Smart+Materials+and+applications+5

 Magnetorheological fluid (MR fluid) is type of smart fluid  in a carrier fluid, usually a type oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications. MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are primarily on the micrometer scale and are too dense for brownian motion to keep them suspended (in the lower density carrier fluid). f errofluid 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. MR fluid is a special material used for building smart structures that can withstand earthquakes. The Magnetorheological fluid can change from liquid to solid and vice versa. It makes the buildings of which it is a component particularly flexible so they can adapt to external force that would have made other more rigid buildings snap. A magnetorheological fluid is a liquid that hardens near a magnet, and becomes liquid again when you remove the magnet. The rheological behaviour of MR fluids is of ten represented as a **Bingham plastic model ** with a variable yield strength depending on applied magnetic field H .The flow is governed by the equation:
 * __ Magnetorheological fluid __**
 * __ What is a __****__ Magnetorheological fluid ? __**

**__Smart Structures: __**

Modern dampeners are constructed with the Magnetorheological fluid which can change from liquid to solid with the application of a magnetic field. The Magnetorheological fluid inside modern dampeners are kept solid in normal conditions, but change to liquid and back as sensors activate and deactivate a magnetic field during an earthquake, allowing the dampeners to absorb the shockwaves and reducing damage to the structure.The Magnetorheological fluid inside the dampeners changes a building from a rigid structure that must absorb the shockwaves to a 'smart' structure which adapts instead.

__ Propeties of MR Fluid: __
The magnetic properties of three MR fluid samples were assessed using a magnetic. The fluid samples contained three different iron loadings: 10%, 20% and 30% by volume. Magnetic induction increases with field strength until saturation of the fluid occurs. This can be seen to occur at an intrinsic induction. For example, the MR fluid sample loaded with 30% iron can be seen to saturate at about (0.3)(2.1 T) D 0.63 T. A very gradual increase in fluid induction is observed after saturation of the fluid. This effect—which has been observed during testing of other MR fluids (Carlson 1996) is attributed to particle restructuring over time (Felici //et al// 1994) and increasing field. Permeabilities of these fluids were found to be between approximately 2 _0 and 6 _0 when measured at very low fields. These permeabilities are consistent with results of similar experiments conducted by G¨okt¨urk //et al// (1993). Parameter k, was adjusted to fit the model to the experimental data. Parameter k is used to account for magnetic interactions other than those between adjacent particles within a chain. Values of k used to fit the model were 5.0, 4.6, and 4.4 for the 30%, 20% and 10% iron by volume MR fluid samples, respectively. Since these values are accurate within about 10%, it can be seen that the model, when applied to MR fluids, is relatively insensitive to the parameter k. A MR fluid is a colloidal suspension of polarised particles. Its viscosity is close to the viscosity of engine oil. In the presence of the magnetic field this viscosity changes. The carrier fluid is usually mineral or silicone oil which combines low tendency to evaporation with resistance to temperature: depending upon a need, either high or low. <span style="font-family: 'Times New Roman','serif';">particles distributed in the carrier oil are micro-magnets. These are coated with a surfactant like oleinic acid which prevents the clustering of micro-magnets. MR fluids may operate within a temperature range from - 65°C to 200°C. High temperature causes solidification of the fluid. <span style="font-family: 'Times New Roman','serif';">The properties of the fluid make it possible to use it in two operational modes. The first mode consists in the regulation of the flow resistance which is used in vibration dampers. In this case, the fluid flows through holes in coils located on the surface of the piston. In the second mode, the fluid is subjected to shearing stresses between two surfaces in a relative motion.

It is composed of three ingredients: carbonyl iron particles, 'soft' iron particles which are only 3-5 micrometers in diameter; a 'carrier' liquid, usually hydrocarbon oil; and additives which enhance lubricity, modifies the fluid's thickness or viscosity, keeps the particles suspended in the liquid, and slows down the gravitational setting of the iron particles.The carbonyl iron particles provide the means for changing the fluid into solid; applying a magnetic field to the Magnetorheological fluid forces the particles to line up so the liquid becomes solid. The solidity of the fluid is influenced by the strength of the magnetic field – the stronger the field, the 'harder' the Magnetorheological fluid becomes. Removing the magnetic field unlocks the particles and turns the solid back to liquid. MR Fluid has been around since the 1940s. The technology that controls the magnetic field required to adjust the MR Fluid solidity to the level of force it is adapting to has been developed only since the 1990s.
 * __<span style="color: #111111; font-family: 'Times New Roman','serif'; font-size: 16px;">MR Fluid Components: __**

**__ Applications: __** ** MR Fluids are also being used in: ** - Exercise machines. -Washing machines. -Car shock absorbers. -Artificial legs. -prosthetic limb. -Bridges. -Create scrolling Braille displays. -reconfigurable molds. -D esign of clutches and brakes.
 * __<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Difference between shear-thickening fluids and MR Fluids: __**



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 * __<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">In these Videos will illustrate the MR Fluid __**


 * __<span style="font-family: 'Times New Roman',serif; font-size: 16px;">References: __**

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