on the spokes, radically slowing them. Even if the spokes break through the aligned particles, the persistent magnetic field causes them to quickly realign, ready to catch the next incoming spoke. This arrangement provides a powerful rotary brake.
Research groups are investigating other possible applications for magnetorheological fluid, such as body armors which are flexible but quickly become rigid upon contact with an incoming bullet.