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STUDY OF NANO AND MICRON PARTICLES ON THE RHEOLOGICAL PROPERTIES OF THE
MAGNETORHEOLOGICAL FLUIDS
Seyed Mohsen Kamali 1 , Mehdi Jolaey 2* , Mohammad Haj heydari3 , Mohsen Dehghan Manshadi4
1- Department of Mechanical Engineering, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran
(M.kamali@iaufb.ac.ir).
2- Corresponding Author; Department of Mechanical Engineering, Najaf Abad Branch, Islamic Azad University, Najaf
Abad, Iran (mehdi.jolaei@iaukhsh.ac.ir).
3- Department of Mechanical & Mechatronic Engineering, koosha College of applied Scince and Technology, Tehran
Branch, Tehran, Iran (mohammad.hajhaydari@kt-uast.ac.ir).
4- Department of Mechanical Engineering, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran
(M.Dehghanmanshadi@iaufb.ac.ir).
ABSTRACT
In this paper the effect of nano and micron scale particles on the rheological properties of the magnetorheological
fluids. the Conventional magnetorheological fluids or common MRFs are suspensions of Carbonyl Iron
microparticles in a Castor oil as carrier fluid. the Bidisperes fluids (BD) are suspensions of mixtures of Carbonyl
Iron microparticles and Silica Nanoparticles in the Castor oil. Ferrofluids (FF) are suspensions of nano–sized Fe3O4
in the Castor oil. All three fluids had a solids loading of a 60%wt. The yield stress and postyield viscosity of the MR
fluid were characterized using a Bingham-plastic model. The goal of this study is to find an optimal composition of
the MR fluids that provides the best combination of high yield stress and low settling rate based on empirical
measurements.
KEY WORDS: Magnetorheological fluids, Yield Stress, Postyield Viscosity, Nanoparticles.
INTRODUCTION
Magnetorheological (MR) fluids are suspensions of surfactant coated micron sized magnetically permeable particles
dispersed in a non magnetizable carrier fluid (Jolly et al., 1996) Their nanosized counterparts are called ferrofluids
(Rosenzweig, 1985) Both these fluids fall under the category of smart materials, whose rheological properties are
controllable with a magnetic field. Over the last few decades, both MR (Xu et al., 2012) and ferrofluids (Ramos et al.,
2010). have been a topic of intense research due to their technological applications in diverse fields besides basic
scientific understanding. As rheological properties of dispersions are important from their application point of view,
more researchers venture into this research field,42,58,59 especially after nanofluids became popular because of their
interesting thermal properties (Philip et al., 2008). Carbonyl iron (CI) is one of the most commonly used particle to
prepare MR fluids due to its high magnetic permeability, soft magnetic property and common availability. Mineral oil,
silicone oil, polyesters, polyethers, synthetic hydrocarbons, vegetable oils, water etc. are used as carrier fluids. MR
fluids show large change in their rheological properties (reversibly change from a liquid like state to a solid like state)
when subject to an external magnetic field. This property of MR fluids is being exploited for several applications in
dampers, shock absorbers, torque transducers, clutches, brakes, ultrafine polishing technology etc (Park et al., 2010).
Despite these advantages, MR fluids suffer from serious problems arising due to sedimentation of the micron sized
particles. However, ferrofluids are relatively stable, owing to smaller sized suspended ferromagnetic particles that are
chemically modified (Muthukumaran et al., 2012). Several techniques have been adopted to enhance the stability of
these fluids like addition of surfactants to provide electrostatic or steric repulsion, addition of magnetic nanoparticles,
addition of thickening agents like carbon fibers, silica nanoparticles, use of ionic carrier liquids etc (López-López et al.,
2009) have studied the magnetorheological properties of magnetic carbonyl iron nanoparticle added MR fluids They
reported enhancement in yield behaviors of the CI nanoparticle added suspension compared to the pure MR suspension.
The added nanoparticles are found to aid tight packing of the CI particles when a field is applied. Single walled carbon
nanotubes (SWNT) were added to soft magnetic carbonyl iron suspension and the yield stress scaling and viscoelastic
properties were studied. SWNTs added suspensions showed increased yield stress than pure CI suspensions and were
also able to reduce the sedimentation of the CI particles. Choi et al. (2010) have also studied MR suspensions
containing fumed silica which was found to reduce the sedimentation of micron-sized CI particles and improve the
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flocculation stability. tried to reduce the sedimentation problem by using organoclay additive that formed a three
dimensional network thus preventing sedimentation. Polymer coatings on the CI particles were found to decrease their
sedimentation rate due to decreased density of the CI-polymer composite. Poly methyl methacrylate (PMMA)
encapsulated CI particles showed larger yield stress as well as enhanced anti-sedimentation stability. MR suspensions
containing magnetizable fibers have good sedimentation stability as against the spheres.They also have larger
magnetorheological response than the sphere based suspensions. The objective of this work is to study the
magnetorheological properties of common MRF, bidespers and ferrofluid. two key experiments were conducted in this
study: (a) settling tests were conducted using a laser scattering device to track mudline formation in a column of MR
fluid in the absence of field, and (b) rheological tests were conducted using a parallel disk rheometer to characterize
key rheological properties including Bingham yield stress, postyield viscosity, and elastic limit yield stress.
MATERIALS AND METHODS
Common MR fluid used in our study was prepared by dispersing surfactant coated Carbonyl Iron particles of size 3-12
microns into castor oil (carrier oil). Bidesperse used in our study was prepared by dispersing surfactant coated Carbonyl
Iron particles of size 3-12 microns into silica nanoparticles added castor oil. The surfactant employed was sodium
sulphonate of castor oil and was synthesized from the carrier fluid. The details of the carrier fluid, Carbonyl Iron and
synthesis of surfactant can be found elsewhere. Fumed silica particles having an size of 20-70 nm was used to form a
pseudo cross-linking network to reduce the settling behavior of Carbonyl Iron particles thus enhancing the stability of
the MR fluid. The silica nanoparticles also play a role in enhancing the packing density when the MR fluid solidifies
under magnetic field which in turn increases the yield stress of the system. Ferofluid used in our study was prepared by
nanoparticles of Magnetite (Fe3O4) added castor oil. Magnetite nanoparticles used in our study are synthesized by a
simple co-precipitation technique. sodium sulphonate was utilized as a surfactant for producing nanofluidic dispersions.
sodium sulphonate (2wt% – optimized) was mixed in castor oil using a high speed emulsifier .Fluid chain formation in
magnetorheological fluids. at speeds close to 11,000 rpm. Silica nanopowder obtained from the microwave plasma
synthesis technique, or commercially available micron sized powder, were added to the oil and the mixing was
continued. The mixing speed was kept constant at 11,000 rpm for a constant mixing time of 20min for all the MR fluid
specimens. All MR fluid had a 60wt% solids powder that in BD 22wt% of solids powder are nanosilica and 38wt% of
solids powder are micron sized of Carbonyl Iron. Each sample was prepared with 77.25g of solids powder, 1.5g of
sodium sulphonate used as the surfactant, and 50g of castor oil as the carrier fluid. Figure 1. Shows the Microscopic
image of carbonyl iron based magnetorheological fluid stabilized with silica nanoparticles in the absence of magnetic
field.
Figure 1. Microscopic image of Bidisperse.
RHEOLOGICAL TESTING
The rheological results obtained in this study were obtained using a Paar Physica MCR300 parallel disk rheometer. In
this study, a standard gap of 1mm was used to separate the parallel disks. The magnetic circuit is designed so that the
magnetic field lines are perpendicular to the parallel disks. The MR cell (TEK 70MR) is capable of continuously
varying the magnetic field applied to the MR fluid sample. The MR cell also included a water-based heating/cooling
system, so that a temperature of 25˚C was maintained for all data reported here. As the top disk rotates above the
stationary bottom disk or platen, a load cell measures the torque and a shaft encoder measures the angular rate. The
MCR300 software then computes the shear stress versus shear rate flow curve for the MR fluid sample consisting of
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0.31mL from each sample. Two classes of tests were performed as a function of magnetic field: (a) constant RPM tests
to measure the flow curve (shear stress vs. shear rate) and (b) oscillatory tests where the amplitude of oscillation varied
and the complex modulus was measured. Several practical difficulties were encountered in performing these tests: (a)
lack of a fluid container, (b) expulsion of fluid from the MR cell at high shear rates, and (c) sedimentation. Since the
rheometer uses a parallel disk configuration, there was no fluid container, and the fluid remained between the disks
only as the result of surface tension around the circumference of the disks. When the sample was placed between the
two disks of the rheometer, the sedimentation process begins immediately so that the homogeneity of the MR fluid,
and, thus, the consistency of the rheological characterization, was problematic. To ensure as much as possible that
homogeneous MR fluid samples were tested, an MR fluid sample bottle was placed in an ultrasonic mixing device and
agitated for 15min. A sample was then taken from the center of the bottle volume. The rheometer test was begun
immediately after preparation. When rheometer tests were conducted under a magnetic field, sedimentation was
prevented by applying a low magnetic field until the start of the test. Applying magnetic field in this way was efficient
at mitigating sedimentation, but utilization of this strategy was not possible when testing in the zero field condition.
Figure 2 shows a calibration curve for the input current to the viscometer and the magnetic flux Figure 2 .Calibration of
magnetic flux density (B, T) as a function of the input current at the gap between the parallel disks of the viscometer.
Figure 3.Measurement of particulate settling rate in MR fluids using a z-axis translating laser light scattering device to
measure mudline formation in a column of MR fluid in the absence of magnetic field. density at the gap. A thin Hall
sensor (F.W. Bell FH301) was placed in the MR fluid between the plate and the upper disk while current was applied.
Similar calibration data were obtained for all of the MR fluid samples. A nominal applied current of 2A (near the
maximum allowable current applied to the electro-magnet in the rheometer) corresponds to a magnetic flux density of
0.38T.
Figure 2.Calibration of magnetic flux density (B, T) as a function of the input current at the gap between the
parallel disks of the viscometer.
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Figure 3. Measurement of particulate settling rate in MR fluid using a z-axis translating laser light scattering device to
measure midline formation in a column of MRF in the absence of magnetic field.
RHEOLOGICAL FLOW CURVES
Using the parallel disk rheometer, rheological flow curves of shear stress versus shear rate were measured. In these
tests, approximately a 0.31mL sample of MR fluid was placed between the base platen disk and upper disk, where
rotation of the upper disk was accomplished via shear rate control. The range of shear rates tested was from 0.1 to 1500
s-1. The maximum shear rate was1500 s-1, because the fluid was expelled from between the disks at higher RPM. For
each fluid sample, 20 measurements were taken from 0.1 to 10 s-1, 20 points from 10 to 100 s-1, 20 points from 100 to
1000 s-1, and 10 points from 1000 to 1500 s-1. The only exception was for a relatively low current of 0.2A, when the
fluid was expelled from between the disks for shear rates above 1300 s-1.
For each test, the shear rate was held constant for 5 s until the measured shear stress reached a steady-state value, to
ensure consistency in the measurements. These measurements were taken over a range of current from 0.2 to 2.0A, in
increments of 0.2A. Also, during these tests, a temperature control system consisting of a chilled water system
maintained a constant temperature of 25˚C. Figure 6 shows typical flow curves for three of the tested fluid
compositions. In each case, the measurements are shown as circles. For these constant RPM rotational tests, a steadystate flow curve, or a shear stress, τ, versus shear rate, γ·, is measured by the rheometer. From these results, the
rheological properties of each MR fluid can be characterized with respect to a rheological model. In this study, the
Bingham-plastic model was used to characterize the flow curves. This is a generalized model for viscoplastic flow with
yield stress. The equation for the constitutive behavior is τ ‗ τy + μγ· γ· >0 (2)
The two parameters of the Bingham-plastic model are the yield stress, τy, and the postyield viscosity, μ. The Binghamplastic model has also been used to model Poiseuille flow in ER and MR dampers 81,82 .The Bingham-plastic model was
fit to the data using a weighted least squares error minimization by selection of the parameters (μ, τ y) for each fluid
tested at all values of applied field. These results are also shown in Figure 4(a)–(c). The measured shear rates were used
as the weights and resulted in the model being a better fit to the high shear rate data. Figure 5 shows the trends of the
Bingham-plastic yield stress as a function of magnetic field (Figure 5). As expected, the dynamic yield stress has a
strong dependence on the applied field. The trends of the Bingham-plastic (high shear rate) post-yield viscosity as
functions of magnetic field (Figure 6) are also shown. The variation in the plastic viscosity does not appear to follow
any precise trend, and further tests and refinements to our characterization are ongoing.
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Figure 4. Experimental data with Bingham-plastic model superimposed.
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Figure 5..Dynamic yield stress with the Bingham-plastic model: yield stress vs field
OSCILLATORY RHEOMETRY
The second category of rheological tests was an
amplitude sweep using oscillatory rheometry. Such a test is used to measure the viscoelastic characteristics of the fluid,
which are important to identify the micro-structure of the MR suspension in the presence of a magnetic field. A
viscoelastic fluid can be modeled by a complex modulus, G*=Gʹ + jGʺ. Gʹ is the in-phase elastic or storage modulus,
whereas Gʺ is the quadrature or loss modulus. But this characterization is valid only if the complex modulus is
measured in the linear viscoelastic (LVE) regime. Thus, the main purpose of the amplitude sweep test is to determine
the limits of the LVE range. As shown in Figure 7, the test was initiated for a low value of amplitude, and the
amplitude was slowly increased while tracking the storage and loss moduli.
Figure 6. Post-yield viscosity using a Bingham-plastic model: post-yield viscosity vs field
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Figure 7. Method of determining the elastic limit yield stress
The elastic limit yield stress can be determined Figure 5. Dynamic yield stress with the Bingham-plastic model: yield
stress vs field. as the point at which Gʹ deviates significantly (e.g., 1, 5, 10%) from the plateau value in the LVE
range84. In our study, the values of the storage modulus were recorded, and the limit of the LVE range was determined
to be 90% of the initial plateau value. At the edge of the LVE, the chain formations are disrupted and begin to break.
The shear strain and shear stress at this LVE threshold value are also measured, so that the elastic limit yield stress can
be determined85. The yield strain occurs in the range of 0.2 – 0.8%, which is comparable to values of yield strain
reported in the literature86,87. The elastic limit yield stress, plotted in Figure 8, is much lower than the measured
dynamic yield stress (Figure 5). This is also contrary to the measurements of the Bingham dynamic yield stress from
Figure 5. However, it should be noted that the dynamic yield stress is a more appropriate measure of MR fluid
performance for practical devices employing higher shear rates.
Figure 8. Elastic limit yield stress of MR fluids using oscillatory rheometry: elastic-limit yield stress vs magnetic flux
density.
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CONCLUSIONS
A key goal was to assess a kinds of micron and nano materials on rheological properties of MR fluids. Two key
experiments were conducted in this study: (a) settling tests were conducted using a laser scattering device to track
mudline formation in a column of MR fluid in the absence of field, and (b) rheological tests were conducted using a
parallel disk rheometer. Settling tests, as well as qualitative observation, Figure 3. Measurement of particulate settling
rate in three MR fluids. The Common MRF manifested a mudline in just over 50min and the Bidisperse manifested a
mudline in over 800min and the Ferrofluid exhibited mudline formation in over 1000min as measured by a laser
scattering device. then Ferrofluid had a least settling rate and Common MRF, Bidisperse had a more settling rate. Thus
Bidisperse had a stability and Ferrofluid had a upper stability. Rheological tests conducted using the parallel disk
rheometer also indicated that the nature of the flow curves changed as a function of kind of MR fluid. A key
observation was that because of increased shear thinning as nanoparticle increased, comparison of the bulk properties
of all of the MR fluid samples was problematic. Therefore, a Bingham-plastic model was used to characterize the
dynamic yield stress and post-yield viscosity of the MR fluid samples. At peak values of current (I=2A), the dynamic
yield stress was just over 15kPa for the Ferrofluid, increasing up to nearly 18.5kPa for the Bidisperse fluid and for
Common MRF is 21kPa. then the Common MRF had a most of the yield stress and Ferrofluid was least of the yield
stress. the elastic limit yield stress as measured using oscillatory rheometry, which the Ferrofluid is least and the
Common MRF had a most of the Elastic limit yield stress.
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