MAGNETORHEOLOGICAL FLUIDS

-K.Sai Malleswar

CONTENTS1. What is Magneto rheological fluid?

2. Aim

3. Objective and Work plan

4. Material Behavior and Properties

5. Common MR fluid surfactants

6. How a MR fluid works?

7. Modes of operation

8. Applications

i. Mechanical Engineering

ii. Military and Defense

iii. Optics

iv. Automotive and Aerospace

9. Recent advances

10. Limitations

11. Bibliography

WHAT IS MAGNETO RHEOLOGICAL FLUID ?

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.

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 which 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.

AIM:

Magnetorheological effect is one of the direct influences on the mechanical properties of a fluid. It represents a reversible increase, due to an external magnetic field of effective viscosity.

MR fluids and devices have the potential to revolutionize the design of hydraulic systems, actuators, valves, active shock and vibration dampers, and other components used in mechanical systems. The key to success in all of these implementations is the ability of MR

fluid to rapidly change its rheological properties upon exposure to an applied magnetic field .MR fluids find a variety of applications in almost all the vibration control systems. It is now widely used in automobile suspensions, seat suspensions, clutches, robotics, design of buildings and bridges, home appliances like washing machines etc.

At present, there is a compelling need to develop new and improved MR fluids, to lower their production cost through improved manufacturing processes, and to develop MR fluid-based application devices that will demonstrate the engineering feasibility of the MR fluids concept and will highlight the implementation challenges. For this purpose, the present project is undertaken.

Objective:

>>Ultra precision polishing of Si 3 N 4 ceramics using magnetorheological fluids and diamond abrasives: Silicon nitride (Si3N4) ceramics have features such as low density, high strength, wear resistance, etc.. Moreover, Si3N4 has been given much attention as a new structural material because it has excellent fracture strength, fracture toughness, and thermal shock resistance comparedwith other fine ceramics.

Aerospace applications of silicon nitride: Gas turbine manufacturing Silicon nitride bearings are used in main engines of NASA’s space

Shuttle. Thruster in rocket engine

Si3N4 ceramics are sensitive to defects resulting from the grinding and polishing processes due to their inherent brittleness. Failure begins inregions of surface irregularities, such as scratches,pits and microcracks. So, it is important to fabricate Si3N4 ceramics with a superior quality and finish with minimum defects in order to obtain reliability in performance.

In order to achieve this, magnetorheological fluids can be used along with diamond abrasives. The MR polishing method can provide excellent surface roughness compared with the existing lapping methods.

WORK PLAN:

To prepare the required fluids for MR polishing, carbonyl iron (CI) powder, which is sensitive to magnetic fields, is to be used. The fluids consist of approximately 50 wt% magnetic particles (CI powders, 2 μm), abrasive diamond particles (0.1-2000 μm), and DI water. A dispersion stabilizer (glycerin) is to be added to the aforementioned materials as it enhances the cohesion of the magnetic fluids and facilitates proper mixing of the polishing slurry and magnetic particles. However, excessive use of a stabilizer may deteriorate the finishing quality for certain materials.

The MR fluid is supplied to the gap between a workpiece and a moving wall to polish the workpiece. When a proper magnetic field is applied to the MR fluid, the viscosity and stiffness of the fluid increase by more than several tens of times within milliseconds. Thus, the MR fluid can rotate continuously as long as it adheres to the wheel surface resulting

from the applied magnetic field . For polishing purposes, a suitable abrasive slurry (a mixture of DI water and abrasive particles, which are generally of non-magnetic materials) is incorporated into the fluid, which is supplied to the narrow gap between the wheel and the workpiece. The experiments can be performed by changing therotating speed of the polishing wheel and the strength of the applied magnetic fields.

Imposed magnetic field and shear direction

in MR polishing

Material behaviorTo understand and predict the behavior of the MR fluid it is necessary to model the fluid mathematically, a task slightly complicated by the varying material properties (such as yield stress). As mentioned above, smart fluids are such that they have a low viscosity in the absence of an applied magnetic field, but become quasi-solid with the application of such a field. In the case of MR fluids (and ER), the fluid actually assumes properties comparable to a solid when in the activated ("on") state, up until a point of yield (the shear stress above which shearing occurs). This yield stress (commonly referred to as apparent yield stress) is dependent on the magnetic field applied to the fluid, but will reach a maximum point after which increases in magnetic flux density have no further effect, as the fluid is then magnetically saturated. The behavior of a MR fluid can thus be considered similar to a Bingham plastic, a material model which has been well-investigated.

However, a MR fluid does not exactly follow the characteristics of a Bingham plastic. For example, below the yield stress (in the activated or "on" state), the fluid behaves as a viscoelastic material, with a complex modulus,that is also known to be dependent on the magnetic field intensity. MR fluids are also known to be subject to shear thinning, whereby the viscosity above yield decreases with increased shear rate. Furthermore, the behavior of MR fluids when in the "off" state is also non-Newtonian and temperature dependent, however it deviates little enough for the fluid to be ultimately considered as a Bingham plastic for a simple analysis.

Thus our model of MR fluid behavior becomes:

Where τ = shear stress; τy = yield stress; H = Magnetic field intensity η =

Newtonian viscosity; is the velocity gradient in the z-direction.

Shear strengthLow 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 sedimentationFerroparticles 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.

Common MR fluid surfactants

MR fluids often contain surfactants including, but not limited to:

oleic acid tetramethylammonium hydroxide citric acid soy lecithin

These surfactants serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of MR fluids. The ideal MR fluid would never settle, but developing this ideal fluid is as highly improbable as developing a perpetual motion machine according to our current understanding of the laws of physics. Surfactant-aided prolonged settling is typically achieved in one of two ways: by addition of surfactants, and by addition of spherical ferromagnetic nanoparticles. Addition of the nanoparticles results in the larger particles staying suspended longer since to the non-settling nanoparticles interfere with the settling of the larger micrometre-scale particles due to Brownian motion. Addition of a surfactant allows micelles. to form around the ferroparticles. A surfactant has a polar head and non-polar tail (or vice versa), one of which adsorbs to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular micelle, respectively, around the particle. This increases the effective particle diameter. Steric repulsion then prevents heavy agglomeration of the particles in their settled state, which makes fluid remixing (particle redispersion) occur far faster and with less effort. For

example, magnetorheological fluids will remix within one cycle with a surfactant additive, but are nearly impossible to remix without them.

While surfactants are useful in prolonging the settling rate in MR fluids, they also prove detrimental to the fluid's magnetic properties (specifically, the magnetic saturation), which is commonly a parameter which users wish to maximize in order to increase the maximum apparent yield stress. Whether the anti-settling additive is nanosphere-based or surfactant-based, their addition decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids on-state/activated viscosity, resulting in a "softer" activated fluid with a lower maximum apparent yield stress. While the on-state viscosity (the "hardness" of the activated fluid) is also a primary concern for many MR fluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity, maximum apparent yields stress, and settling rate of an MR fluid.

How it works?The magnetic particles, which are typically micrometer or nanometer scale spheres or ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances, as below.

When a magnetic field is applied, however, the microscopic particles (usually in the 0.1–10 µm range) align themselves along the lines of magnetic flux, see below. When the fluid is contained between two

poles (typically of separation 0.5–2 mm in the majority of devices), the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity. Importantly, mechanical properties of the fluid in its “on” state are anisotropic. Thus in designing a magnetorheological (or MR) device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the motion to be restricted.

Modes of operation and applicationsAn MR fluid is used in one of three main modes of operation, these being flow mode, shear mode and squeeze-flow mode. These modes involve, respectively, fluid flowing as a result of pressure gradient between two stationary plates; fluid between two plates moving relative to one another; and fluid between two plates moving in the direction perpendicular to their planes. In all cases the magnetic field is perpendicular to the planes of the plates, so as to restrict fluid in the direction parallel to the plates.

Flow mode

Shear Mode

Squeeze-Flow Mode

The applications of these various modes are numerous. Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied. Shear mode is particularly useful in clutches and brakes - in places where rotational motion must be controlled. Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces. This

particular flow mode has seen the least investigation so far. Overall, between these three modes of operation, MR fluids can be applied successfully to a wide range of applications

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.

Applications

The application set for MR fluids is vast, and it expands with each advance in the dynamics of the fluid.

>>Mechanical EngineeringMagnetorheological dampers of various applications have been and continue to be developed. These dampers are mainly used in heavy industry with applications such as heavy motor damping, operator seat/cab damping in construction vehicles, and more.

As of 2006, materials scientists and mechanical engineers are collaborating to develop stand-alone seismic dampers which, when positioned anywhere within a building, will operate within the building's resonance frequency, absorbing detrimental shock waves and oscillations within the structure, giving these dampers the ability to make any building earthquake-proof, or at least earthquake-resistant.

>>Military and DefenseThe U.S. Army Research Office is currently funding research into using MR fluid to enhance body armor. In 2003, researchers stated they were five to ten years away from making the fluid bullet resistant.[8] In addition, Humvees, certain helicopters, and various other all-terrain vehicles employ dynamic MR shock absorbers and/or dampers.

>>OpticsMagnetorheological Finishing, a magnetorheological fluid-based optical polishing method, has proven to be highly precise. It was used in the construction of the Hubble Space Telescope's corrective lens.

>>Automotive and AerospaceIf the shock absorbers of a vehicle's suspension are filled with MR fluid instead of plain oil, and the whole device surrounded with an electromagnet, the viscosity of the fluid (and hence the amount of damping provided by the shock absorber) can be varied depending on

driver preference or the weight being carried by the vehicle - or it may be dynamically varied in order to provide stability control. This is in effect a magnetorheological damper. The MagneRide magnetic ride control (a kind of active suspension) is one such system which permits the damping factor to be adjusted once every millisecond in response to conditions. GM is the origin company of this technology as applied to automobiles. As of 2007, BMW manufactures cars using their own proprietary version of this device, while Audi and Ferrari offer the MagneRide on various models. All Corvettes made since 2005 have also employed a dynamic MR suspension system.

>>Vehicle Suspension Dampers

The MR damper has a built-in MR valve across which the MR fluid is forced. The piston of the MR damper acts as an electromagnet with the required number of coils to produce the appropriate magnetic field. Also the MR damper has a run-through shaft to avoid an accumulator.

>>MR Transmission Clutches

They are used in automotive power train to transmit torque from the engine to the transmission and the vehicle. The MR sponge clutch may be used to provide launch control of an automobile thereby achieving

smooth vehicle launch. The MRF clutch thus may replace the existing torque converters and therefore help increase the fuel economy.

General Motors and other automotive companies are seeking to develop a magnetorheological fluid based clutch system for push-button four wheel drive systems. This clutch system would use electromagnets to solidify the fluid which would lock the driveshaft into the drive train.

Magnetorheological dampers for use in military and commercial helicopter cockpit seats, as safety devices in the event of a crash, are under development. This decreases the shock delivered to each passenger's spinal column thereby decreasing the rate of permanent injury during a crash.

Porsche has introduced magnetorheological engine mounts in the 2010 Porsche GT3 and GT2. At high engine revolutions, the magnetorheological engine mounts get stiffer to provide a more precise gearbox shifter feel by reducing the relative motion between the power train and chassis/body.

Recent AdvancesRecent studies which explore the effect of varying the aspect ratio of the ferromagnetic particles have shown several improvements over conventional MR fluids. Nanowire-based fluids show no sedimentation after qualitative observation over a period of three months. This observation has been attributed to a lower close-packing density due to decreased symmetry of the wires compared to spheres, as well as the structurally supportive nature of a nanowire lattice held together by remnant magnetization. Further, they show a different range of loading of particles (typically measured in either volume or weight fraction) than conventional sphere- or ellipsoid-based fluids. Conventional commercial fluids exhibit a typical loading of 30 to 90 wt%, while nanowire-based fluids show a percolation threshold of ~0.5 wt% (depending on the aspect ratio). They also show a maximum loading of ~35 wt%, since high aspect ratio particles exhibit a larger per particle excluded volume as well as inter-particle tangling as they attempt to rotate end-over-end, resulting in a limit imposed by high off-state apparent viscosity of the fluids.

This new range of loadings suggest a new set of applications are possible which may have not been possible with conventional sphere-based fluids.

Newer studies have focused on dimorphic magnetorheological fluids, which are conventional sphere-based fluids in which a fraction of the spheres, typically 2 to 8 wt%, are replaced with nanowires. These fluids exhibit a much lower sedimentation rate than conventional fluids, yet exhibit a similar range of loading as conventional commercial fluids, making them also useful in existing high-force applications such as damping. Moreover, they also exhibit an improvement in apparent yield stress of 10% across those amounts of particle substitution.

LimitationsAlthough smart fluids are rightly seen as having many potential applications, they are limited in commercial feasibility for the following reasons:

High density, due to presence of iron, makes them heavy. However, operating volumes are small, so while this is a problem, it is not insurmountable.

High-quality fluids are expensive. Fluids are subject to thickening after prolonged use and need

replacing. Settling of ferro-particles can be a problem for some applications.

Commercial applications do exist, as mentioned, but will continue to be few until these problems (particularly cost) are overcome.

BIBLIOGRAPHY

1. www.wikipedia.com2. www.google.com3. "Mechanical properties of magnetorheological fluids under

squeeze-shear mode" by Wang, Hong-yun; Zheng.4. "Physical Properties of Elongated Magnetic Particles" by Fernando

Vereda, Juan de Vicente.5. “Influence of particle shape on the properties of magneto

rhelogical fluids.” By R.c.bell and E.D.Miller.