Magnetorheological fluid is a smart fluid that can switch between liquid and near-solid states under a magnetic field, making it useful for braking. It contains ferroparticles that align under a magnetic field, increasing viscosity. It has potential for various applications, including body armor.
A magnetorheological fluid is a fascinatingly intelligent 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 braking applications. The term “magnetorheological fluid” comes from a combination of magneto, meaning magnetic, and reo, the prefix for studying the deformation of matter under applied stress. Magnetorheological fluids are not currently widely used, but are considered a futuristic type of material.
The method of operation of a magnetorheological fluid is simple. A magnetorheological fluid consists of micrometre-sized ferroparticles, particles such as 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 activated, however, the ferroparticles line up in vertical chains along the flux lines of the field, restricting fluid flow and increasing the viscosity to that of a weak plastic.
Because the magnetorheological fluid’s strength comes from aligned ferroparticles that make up only a minority of the overall blend, there are definite limits to how strong it can be, but the significant difference between the “off” and “on” modes makes it attractive for use. in a variety of applications where conventional brakes are ineffective. Typically, the magnetorheological fluid is held between two small plates, only a few millimeters apart, which maximizes the braking properties of the mixture. The system must be arranged so that the magnetic flux lines are perpendicular to the direction of the movement to be stopped.
For circular motion, one of the best target applications, imagine a rotor with freely rotating spokes in a magnetorheological fluid. When the rotor needs to be stopped, a series of small magnets produce lines of magnetic flux that radiate away from the central rotor, creating chains of ferroparticles in between as beams pointing away from the sun. These beams catch the beams, slowing them down radically. Even as the beams break up aligned particles, the persistent magnetic field causes them to quickly realign, ready to catch the next incoming beam. This arrangement provides a powerful rotary brake.
Research groups are studying other possible applications for the magnetorheological fluid, such as body armor that is flexible but quickly becomes stiff upon contact with an incoming projectile.
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