Magneto rheological fluid

Magneto-rheological fluids
SIET, VIJAYAPURA EEE DEPT Page 1
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.

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 PRINCIPLE
2.1 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

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

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

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