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Rheological properties of soft magnetic flake shaped iron particle based magnetorheological

fluid in dynamic mode

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2014 Smart Mater. Struct. 23 015002

(http://iopscience.iop.org/0964-1726/23/1/015002)

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Smart Materials and Structures

Smart Mater. Struct. 23 (2014) 015002 (8pp) doi:10.1088/0964-1726/23/1/015002

Rheological properties of soft magneticflake shaped iron particle basedmagnetorheological fluid in dynamicmode

R V Upadhyay, Zarana Laherisheth and Kruti Shah

P D Patel Institute of Applied Sciences, Charotar University of Science and Technology,CHARUSAT-Campus, Changa 388 421, India

E-mail: rvu.as@charusat.ac.in

Received 14 October 2013, in final form 1 November 2013Published 6 December 2013

AbstractIn this work, the effect of particle shape (flakes) on the magnetorheological (MR) properties ofan iron based MR fluid, constituted of two different volume fractions of particles dispersed ina liquid carrier, is studied. To compare the MR effect, spherical iron carbonyl particle basedMR fluid is studied. In both MR fluids, linear viscoelastic behavior has been extensivelyinvestigated using small amplitude oscillatory analysis and magnetic sweep tests, in thepresence and absence of a magnetic field (H). The amplitude sweep tests reveal thatflake-based MR fluid shows a higher storage modulus compared to sphere-based MR fluid andsaturates at a lower magnetic field strength. The variation of storage modulus with magneticfield strength shows an Hn dependence, where n varies from 2.2 to 2.4 for 20% volumefraction while it varies from 1.6 to 2 for a dilute sample. In the case of sphere-based MR fluid,at 20% volume fraction the variation of storage modulus is nearly linear with the magneticfield at low strain amplitude, and with increasing strain amplitude shows H2 dependence. Atlower volume fraction in both cases, the loss modulus increases linearly with the magneticfield strength. The observed enhancement in the MR effect in the flake-based MR fluid islikely due to the stronger particle–particle interaction which results in higher friction betweenthe particles. The sedimentation rate decreases by nearly 50% when flakes are used. The studyreveals that one can use the irregular shaped particles for MR applications at low fields(∼80 kA m−1).

(Some figures may appear in colour only in the online journal)

1. Introduction

A tunable and reversible transition from a solid-like toa liquid-like state upon magnetic field is the basis ofvarious industrial applications of rheological suspensions,known as magnetorheological (MR) fluids [1–12]. Typically,MR fluids are suspensions of magnetic microspheres in anon-magnetic medium; their MR properties with referenceto the magnetic field induced effect, the effect of magneticvolume fraction, size effects, etc, are reported extensively[1, 2, 7–9]. In the last few years, more attention has been

paid to (i) improving the stability against sedimentation andre-dispersion [13–16] and (ii) the development of new MRsuspensions with enhanced MR effect, such as high yieldstress [9, 15, 17, 18]. Carbonyl iron (CI) with micron sizedparticles is widely used in MR fluids due its soft magneticcharacteristics and high magnetization values. However, thisfluid has an issue with sedimentation of iron particles dueto the density mismatch. Therefore, to improve the stabilityagainst sedimentation, many researchers have studied variousmethods like coating the magnetic particles with polymers,the addition of thixotropic additives, non-spherical particles,

10964-1726/14/015002+08$33.00 c© 2014 IOP Publishing Ltd Printed in the UK

Smart Mater. Struct. 23 (2014) 015002 R V Upadhyay et al

core–shell composite particles with CI magnetic cores, theaddition of nanomagnetic particles into micro-particle basedsuspensions, etc [19–23].

Very few reports exist on the influence of shape on theMR properties of MR fluids [24–26]. Bell et al [25] havereported an increase in the yield stress with the magneticfield of a suspension containing magnetic wires compared tothat of a spherical particle suspension. Extensive experimentaland theoretical work is reported by Lopez-Lopez et al [27]and Kuzhir et al [28] on the MR properties of magneticcobalt fibers dispersed in a carrier liquid. It was found thatthe suspension developed a much larger value of the yieldstress than the equivalent suspension of cobalt microspheres.They proposed that the existence of field dependent solidfriction between the fibers as well as shape anisotropyis the primary physical basis for the observed enhancedmagnetorheological effects. A similar study was also carriedout on rod-shaped particles by Vincent et al [29], and theyalso assigned the enhanced MR effect as due to the friction. Itis believed that stronger MR fluids will use these anisotropicparticles.

The aim of this work is to study in detail the MRproperties of a suspension of magnetic flakes. The effectivesize of the particles is large enough so that magnetic forcesovercome thermal movement (non-Brownian particles) andalso small enough to prevent sedimentation. Additionally, theparticles are anisotropic; therefore, lower volume fractionsare required to have a gap-spanning structure. In thisstudy, the main goal is to understand the MR properties,elastic and viscous contributions, without destroying themicrostructure. For this reason, we have performed theexperiments in an oscillatory shear flow using a parallel-platemagnetorheometer under the presence of a magnetic field.These results are also compared with an MR suspensionhaving spherical microspheres.

2. Experimental methods

2.1. Materials and preparation

The driving force of the sedimentation is the gravity, whichdominates in comparison with Brownian motion for particleslarger than 0.1 µm. At the same time, the fluid also needsproper magnetic properties, which can only be provided bylarger sized particles. Therefore, we have chosen carbonyliron particles because of their high magnetic permeability andlow coercivity. Two types of MR suspension were prepared:one contained spherical iron particles (S) (CM grade, BASF,Germany) and the other one contained flake shaped ironparticles (F) (Industrial Metal Powder, Pune, India). Theessential material characteristics of the two differently shapedparticles according to the product information are as follows:the average size of the particles (d50) is 7–8 µm and theamount of α-iron is greater than 97%. The particle density wasabout 7.86 g cm−3. Commercially available paraffin liquidwith a viscosity of 132 mPa s and density of 0.89 g cm−3

at 25 ◦C was used as the suspending medium.

The MR fluids contained two different iron loadings(MRF-S: MR fluid with spherical iron particles; MRF-F: MRfluid with flake shaped particles), 10% and 20% by volume(ϕ), and were direct mixtures of iron particles and paraffinliquid. In both cases, a small amount of stabilizing additivewas added and mixed at room temperature and stirred andhomogenized until good dispersion was achieved.

2.2. Particle characterization

The structure and morphology of the particles werecharacterized using a powder x-ray diffractometer (D2-Phaser, Bruker, Germany) and scanning electron microscopy(SEM). The x-ray patterns were refined using Topaz software.The quasistatic magnetic properties of both kinds of particleswere measured using a vibrating sample magnetometer(VSM) (LakeShore Model 7404, USA). Both kinds ofparticles were projected into a magnetic field of strength±17 kOe and the saturation magnetization and coercivitywere measured.

2.3. Dispersion stability

A sample of each MR fluid was transferred to a 10 mlgraduated (scale division: 0.1 ml) cylinder, and placed instatic storage. The volume of supernatant liquid was recordedwith time from the phase boundary between the concentratedsuspension and the supernatant liquid. The sedimentation ratiois defined as follows:

sedimentation ratio (%)

= (volume of supernatant liquid/volume of entire

suspension)× 100%.

2.4. Magnetorheological properties

Constant temperature (25 ± 1 ◦C) magnetorheologicalmeasurements were performed using a commercial rheometer(MCR 301, Anton-Paar, Germany) with a controlled magneticfield supported by an MR device (MRD70/1T). A parallel-plate measuring system with a diameter of 20 mm andgap of 1 mm was used. The parallel plate used for MRcharacterization was of titanium, and the magnetic field wasapplied normal to the direction of flow.

A magnetorheological fluid exhibits linear viscoelastic(LVE) range properties when the applied strain amplitude issmaller than the yield strain. Hence, prior to carrying outviscoelastic measurements, the storage and loss moduli weremeasured as a function of strain at an angular frequency (ω)of 10 rad s−1, in the presence of the following magneticfields: 0 A m−1, 8.9 kA m−1, 26.2 kA m−1, 44.3 kA m−1,59.3 kA m−1 and 72.3 kA m−1. The moderate magneticflux density used in oscillation mode is to ensure sufficienthomogeneity of the magnetic field perpendicular to the shearflow direction. Then, magneto-sweeps were carried out at astrain amplitude of γ0 = 0.005% (this value is well belowthe LVE in all cases) and at ω = 10 rad s−1 (frequency,f = 1.6 Hz). The low value of frequency (f = 1.6 Hz) is

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Figure 1. SEM image of flakes (a) (the inset shows a magnified view of two flakes) and (b) x-ray powder diffraction patterns for flakes.

to make sure that there are insufficient inertial forces in thesample during oscillation. According to Ferry [30], the inertialforces are small when the sample thickness is much smallerthan (2πη/ρf )1/2. Here, ρ is the density of the sample inkg m−3 and η is the viscosity of the sample. Considering thevalues of dynamic viscosity of 2.27 Pa s (at a shear rate of100 s−1), density of 2200 kg m−3 and oscillation frequency of1.6 Hz, the sample thickness (or gap between the two parallelplates) should be much smaller than 63.6 mm in order tohave negligible inertial effects. In the present situation wehave considered a 1 mm gap; thus the inertial effects arenegligible.

The experimental protocol used for field dependent LVEwas as follows: (i) pre-shear at a constant 20 s−1 for 60 s;(ii) left to stabilize under zero shear for 60 s and thenthe desired magnetic field is applied and there is another60 s wait; (iii) strain is increased from 0.001 to 100% ata rate of ten points per decade. For the magneto-sweep, asimilar protocol to that defined above is used until step (ii).Then, the dynamic-mechanical shear conditions, the shearstrain of 0.005% and ω = 10 rad s−1 are kept constant andthe magnetic field is increased logarithmically from 0.1 to174 kA m−1 at a rate of ten points per decade. In all casesfresh new samples were used for each measurement.

3. Results and discussion

3.1. Structure and morphology

Figure 1(a) shows the size and surface morphology of theflake shaped iron particles (F) observed by SEM. The averagesize of the particles (inset of figure 1(a)) is 2 µm. Theparticles are of different shapes and sizes, ranging from 2 to10 µm. The Microtech particle size analyzer gives a d50 sizeof around 6–7 µm. Figure 1(b) shows typical powder x-raydiffraction patterns for flake shaped (F) particles recordedat room temperature. All the diffraction peaks correspond to

body centered cubic structure having a lattice parameter of0.2867 ± 0.0001 nm.

3.2. Magnetic properties of the particles and MR fluids

The magnetic properties of the powder samples andfluid samples are depicted in figure 2(a). The saturationmagnetization (Ms) and low field initial susceptibility (χi)were obtained from the best fit to the Frohlich–Kennellylaw [31],

M(H) = (χiMsH)/(Ms + χiH),

where M and H are the magnetization and magnetic fieldintensity, respectively. From the fit, the Ms values for thesphere (S) and flake (F) samples are 1650 ± 15 kA m−1

and 1250 ± 15 kA m−1, respectively. The value of χi,for both the samples, is 7.5. The magnetization curves forMRF-F and MRF-S fluid samples containing different volumefractions of magnetic loading, 1%, 10% and 20%, are alsoshown in figures 2(b) and (c). Also shown are the theoreticalmodel results computed from the Frohlich–Kennelly law. Themagnetization increases with volume fraction and it seemsto saturate at about ϕMs. For example, the MR fluid sampleloaded with 20% spherical iron particles saturates at around(0.2) × (1650 kA m−1) = 330 kA m−1. The initial low fieldsusceptibility of the fluid is 3.

3.3. Sedimentation stability

Sedimentation stability tests were conducted on both fluidswith 10% volume fraction. The sedimentation for a 20%sample was extremely slow and almost identical for boththe samples; the rate of sedimentation was about 0.1%per day (thus it is not shown here). Figure 3 shows thesedimentation ratio as a function of time for both MR fluids.The MR fluid with anisotropic iron particles shows bettersedimentation stability than that with spherical iron particles.

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Figure 2. Magnetization curves of the particles (a), flake-based MR fluid (MRF-F) for different volume fractions of iron particles (b) andsphere-based MR fluid (MRF-S) for different volume fractions (c) as used in the experiments. The drawn line corresponds to theFrohlich–Kennelly law fitted to the data.

Figure 3. Sedimentation percentages of MR fluids (10% volumefraction) based on flakes (F) and spheres (S). The increase is almosttwo times higher for the sphere-based MR fluid compared to theflake-based one.

This is expected as the anisotropic shape hinders settling. Inboth cases, re-dispersion was easy, because of the formationof less dense sediment. The reason for the observed retentionin sedimentation for the spherical sample during the first 1.5 his not clear at present.

3.4. Magnetorheological effect

3.4.1. Amplitude sweep mode.. A steady shear rheologicalstudy (not reported here) has shown that MR fluid exhibitsBingham plastic behavior with a yield stress. This is acharacteristic feature of MR fluids and is caused by thedevelopment of a robust chain structure [2]. To investigatethe viscoelastic behavior, as well as the process of chainformation in MR fluid, an oscillatory test (amplitude sweepand magneto-sweep) was performed. The variation in thestorage modulus (G′) and loss modulus (G′′) with increasingmagnetic flux density allows the development of chainstructure in the MR fluid [1–5]. It is expected in an MRsuspension that, with increasing magnetic field, the storagemodulus and loss modulus will increase.

Figures 4(a) and 5(a) show the dependence of G′ and G′′

on the strain amplitude at 25 ◦C with varying magnetic fieldstrength for both the MR suspensions with different volumefractions. In all cases, the storage modulus for flakes is largerthan the storage modulus for spheres. The observed largestorage modulus G′, under the magnetic field, is associatedwith the field induced structures and is typically at leastone order of magnitude larger than the loss modulus, G′′.For an extremely small shear strain amplitude, the elasticpart of the viscoelastic modulus (G′) is independent of theapplied strain. This region is known as the linear viscoelasticregion. In this region, magnetic induced structures remainundisturbed. As the strain amplitude increases, the chainstructure starts to break, and as a result the system showsdeviation from nonlinearity, as well as from viscoelasticbehavior. The viscous modulus and elastic modulus intersectat G′ = G′′ and above this point the system starts to flow. Thestrain at which this condition is satisfied also increases withincreasing magnetic field.

Figures 4(b) and 5(b) show the variation of the storagemodulus with applied magnetic field for various strainamplitudes. The value of the storage modulus is largercompared to sphere-based MR fluid by a factor of two at lowstrain amplitude (≤0.015%) and a factor of four for higherstrain amplitude (>0.2%). The variation of G′ with the fieldand volume fraction is different for the MRF-S and MRF-Ffluids. The elastic part, storage modulus G′, can be viewed asthe magnetic induced part because it is the energy stored in thematerial. The viscous, loss modulus G′′ can be viewed as thefriction part. Different models have been proposed to explainthe variation of G′ or yield stress with field [32–36].

Both macroscopic (employing the magnetic energyminimization principle) and microscopic (taking into accountinter-particle interactions) models were applied effectivelyin the variation of the yield stress. However, microscopicmodels, which include the anisotropic polar magnetostatic in-teractions between particles, give reasonable agreement withexperiments compared to macroscopic models. According toRamos et al [35], at small field strength, the yield stress isproportional to the square of the magnetic field strength; asimilar quadratic dependence is also predicted by Vincent

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Figure 4. Amplitude sweep results for MRF-F fluid for different volume fractions at different magnetic fields. (a) Variation of G′ (filledsymbols) and G′′ (open symbols) at different strain amplitudes for two different volume fractions. The variation is only shown for typicalmagnetic field strength values. (b) Magnetic field dependent maximum values of the storage modulus at different strain amplitudes (γ ) for20 and 10% volume fractions with a magnetic field. The solid line is a fit to Hn dependence and the dotted line is a guide to the eye.(c) Variation of G′′max (value at peak) with field (H).

Figure 5. Amplitude sweep results for MRF-S fluid for different volume fractions at different magnetic fields. (a) Variation of G′ (filledsymbols) and G′′ (open symbols) at different strain amplitudes for two different volume fractions. The variation is only shown for typicalmagnetic field strength values. (b) Magnetic field dependent maximum values of the storage modulus at different strain amplitudes (γ ) for20 and 10% volume fractions with a magnetic field. The solid line is a fit to Hn dependence and the dotted line is a guide to the eye.(c) Variation of G′′max (value at peak) with field (H).

et al [34]. At intermediate field, it becomes H3/2 dependenceand it is independent of the field strength at high fieldstrengths. According to Ginder et al [33], at intermediatefield quadratic dependence is no longer observed and G′ isproportional to H.

For MRF-S-20%, below 0.01% shear strain G′ varieslinearly with the magnetic field (figure 5(b)), while forhigher strain amplitude it exhibits a quadratic dependence. Incontrast, MRF-F-20% shows a slight departure from quadraticdependence, Hn (2.2 < n < 2.4), for strain amplitude greater

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Figure 6. Magneto-sweep results for flake-based and sphere-based MR fluids. The open symbols are for spherical particle based fluid andthe filled symbols are for flake-based MR fluid.

than 0.2% (figure 4(b)). At low strain amplitude, G′ tendsto saturation for H > 80 kA m−1 (figure 4(b)). On dilution,irrespective of strain amplitude, both the MR fluids (i.e.MRF-S and MRF-F) exhibit Hn behavior with n valuesbetween 1.6 and 2. This observed increase could be due to thecontribution of friction. The observed increase in the G′ valuewith the field is an indication of chain or column structuresin a system. On increasing the strain, this structure starts tobreak and finally it starts to flow. Under the assumption thatstress is purely a magnetic force, the shear stress due to thestorage modulus is calculated (τ = G′γ ). This value was oneorder of magnitude lower than the total shear stress obtainedfrom steady shear analysis. The discrepancies are likely due tothe assumption in this theory that only magnetic interactionscontribute to stress, and not other interactions. Interactionslike van der Waals, steric, etc, are more pronounced whennonisotropic magnetic particles are used (as is the case inthe present investigation). Thus, the observed low field G′

increase in MRF-F-20% (figure 4(b)) is possibly due tothe other forces contributing to the formation of strongerstructures in the MR fluid.

The variation of G′′ with strain amplitude shows apeak at a strain value which shifts to a higher value ofstrain with increasing field. At low strain and high volumefraction (figures 4(a), 5(a)) G′′ shows extremely weak fielddependence, but with increasing strain amplitude the value ofG′′ increases and reaches a maximum (G′′max) and then startsto decrease. It is known that the dissipation effect in MR fluid,which gives the G′′ variation, consists of two components, onethe viscous force in the carrier liquid and the other due tothe friction between the particles in the structure. The viscous

effect shows weak field dependence but the observed valueof G′′ cannot be accounted for due to the viscous effect only.Therefore, the observed higher value of G′′ in both MR fluidsindicates the dominance of the friction contribution. The valueof G′′ increases with increasing magnetic field. The observedhigher value of G′′ in MRF-F fluid compared to MRF-S fluidis likely due to the lowering of the demagnetization energyfor flakes (compared to spherical particles) which enhancesthe field induced torque. The variation of G′′max with thefield, for MRF-F and MRF-S MR fluids having magneticvolume fractions of 10% and 20% is shown in figures 4(c) and5(c), respectively. The observed saturation behavior indicatesthat structuration is easily achieved, and particle arrangementprevents plate motion.

3.4.2. Magneto-sweep mode.. Figures 6(a)–(c) show thedependence of G′ and G′′, and the loss factor, tan(δ) =G′′/G′, on the magnetic field strength, for a constant strainamplitude of γ = 0.005% (=5 µrad with 1 mm gap) anda constant angular frequency of ω = 10 s−1 (f = 1.6 Hz).It is clear from figures 6(a) and (b) that, regardless ofthe magnetic field strength, the storage modulus (G′) andloss modulus (G′′) values are higher for flakes compared tospherical-based MR fluid, similarly to what was observedin amplitude sweep mode. An increase in the values ofthe storage and loss moduli suggests structuration in theMR fluid with the field. Three regions are observed on themagneto-sweep curve (figure 6(a)). (i) A low field region(<3 kA m−1): both modules are constant, indicating weakinter-particle magnetostatic interactions which inhibit theformation of gap-spanning aggregates. In this region, the

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complex viscosity decreases slightly (figure not shown herefor brevity) and simultaneously the loss factor increases. Thisclearly shows that at low field the particle arrangement favorsflow behavior. (ii) An intermediate field region (2 kA m−1 <

H < 70 kA m−1): G′ and G′′ both increase while the lossfactor is at a maximum, as shown in figure 6(c). At this point,the chain structure predominates over the base fluid. Thismarks the transition from liquid behavior to a quasi-gel. Onfurther increasing the field the loss factor starts to decrease.The elastic modulus increases more than the viscous portionas the magnetic field increases. This hinders the orientationof magnetic particles along the field direction. (iii) A highfield region (>80 kA m−1): on further increase in magneticintensity, rigid chains are formed, and the sample becomesincreasingly ‘immobile’; the loss factor decreases further andG′ and G′′ have constant values.

4. Conclusion

This study shows that flake shaped particles of pure iron canbe used as the dispersing phase in an MR fluid. The advantageof using flakes is that they reduce the sedimentation ratewithout compromising the magnetic properties. The relativechanges in the magnetorheological effects are observedvia oscillatory and magneto-sweep tests using a rotationalrheometer. The observed increase in storage modulus andelastic modulus with magnetic intensity reflects the strongrobust chain structure. The greater control of G′ over G′′ is anindicator of a solid-like character of the MR fluid. The fieldrequired for structuration of the liquid is reduced (comparedto spherical particles), which may be due to the irregular shapeof the particles. The observed enhancement in the storagemodulus is likely due to the field dependent solid frictionbetween the flake shaped particles. This study reveals thatone can use irregular shaped particles for MR applications at80 kA m−1 field.

Acknowledgment

This work was carried out under project No. DST-161-Gsanctioned by the Department of Science and Technology,Government of India, New Delhi.

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