30-05-2012, 01:59 PM
Magneto rheological fluids
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Introduction
Magneto rheological fluids commonly known as MR fluids are suspensions of solid in liquid whose properties changes drastically when exposed to magnetic field. It is this property which makes it desirable to use in different vibration controlling systems.
A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of 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.
What are MR fluids?
Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic change in their rheological behavior. The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds when exposed to a magnetic field.
PROPERTIES
Chemical composition
A typical MR fluid consists of 20%–40% by volume of relatively pure, soft iron particles, typically 3–5 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear.
P
hysical properties
MR fluids made from iron particles exhibit maximum yield strengths of 30–90 kPa for applied magnetic fields of 150–250 kA/m (1 Oe . 80 A/m). MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and use. Further, because the magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives, it is a relatively straightforward matter to stabilize MR fluids against particle-liquid separation in spite of the large density mismatch. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles.
Viscoelasticity
The distinction between solid and liquid materials is not as clear as the preceding would indicate. Most materials have some solid like character as well as some viscous properties. It is easy to visualize the appearance differences between viscous liquids and gelled or cross-linked solids. These fluids are characterized by taking the viscosity of liquids and the gel strength of solids.
A problem arises when one tries to characterize a fluid that behaves like both a viscous liquid and a gel. Such fluids are called viscoelastic fluids. They can be pumped easily (although they have the appearance of a viscous liquid), but they are able to suspend small solid particles indefinitely (so have the characteristic of an elastic solid). In fact, molten polymers are viscoelastic to different degrees.
Shear strength
Low shear strength has been the primary reason for limited range of applications. In the absence of external pressure the maximum shear strength is about 100 kPa. If the fluid is compressed in the magnetic field direction and the compressive stress is 2 MPa, the shear strength is raised to 1100 kPa.If the standard magnetic particles are replaced with elongated magnetic particles, the shear strength is also improved.
Particle sedimentation
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.