Smart_Materials7

 Magnetorheological (MR) fluids are materials that respond to an applied magnetic field with a change in rheological behavior. Typically, this change is manifested by the development of a yield stress that monotonically increases with applied field. Interest in magnetorheological fluids derives from their ability to provide simple,quiet,rapid-response interfaces between electronic controls and mechanicalsystems. That magnetorheological fluids have the potential to radically change the wayelectromechanical devices are designed and operated has long been recognized. MR fluids are considerably less well known than their electrorheological (ER) fluid analogs. Both fluids are non-colloidal suspensions of polarizable particles having a size on the order of a few microns.
 * **Introduction : **



The initial discovery and development of MR fluids and devices can be credited to Jacob Rabinow at the US National Bureau of Standards (Rabinow, 1948a, 1948b, 1951) in the late 1940s. Interestingly, this work was almost concurrent with Winslow's ER fluid work. The late 1940s and early 1950s actually saw more patents and publications relating to MR than to ER fluids. Except for a flurry of interest after their initial discovery, there has been scant information published about MR fluids. Only recently has a resurgence in interest in MR fluidsbeen seen (Shtarkman, 1991; Kordonsky, 1993; Weiss et al., 1993; Carlson et al.,1994; Carlson, 1994; Carlson and Weiss, 1994). While the commercial success of ER fluids has remained elusive, MR fluids have enjoyed recent commercial success. A number of MR fluids and various MR fluid-based systems have been commercialized including an MR fluid brake for use in the exercise industry (Anon., 1995; Chase, 1996), a controllable MR fluid damper for use in truck seat suspensions (Carlson, Catanzarite and St.Clair, 1995; Lord, 1997) and an MR fluid shock absorber for oval track automobile racing.
 * History :


 * **What characterizes the material from other smart and ordinary materials: **

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).  Ferrofluid particles are primarily  nano-particles that are suspended by Brownian motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.

Magnetic, rheological, tribological and settling properties of four MR fluids are discussed. The basic composition of these four fluids is given in Table 1.

Table 1. Basic composition and density of four MR fluids (Lord, 1998). by Volume || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Carrier Fluid || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Density (g/ml) || <span style="font-family: Calibri,sans-serif; font-size: 11pt;"> oil || 3.64 ||
 * MR Fluid || Percent Iron
 * <span style="font-family: Calibri,sans-serif; font-size: 11pt;">MRX-126PD || 26 || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Hydrocarbon oil || 2.66 ||
 * <span style="font-family: Calibri,sans-serif; font-size: 11pt;">MRX - 140ND || 40 || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Hydrocarbon
 * <span style="font-family: Calibri,sans-serif; font-size: 11pt;">MRX-242AS || 42 || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Water || 3.88 ||
 * <span style="font-family: Calibri,sans-serif; font-size: 11pt;">MRX- 336AG || 36 || <span style="font-family: Calibri,sans-serif; font-size: 11pt;">Silicone oil || 3.47 ||




 * 1) ** Rheological Properties **

The rheological properties of controllable fluids depend on concentration and density of particles,particle size and shape distribution,properties of the carrier fluid, additional additives, applied field,temperature, and other factors. The inter-dependency of all these factors is very complex, yet is important in establishing methodologies to optimize the performance of these fluids for particular applications.

2. **<span style="font-family: Calibri,sans-serif; font-size: 12pt;">Magnetic Properties **

Understanding the magnetic properties of MR fluids is important for designing MR fluid-based In many such devices, the MR fluid represents the largest magnetic reluctance within the magnetic circuit. These magnetic properties may also prove useful in providing insight into the character and formation of particle structures within the fluid.

3**.** **<span style="font-family: Arial,sans-serif; font-size: 12pt;">Particle sedimentation ** <span style="font-family: Arial,sans-serif; font-size: 11pt;"> Ferroparticles settle out of the suspension over time due to the inherent density difference between the particles and their carrier fluid. The rate and degree to which this occurs is one of the primary attributes considered in industry when implementing or designing an MR device.[|Surfactants] are typically used to offset this effect, but at a cost of the fluid's magnetic saturation, and thus the maximum yield stress exhibited in its activated state.

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