Magneto rheological fluid

Magneto-rheological fluids

CHAPTER 1: INTRODUCTION

1.1Background

The properties of smart fluids have been known for around sixty years, but were subject to

only sporadic investigations up until the 1990s, when they were suddenly the subject of renewed

interest, notably culminating with the use of an MR fluid on the suspension of the 2002 model of

the Cadillac Seville STS automobile and more recently, on the suspension of the second-

generation Audi TT. Other applications include brakes and seismic dampers, which are used in

buildings in seismically-active zones to damp the oscillations occurring in an earthquake. Since

then it appears that interest has waned a little, possibly due to the existence of various limitations

of smart fluids which have yet to be overcome.

A magneto rheological 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 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. Extensive

discussions of the physics and applications of MR fluids can be found in a recent book.

MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are

primarily on the micrometre-scale and are too dense for Brownian motion to keep them

suspended (in the lower density carrier fluid). Ferrofluid 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.

A magneto rheological damper or magneto rheological shock absorber is a damper

filled with magneto rheological fluid, which is controlled by a magnetic field, usually using an

electromagnet. This allows the damping characteristics of the shock absorber to be continuously

controlled by varying the power of the electromagnet. This type of shock absorber has several

applications, most notably in semi-active vehicle suspensions which may adapt to road

conditions, as they are monitored through sensors in the vehicle, and in prosthetic limbs.

SIET, VIJAYAPURA EEE DEPT


A smart fluid is a fluid whose properties can be changed by applying an electric field or

a magnetic field.

The most developed smart fluids today are fluids whose viscosity increases when a magnetic

field is applied. Small magnetic dipoles are suspended in a non-magnetic fluid, and the applied

magnetic field causes these small magnets to line up and form strings that increase the viscosity.

These magnetorheological or MR fluids are being used in the suspension of the 2002 model of

the Cadillac Seville STS automobile and more recently, in the suspension of the second-

generation Audi TT. Depending on road conditions, the damping fluid's viscosity is adjusted.

This is more expensive than traditional systems, but it provides better (faster) control. Similar

systems are being explored to reduce vibration in washing machines, air conditioning

compressors, rockets and satellites, and one has even been installed in Japan's National Museum

of Emerging Science and Innovation in Tokyo as an earthquakeshock absorber.

Some haptic devices whose resistance to touch can be controlled are also based on these MR

fluids.

Another major type of smart fluid are electrorheological or ER fluids, whose resistance to flow

can be quickly and dramatically altered by an applied electric field. Besides fast acting clutches,

brakes, shock absorbers and hydraulic valves, other, more esoteric, applications such as

bulletproof vests have been proposed for these fluids.

Other smart fluids change their surface tension in the presence of an electric field. This has been

used to produce very small controllable lenses: a drop of this fluid, captured in a small cylinder

and surrounded by oil, serves as a lens whose shape can be changed by applying an electric field.



1.1 MOTIVATION

Microrheology involves forcing probes externally and can be extended out of

equilibrium to the non linerar regime. Here we review the development, present state and

future directions of this field. We organise our review around the generalised stokes-

Einstein relation, which plays a central role in the interpretation of microrheology.

1.2 Motion control MR-Fluid

As motion control systems become more refined, vibration characteristics

become more important to a systems overall design and functionality engineers, however,

have tended to look at motion control and vibration as separate issues. Motion control, it

might be said, presents fairly familiar design engineering problems while vibration

suggests more subtle problems. Few design engineers have either the hands-on

experience or the training to address both sets of problems in a single design solution.



CHAPTER 2: WORKING PRINCIPLE2.1 Working

Fig 2.1: Working

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

2.2 Direction of magnetic flux

Fig 2.2: Direction of magnetic flux

To 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. 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, the

fluid actually assumes properties comparable to a solid when in the activated ("on") state, up

until a point of yield. 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


https://en.wikipedia.org/wiki/File:Smart_fluid_off_state.jpg

https://en.wikipedia.org/wiki/File:Smart_fluid_on_state.jpg


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 in the shear mode becomes:

Where = shear stress; = yield stress; = Magnetic field intensity = Newtonian viscosity;

is the velocity gradient in the z-direction.

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.

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.

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 occur far faster and with less

effort. For example, magneto rheological dampers 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, 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.



CHAPTER 3: MODES OF OPERATION

3.1 Flow mode

Fig 3.1: Flow mode

The fluid is located between a pair of stationary poles. The resistance to the fluid flow is

controlled by modifying the magnetic field between the poles, in a direction perpendicular to the

flow (Fig. 3.1). Devices using this mode of operation include servo-valves, dampers, shock

absorbers and actuators.

3.2 Shear mode

Fig 3.2 Shear mode

The fluid is located between a pair of moving poles (translation or rotation motion). The relative

displacement is parallel to the poles. The apparent viscosity, and thus the “drag force” applied by

the fluid to the moving surfaces can be controlled by modifying the magnetic field between the

poles. Devices using this mode of operation include clutches, brakes, locking devices


https://en.wikipedia.org/wiki/File:Mr_fluid_flowmode.jpg

https://en.wikipedia.org/wiki/File:Mr_fluid_shearmode.jpg


3.3 Squeeze-flow mode

Fig 3.3: Squeeze-flow mode

The fluid is located between a pair of moving poles. The relative displacement is perpendicular

to the direction of the fluid flow .The compression force applied to the fluid is varying

periodically. Displacements are small compared to the other modes but resistive forces are high.

As for the two other modes, the magnitude of these resistive forces can be controlled by

modifying the magnetic field between the poles. While less well understood than the other

modes, the squeeze mode has been explored for use in small amplitude vibration and impact

dampers.

3.4 Recent advances

Recent 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).


https://en.wikipedia.org/wiki/File:Mr_fluid_squeezeflowmode.jpg


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. These new

ranges of loading 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 magneto rheological 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.

Another way to increase the performance of magneto rheological fluids is to apply a pressure to

them. In particular the properties in term of yield strength can be increased up to ten times in

shear mode and up five times in flow mode. The motivation of this behaviour is the increase in

the ferromagnetic particles friction, as described by the semi empirical magneto-tribological

model by Zhang et al. Even though applying a pressure strongly improves the magneto

rheological fluids behaviour, particular attention must be paid in terms of mechanical resistance

and chemical compatibility of the sealing system used.



CHAPTER 4: APPLICATIONS OF MR-FLUID

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

the fluid

4.1 Mechanical engineering

Magneto rheological 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.

4.2 Military and defense

The 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. In addition, HMMWVs, and various other all-terrain vehicles employ

dynamic MR shock absorbers and/or dampers.

4.3 Optics

Magneto rheological finishing, a magneto rheological 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.



4.4 Automotive

If the shock absorbers of a vehicle's suspension are filled with magneto rheological fluid

instead of a plain oil or gas, and the channels which allow the damping fluid to flow between the

two chambers is surrounded with electromagnets, the viscosity of the fluid, and hence the critical

frequency of the damper, 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

across vastly different road conditions. This is in effect a magneto rheological damper. For

example, the MagneRideactive suspension system permits the damping factor to be adjusted

once every millisecond in response to conditions. General Motors has developed this technology

for automotive applications. It made its debut in both Cadillac as "Magneride and Chevrolet

passenger vehicles (All Corvettes made since 2003 with the F55 option code) as part of the

driver selectable "Magnetic Selective Ride Control (MSRC)" system in model year 2003. Other

manufacturers have paid for the use of it in their own vehicles, for example Audi and Ferrari

offer the MagneRide on various models.

General Motors and other automotive companies are seeking to develop a magneto rheological

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.

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.

4.5 Aerospace

Magnetorheological dampers are under development for use in military and commercial

helicopter cockpit seats, as safety devices in the event of a crash. They would be used to decrease

the shock delivered to a passenger's spinal column, thereby decreasing the rate of permanent

injury during a crash.



4.6 Human prosthesis

Magnetorheological dampers are utilized in semi-active human prosthetic legs. Much like

those used in military and commercial helicopters, a damper in the prosthetic leg decreases the

shock delivered to the patients leg when jumping, for example. This results in an increased

mobility and agility for the patient.



CHAPTER 5: ADVANTAGES & DISADVANTAGES

5.1 Advantages

Flow mode can we use in dampers and shock absorber.

Shear mode is particular useful in clutches and breaks and in place where rotational motion

must be controlled.

Switch flow mode is suitable for controlling small millimeter order movements.

Can be used in flow channels.

5.2 Disadvantages

Although 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.



CHAPTER 6: FUTURE SCOPE & CONCLUSION

6.1 Future Scope

Mechanical engineering, Magneto rheological 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

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.

The U.S. Army Research Office is currently funding research into using MR fluid to

enhance body armor

Can be used in the construction of the Hubble Space Telescope's corrective lens.

Magneto rheological dampers are under development for use in military and commercial

helicopter cockpit seats, as safety devices in the event of a crash

Magneto rheological dampers are utilized in semi-active human prosthetic legs.

6.2 Conclusion

future technology used in motor damping, operator seat/cab damping in construction

vehicles, and more

Ability to make any building earthquake-proof, or at least earthquake-resistant.

It was used in the construction of the Hubble Space Telescope's corrective lens.

Magneto rheological dampers are utilized in semi-active human prosthetic legs



REFERENCES

[1] Magnetorheology: Advances and Applications (2014), N.M. Wereley, Ed., Royal Society

of Chemistry, RSC Smart Materials, Cambridge, UK. DOI: 10.1039/9781849737548.

[2] "Mechanical properties of magnetorheological fluids under squeeze-shear mode" by

Wang, Hong-yun; Zheng, Hui-qiang; Li, Yong-xian; Lu, Shuang

"Physical Properties of Elongated Magnetic Particles" by Fernando Vereda, Juan de Vicente,

Roque Hidalgo-Álvarez

“Magnetorheology of submicron diameter iron microwires dispersed in silicone oil.” R.C.

Bell, J.O. Karli, A.N. Vavereck, D.T. Zimmerman. Smart Materials

“Influence of particle shape on the properties of magnetorheological fluids.” R.C. Bell, E.D.

Miller, J.O. Karli, A.N. Vavereck, D.T. Zimmerman. Journal of Modern Physics B. Vol. 21,

No. 28 & 29 (2007) 5018-5025.

“Elastic percolation transition in nanowire-based magnetorheological fluids.” D.T.

Zimmerman, R.C. Bell, J.O. Karli, J.A. Filer, N.M. Wereley, Applied Physics Letters, 95

(2009) 014102.

“Dimorphic magnetorheological fluids: exploiting partial substitution of micro-spheres by

micro-wires.” G.T. Ngatu, N.M. Wereley, J.O. Karli, R.C. Bell. Smart Materials and

Structures, 17 (2008) 045022.

"Study on the mechanism of the squeeze-strengthen effect in magnetorheological fluids " X.

Z. Zhang, X. L. Gong, P. Q. Zhang, and Q. M. Wang, J. Appl. Phys. 96, 2359 (2004).

A. Spaggiari, E. Dragoni "Effect of Pressure on the Flow Properties of Magnetorheological

Fluids" J. Fluids Eng. Volume 134, Issue 9, 091103 (2012).



HowStuffWorks "How Smart Structures Will Work"

Instant Armor: Science Videos - Science News - ScienCentral

G.J. Hiemenz,Y.-T. Choi, and N.M. Wereley (2007). "Semi-active control of vertical

stroking helicopter crew seat for enhanced crashworthiness." AIAA Journal of Aircraft,

44(3):1031-1034 DOI: 10.2514/1.26492

N.M. Wereley, H.J. Singh, and Y.-T. Choi (2014). "Adaptive Magnetorheological Energy

Absorbing Mounts for Shock Mitigation." Magnetorheology: Advances and Applications,

N.M. Wereley, Ed., Royal Society of Chemistry, RSC Smart Materials, Cambridge, UK.

Chapter 12, pp. 278-287, DOI: 10.1039/9781849737548-00278.


Magneto rheological fluid

Engineering

Transcript of Magneto rheological fluid