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Study of the effect of particle volume fraction on the microstructure of magnetorheological fluids using ultrasound: Transition between the strong-link to the weak-link regimes
Ultrasonics 61 (2015) 10–14
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short Communication
Study of the effect of particle volume fraction on the microstructure of magnetorheological fluids using ultrasound: Transition between the strong-link to the weak-link regimes Jaime Rodríguez-López ⇑, Pedro Castro, Luis Elvira, Francisco Montero de Espinosa Institute of Physical and Information Technologies, CSIC, 28006 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 8 April 2015 Keywords: Magnetorheological fluids Ultrasonic characterization Strong-link regimen Weak-link regimen Magnetic field
a b s t r a c t The effect of particle volume fraction on the microstructure of magnetorheological (MR) fluids has been studied using ultrasonic techniques. When no magnetic field is applied, they behave as slurry. However, when magnetic field is applied, important features regarding the change of the microstructure have been found with the help of ultrasonic waves propagating in the direction of the magnetic field. As the volume fraction increases, a rearrangement of particles which decrease the compressibility of the system is detected; nevertheless, the material behaves as a non-consolidated material. Three different particle volume fraction regions are found identifying a critical particle volume fraction predicted in the literature. Ultrasounds are confirmed as an interesting tool to study MR fluids in static conditions. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetorheological (MR) fluids are smart suspensions with tunable properties by applying an external magnetic stimulus. When no magnetic excitation is applied, they are isotropic suspensions of magnetic particles in a carrier fluid; however, when they are subjected to a magnetic field, the particles interact among them and the MR fluid becomes a structured and anisotropic material. The rearrangement of the particles into the MR fluid is the main mechanism responsible of the change of its mechanical properties [1–4]. In these kinds of smart systems, particles play the main active role. Particles which can be oriented by external magnetic fields can be used in these formulations; however, carbonyl iron powder (CIP) is the most widely used due to the reversibility and reproducibility of the magnetorheological effect [5]. Although CIP shows very good performance, it also presents some drawbacks regarding long term practical applications like sedimentation, abrasion or oxidation issues and difficulty to be redispersed. Nevertheless, these problems have been overcome coating them with, for example, organic materials [6], silica based coatings [7], cholesteryl functional groups [8], graphene oxide [9] or Zinc oxide [10] among others.
⇑ Corresponding author. http://dx.doi.org/10.1016/j.ultras.2015.03.011 0041-624X/Ó 2015 Elsevier B.V. All rights reserved.
The microstructure formed under the effect of the magnetic field depends on many factors: the shape, the nature and concentration of the particles, the properties of the carrier oil, the presence of surfactants and thickening agents, the uniformity and intensity of the magnetic field applied and the size of the cell where the fluid is studied [11–16]. These fluids are used in different industrial applications as it is the case of the automotive industry where they are components of high tech breaks and clutches. The MR fluids are also used as seals or earthquake absorbers [3,17–19]. As the properties of the MR fluids are strongly dependent on their inner structure, a better knowledge of the microstructure as a function of the external magnetic field will permit a better understanding of the response of these fluids and, therefore, an improvement of their industrial applications. To this end, different scientific works have been made to infer the microstructure of MR fluids from their optical, rheological or acoustical properties, both in static and dynamic conditions [16,19–25]. Different models have been proposed to explain the structure formation and the yielding process related to the application of a shear stress in MR fluids. Models have been usually applied successfully for very low particle volume fractions [26–28], while for higher volume fractions, simulations are compulsory [15,29,30]. The yielding of MR fluids of iron particles suspended in a low viscosity oil was studied as a function of the particle volume fraction by Segovia-Gutiérrez et al. [23]. In that work, a two-step behavior was observed when they were subjected
J. Rodríguez-López et al. / Ultrasonics 61 (2015) 10–14
to intermediate magnetic fields. This feature was explained as a transition from a strong-link to a weak-link particle regime when particle concentration was increased. Considering the situation on which the magnetic field energy is higher than the thermal energy, the rearrangement of the particles is induced. For low particle volume fraction, the fluid will be in the strong regime: the links between chain aggregates (transversal direction) are stronger than the links which keep particles forming the chain-like structures – longitudinal direction. Consequently, if any breakage under shear stress takes place, the breaking of bonds will occur within a chain-like aggregate. However, when particle volume fraction is high, the chains are thicker and there are lots of transversal connections between them. The chain aggregates are more resilient than the interlinks between different structures so two different breaking mechanisms are found; this is the weak-link regime. The particle volume fraction which gives way from one regime to another is a critical or threshold particle volume fraction which depends on the features of the suspension [23]. Ultrasonic techniques are very convenient to study MR fluids. They are non invasive, universal and give results on real time [31,32]. As ultrasonic propagation is sensitive to the inner structure of the fluid, changes in the mechanical stiffness and density in one direction will be accompanied by changes in the ultrasonic velocity and attenuation along this direction [33–35]. Due to attenuation mechanisms, acoustic filters or active matching layers associated to changes in the velocity have been studied using ultrasound [24,36]. Sedimentation processes, thermal behavior, hysteretic effects, yielding processes and evolution of the inner microstructure of MR fluids have been studied with these techniques [25,37,38]. In the present paper, a through-transmission ultrasonic technique has been used to measure the variations of the velocity of sound as a function of particle volume fraction in static conditions. Three different regions regarding the kind of structure have been found, identifying the particle volume threshold between the strong-link and the weak-link regimes. Pictures of the inner structure at the three regions have been taken.
2. Materials and methods Ten MR fluids of different particle volume fractions – from 0.01 to 0.35 – of carbonyl iron powder, grade CC (BASF), suspended in an epoxy resin with a viscosity of 1000 mPa s have been made. Due to the high viscosity of the host fluid, the suspensions are stable for at least 3 h as can be deduced from the velocity of sound measurement which remains constant. The carbonyl iron powder used has a magnetization saturation of 190 emu/g at 9 kOe at room temperature and a diameter between 3.8 lm and 5.3 lm. This particular grade has been chosen as particles show good ferromagnetic response and they are easily dispersed in the carrier fluid. Two piezoceramics – PZ 27 Ferroperm – with a frequency resonance of 1 MHz were used as the emitter (A) and the receiver (B). An electromagnet with 212-mT magnetic field (C) was used applying the field parallel to the ultrasonic propagation (D) as it is shown in Fig. 1b. The fluids were introduced in a 33 33 6 mm3cell, made of methacrylate, being the ultrasonic path 6 mm. The cell (1) was placed between the two poles of the electromagnet (5). The electromagnet, controlled by a current source (6), provides a very uniform magnetic field which affects the whole measuring cell. The ultrasonic emitter (A) was excited by a function generator – Agilent 33250A – (2). The ultrasonic received signal was acquired by an oscilloscope – Tektronics 1020 – (3) connected to a PC (4). The phase velocity of the ultrasonic waves was obtained from their spectrum phase using a FFT algorithm [39].
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Fig. 1. (a) Experimental set up. (b) Detail of the experimental set up where the cell is shown. The blue arrow (D) indicates the ultrasonic direction of propagation, while the red (C) one shows the direction of the magnetic field applied. Both fields are parallel. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The fluid fabrication procedure consists in mixing thoroughly the particles with the carrier fluid with a stirrer for about 10 min at 100 rpm till the mixture becomes homogenous slurry. After this step, the sample is introduced in a vacuum chamber at 1 kPa for 15 min to remove all the air bubbles. Next, the sample is poured into the cell and kept at 30 °C for 1 h until the sample reaches the thermal and mechanical equilibrium. After this step, the experiments are carried out, maintaining a constant temperature, in order to avoid fluctuations of the velocity induced by thermal variations. 3. Results In Fig. 2, it is shown the longitudinal velocity of sound as a function of the particle volume fraction (c/) when no magnetic field is applied (subscript X0, dark squares) and when the samples are subjected to a 212-mT parallel magnetic field (subscript XB, light circles). When no magnetic field is applied, the typical behavior of a suspension of high dense solid particles in a fluid is obtained. As a general trend, the velocity of sound depends inversely on the square root of the compressibility (j/) and density (q/), c/ = (j/q)1/2. Measuring the velocity and the density, the compressibility j/0 is calculated. As it is shown in Table 1, the higher the particle volume fraction, the lower the compressibility and the higher the density;
Fig. 2. Velocity of sound as a function of the particle volume fraction (c/), when no magnetic field is applied (dark squares) and when the sample is subjected to a 212mT magnetic field parallel to the ultrasonic waves (light circles).
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Table 1 Velocity of sound and its error (c/0 and ec/0 ), compressibility (j/0 ) and density (q/ ) as a function of particle volume fraction (/) when no magnetic field is applied. /
c/0 ec/0 (m/s)
j/0 ± 0.02 1010 (Pa1)
q/ ± 1 (kg/m3)
0 0.01 0.03 0.06 0.09 0.12 0.16 0.2 0.25 0.35
1611 ± 2 1570 ± 2 1516 ± 1 1438 ± 1 1376 ± 2 1310 ± 2 1266 ± 2 1221 ± 2 1187 ± 3 1151 ± 3
3.35 3.34 3.22 3.12 3.02 2.99 2.81 2.70 2.52 2.17
1149 1216 1349 1548 1748 1947 2213 2479 2812 3477
nevertheless, as the change in density is bigger than the change in the compressibility, the velocity of sound decreases with the increase of particle volume fraction. Once the velocities of sound of the MR fluids without magnetic field are obtained, the magnetic field is applied and the velocity of each suspension under this effect – light circles in Fig. 2 – is measured. Fig. 3 shows the measuring procedure used to obtain the velocity of sound (c) in the case of the suspension with 0.06 particle volume fraction as an example. First (I), no magnetic field is applied to check the stability of the suspension for 75 s. After, (II) the 212-mT parallel magnetic field is applied to the sample for 600 s. Finally, (III) the magnetic field is removed. The different sound speed results obtained in I and III highlight a hysteretic behavior. In this work, this method has been used to carry out the measurement of the velocity of sound at a moderate uniform magnetic field – 212 mT – as the changes are more evident. However, as it can be seen in the figure below, Fig. 4, the experimental set up has enough accuracy to measure changes in the velocity of sound at lower magnetic fields, 10 mT. This example corresponds to a MR fluid with 25% of particle volume fraction. The measured velocities of all the samples under the effect of the 212-mT magnetic field are shown in Table 2 with the calculated compressibility. Comparing the results of the velocity of sound when the magnetic field is applied to the case when it is not – Fig. 2, the next results are obtained. When the magnetic field is applied, the inner structures change and the fluid evolves from a homogenous suspension to an ordered material. Nevertheless, from Fig. 2 (light circles), the sound speed trend with the particle volume fraction is very similar to the case of no magnetic field applied. The system behaves closely to a
Fig. 4. Change of the velocity of sound as function of the intensity of magnetic field applied in a MR fluid with 25% particle volume fraction.
Table 2 Velocity of sound and its error (c/B and ec/B ), compressibility (j/B ) and density (q/ ) as a function of the particle volume fraction (/) when a magnetic field is applied. Dc/B ¼ c/B c/0 is the change of the velocity of sound with and without the magnetic field for each volume fraction. eDc/B is the error. /
c/B ec/B (m/s)
j/B ± 0.02 110
q/ ± 1 (kg/m3)
Dc/B eDc/B
1149 1216 1349 1548 1748 1947 2213 2479 2812 3477
0±2 8±2 13 ± 1 15 ± 1 13 ± 1 14 ± 1 14 ± 1 16 ± 1 18 ± 1 25 ± 2
(Pa1) 0 0.01 0.03 0.06 0.09 0.12 0.16 0.2 0.25 0.35
1611 ± 2 1578 ± 2 1529 ± 1 1453 ± 1 1389 ± 2 1324 ± 2 1280 ± 2 1237 ± 2 1204 ± 3 1176 ± 3
3.35 3.30 3.17 3.06 2.96 2.93 2.76 2.64 2.45 2.08
non-completely consolidated material [40]. Due to the high magnetic field applied, there are magnetic interactions among particles which create an inner frame, but the interaction strength is far from that of interparticle interactions in a solid. Nevertheless, the rearrangement of particles due to the magnetic interactions causes always an increase of the velocity of sound and, consequently, a decrease of the compressibility; the sample becomes more rigid. The higher the particle volume fraction, the more noticeable the sound speed changes are. More detailed information can be obtained about the microstructure of MR fluids when, for each volume fraction, the velocity of sound of the MR fluid with magnetic field applied (c/B ) is compared to that with no magnetic field (c/0 ) – Fig. 5(a). Doing so, the effect of the change in the microstructure due to the magnetic field is highlighted. Pictures of the microstructures taken at different volume fractions and with different angles are shown in Fig. 5(b) and (c). Fig. 5 (b) shows the images obtained when the camera is placed parallel to the ultrasonic propagation, and therefore parallel to the magnetic field lines. Fig. 5(c) corresponds to the same structures but the images are taken perpendicularly. Three regions can be distinguished: Three regions can be distinguished:
Fig. 3. Velocity change behavior of the sample having 0.06 particle volume fraction when initially no magnetic field is applied (I), then the magnetic field is switched on (II), and finally it is removed (III).
(a) Low volume fraction region (up to 0.06). There is a linear and strong increase on the change of the velocity of sound. Accordingly to the pictures shown in Fig. 4, in this region, especially at the lowest volume fractions, practically isolated chain-like structures or fibers are formed. As soon as more
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Ó IOP Science 2013
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Fig. 5. (a) Change of the velocity of sound, Dc/B , obtained subtracting c/B c/0 , for each volume fraction. Pictures of the microstructure for each volume fraction taken in parallel (b) and in perpendicular (c) to the acoustic propagation are shown, the latter are reprinted with permission from J. Phys. D: Appl. Phys. 46 065001.
particles are added, the structures become thicker and some of them start to be linked transversally. Due to defects in particles and the effect of close neighbors, perpendicular interactions are expected theoretically. This region seems to be related to the strong-link regime shown by SegoviaGutiérrez et al. [23]. If the particle volume fraction is increased up to 0.06, complex fiber structure interconnections start giving way to the second intermediate region. (b) Intermediate volume fraction region (from 0.06 to 0.09). The change in the velocity of sound starts decreasing slightly. Accordingly to the pictures, structures are more interconnected. Big oriented cavities are shown, which become smaller when the particle volume fraction increases. This second region could be considerate as an intermediate between a fiber-like structure and an interconnected like-porous material. Probably, transversal structures linking parallel structures are not enough significant to create stiff joints, but they are important enough to difficult the sound propagation avoiding the clearance of the fluid regions. As a result, the change of velocity decreases. This region can be related to the critical volume fraction shown by SegoviaGutiérrez et al. [23] in which the transition from the strong-link regime to the weak-link regime occurs. (c) High volume fraction region (from 0.09 up to 0.35). A clear increase of the change of velocity of sound as a function of the particle volume fraction is shown, but weaker than that of the low volume fraction region. Pictures show a concentrated, oriented and linked material with small cavities. There is a frame connected by horizontal and transversal structures. In this region, the contribution of the solid part becomes more important when adding more particles, highlighting the interconnected nature of the structures. This part could be related to the weak-link regime [23].
4. Conclusions In this work, ultrasonic techniques are confirmed as an important tool to study MR fluids, not only in dynamic conditions as it is shown in literature, but also in static conditions. Important features regarding the inner microstructure of these smart responsive
materials are found studying its velocity of sound. When no magnetic field is applied, MR fluids behave like suspensions of particles, while if a magnetic field is applied, due to magnetic interactions, particles reordered into an oriented inner frame. The structure formed is less compressible although it is not completely consolidated, being far from becoming a solid. Moreover, from the different trends of the change in the velocity of sound as a function of volume fraction, different microstructural regions that could be correlated with the strong-link and weak-link regimes described in the literature have been shown. A particle volume fraction region which separates both regions has been clearly identified as a transition region where the inner structures pass from a one directional structure to a three dimensional one. The methodology exposed in this work is a very powerful tool to study the structure of MR fluids under the effect of an external magnetic field. According to the accuracy of the experimental technique, this method opens new opportunities to analyze the effect of different factors, such as the viscosity, the intensity of the applied magnetic field or the kind and size of particles used on the microstructure of MR fluids. This work was supported by a CSIC JAE fellowship and funding from Spanish Ministry of Science and Innovation (DPI2013-46915C2-1-R). References [1] J. Rabinow, The magnetic fluid clutch, AIEE Trans. 67 (1948) 1308–1315. [2] G. Bossis, S. Lacis, A. Meunier, O. Volkova, Magnetorheological fluids, J. Magn. Magn. Mater. 252 (2002) 224–228. [3] A.G. Olabi, A. Grunwald, Design and application of magneto-rheological fluid, Mater. Des. 28 (2007) 2658–2664. [4] J. de Vicente, D.J. Klingenberg, R. Hidalgo-Alvarez, Magnetorheological fluids: a review, Soft Matter 7 (2011) 3701–3710. [5] A.J.F. Bombard, I. Joekes, M.R. Alcântara, M. Knobel, Mater. Sci. Forum 416–418 (2003) 753–758. [6] I.B. Jang et al., Role of organic coating on carbonyl iron suspended particles in magnetorheological fluids, J. Appl. Phys. 97 (2005) 10Q912. [7] P.-B. Nguyen et al., Brake performance of core–shell structured carbonyl iron/ silica based magnetorheological suspension, J. Magn. Magn. Mater. 367 (2014) 69–74. [8] M. Mrlik et al., Cholesteryl-coated carbonyl iron particles with improved anticorrosion stability and their viscoelastic behaviour under magnetic field, Colloid Polym. Sci. 292 (2014) 2137–2143. [9] W.L. Zhang, H.J. Choi, Self-assembly of graphene oxide coated soft magnetic carbonyl iron particles and their magnetorheology, J. Appl. Phys. 115 (2014) 17B508.
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[10] M. Machovsky, M. Mrlik, I. Kuritka, V. Pavlinek, V. Babayan, Novel synthesis of core–shell urchin-like ZnO coated carbonyl iron microparticles and their magnetorheological activity, RSC Adv. 4 (2014) 996–1003. [11] J.E. Martin, J. Odinek, T.C. Halsey, Evolution of structure in a quiescent electrorheological fluid, Phys. Rev. Lett. 69 (1992) 1524–1527. [12] J.E. Martin, K.M. Hill, C.P. Tigges, Magnetic-field-induced optical transmittance in colloidal suspensions, Phys. Rev. E 59 (1999) 5676–5692. [13] E.M. Furst, A.P. Gast, Dynamics and lateral interactions of dipolar chains, Phys. Rev. E – Stat. Phys., Plasmas, Fluids, Relat. Interdiscipl. Top. 62 (2000) 6916– 6925. [14] M. Fermigier, A.P. Gast, Structure evolution in a paramagnetic latex suspension, J. Colloid Interface Sci. 154 (1992) 522–539. [15] M. Mohebi, N. Jamasbi, J. Liu, Simulation of the formation of nonequilibrium structures in magnetorheological fluids subject to an external magnetic field, Phys. Rev. E 54 (1996) 5407–5413. [16] P. Dominguez-Garcia, S. Melle, M.A. Rubio, Morphology of anisotropic chains in a magneto-rheological fluid during aggregation and disaggregation processes, J. Colloid Interface Sci. 333 (2009) 221–229. [17] S.J. Dyke, B.F. Spencer, M.K. Sain, J.D. Carlson, Modeling and control of magnetorheological dampers for seismic response reduction, Smart Mater. Struct. 5 (1996) 565–575. [18] J.D. Carlson, J.L. Sproston, Controllable fluids in 2000-status of ER and MR fluid technology, in: Actuator 2000 – 7th Int. Conf on New Actuators, 2000, pp. 126– 130. [19] D.J. Klingenberg, Magnetorheology: applications and challenges, AIChE J. 47 (2001) 246–249. [20] S. Melle, O.G. Calderón, M.A. Rubio, G.G. Fuller, Microstructure evolution in magnetorheological suspensions governed by Mason number, Phys. Rev. E – Stat., Nonlinear, Soft Matter Phys. 68 (2003) 415031–41503111. [21] R. Tao, Super-strong magnetorheological fluids, J. Phys.: Condens. Matter 13 (2001) R979–R999. [22] X. Tang, X. Zhang, R. Tao, Y. Rong, Structure-enhanced yield stress of magnetorheological fluids, J. Appl. Phys. 87 (2000) 2634–2638. [23] J.P. Segovia-Gutiérrez, C.L.A. Berli, J. de Vicente, Nonlinear viscoelasticity and two-step yielding in magnetorheology: a colloidal gel approach to understand the effect of particle concentration, J. Rheol. 56 (2012) 1429–1448. [24] J. Rodriguez-Lopez, L. Elvira, P. Resa, F. Montero De Espinosa, Sound attenuation in magnetorheological fluids, J. Phys. D: Appl. Phys. 46 (2013).
[25] J. Rodriguez-Lopez, L. Elvira, F. Montero De Espinosa Freijo, G. Bossis, J. De Vicente, Measuring the yield stress in magnetorheological fluids using ultrasounds, Appl. Phys. Lett. 102 (2013). [26] G. Bossis, E. Lemaire, O. Volkova, H. Clercx, Yield stress in magnetorheological and electrorheological fluids: a comparison between microscopic and macroscopic structural models, J. Rheol. 41 (1997) 687–704. [27] J.M. Ginder, L.C. Davis, L.D. Elie, Rheology of magnetorheological fluids: models and measurements, Int. J. Mod. Phys. B 10 (1996) 3293–3303. [28] D.J. Klingenberg, C.F. Zukoski, Studies on the steady-shear behaviour of electrorheological suspensions, Langmuir 6 (1990) 15–24. [29] Y. Ido, T. Inagaki, T. Yamaguchi, Numerical simulation of microstructure formation of suspended particles in magnetorheological fluids, J. Phys.: Condens. Matter 22 (2010). [30] H. Li, X. Peng, Simulation for the microstructure and rheology in bidisperse magnetorheological fluids, J. Comput. 7 (2012) 1405–1412. [31] L.C. Lynnworth, Industrial applications of ultrasound. A review. II. Measurements, tests, and process control using low intensity ultrasound, IEEE Trans. Son. Ultrason. 54–22 (1975) 71–101. [32] J.G. Webster, Measurement, Instrumentation and Sensors, CRC Press, 1998 (29 de diciembre de 1998), ISBN-13: 978-0849383472. [33] J.R. Allegra, S.A. Hawley, Attenuation of sound in suspensions and emulsions: theory and experiments, J. Acoust. Soc. Am. 51 (1972) 1545–1564. [34] M.J.W. Povey, Ultrasonic Techniques for Fluids Characterization, Academic Press, San Diego, California, 1997. [35] A.H. Harker, J.A.G. Temple, Velocity and attenuation of ultrasound in suspensions of particles in fluids, J. Phys. D: Appl. Phys. 21 (1988) 1576–1588. [36] A.J. Mulholland et al., A theoretical analysis of a piezoelectric ultrasound device with an active matching layer, Ultrasonics 47 (2007) 102–110. [37] J. Rodriguez-Lopez, L. Elvira, F.M.D. Espinosa, in: IOP Conference Series: Materials Science and Engineering, first ed., vol. 42, 2013. [38] F. Donado, J.L. Carrillo, M.E. Mendoza, Sound propagation in magneto-rheological suspensions, J. Phys.: Condes. Matter 14 (2002) 2153– 2157. [39] J. Rodriguez-Lopez, L. Elvira Segura, F. Montero De Espinosa Freijo, Ultrasonic velocity and amplitude characterization of magnetorheological fluids under magnetic fields, J. Magn. Magn. Mater. 324 (2012) 222–230. [40] D.L. Johnson, T.J. Plona, Acoustic slow waves and the consolidation transition, J. Acoust. Soc. Am. 72 (1982) 556–565.
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short Communication
Study of the effect of particle volume fraction on the microstructure of magnetorheological fluids using ultrasound: Transition between the strong-link to the weak-link regimes Jaime Rodríguez-López ⇑, Pedro Castro, Luis Elvira, Francisco Montero de Espinosa Institute of Physical and Information Technologies, CSIC, 28006 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 8 April 2015 Keywords: Magnetorheological fluids Ultrasonic characterization Strong-link regimen Weak-link regimen Magnetic field
a b s t r a c t The effect of particle volume fraction on the microstructure of magnetorheological (MR) fluids has been studied using ultrasonic techniques. When no magnetic field is applied, they behave as slurry. However, when magnetic field is applied, important features regarding the change of the microstructure have been found with the help of ultrasonic waves propagating in the direction of the magnetic field. As the volume fraction increases, a rearrangement of particles which decrease the compressibility of the system is detected; nevertheless, the material behaves as a non-consolidated material. Three different particle volume fraction regions are found identifying a critical particle volume fraction predicted in the literature. Ultrasounds are confirmed as an interesting tool to study MR fluids in static conditions. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetorheological (MR) fluids are smart suspensions with tunable properties by applying an external magnetic stimulus. When no magnetic excitation is applied, they are isotropic suspensions of magnetic particles in a carrier fluid; however, when they are subjected to a magnetic field, the particles interact among them and the MR fluid becomes a structured and anisotropic material. The rearrangement of the particles into the MR fluid is the main mechanism responsible of the change of its mechanical properties [1–4]. In these kinds of smart systems, particles play the main active role. Particles which can be oriented by external magnetic fields can be used in these formulations; however, carbonyl iron powder (CIP) is the most widely used due to the reversibility and reproducibility of the magnetorheological effect [5]. Although CIP shows very good performance, it also presents some drawbacks regarding long term practical applications like sedimentation, abrasion or oxidation issues and difficulty to be redispersed. Nevertheless, these problems have been overcome coating them with, for example, organic materials [6], silica based coatings [7], cholesteryl functional groups [8], graphene oxide [9] or Zinc oxide [10] among others.
⇑ Corresponding author. http://dx.doi.org/10.1016/j.ultras.2015.03.011 0041-624X/Ó 2015 Elsevier B.V. All rights reserved.
The microstructure formed under the effect of the magnetic field depends on many factors: the shape, the nature and concentration of the particles, the properties of the carrier oil, the presence of surfactants and thickening agents, the uniformity and intensity of the magnetic field applied and the size of the cell where the fluid is studied [11–16]. These fluids are used in different industrial applications as it is the case of the automotive industry where they are components of high tech breaks and clutches. The MR fluids are also used as seals or earthquake absorbers [3,17–19]. As the properties of the MR fluids are strongly dependent on their inner structure, a better knowledge of the microstructure as a function of the external magnetic field will permit a better understanding of the response of these fluids and, therefore, an improvement of their industrial applications. To this end, different scientific works have been made to infer the microstructure of MR fluids from their optical, rheological or acoustical properties, both in static and dynamic conditions [16,19–25]. Different models have been proposed to explain the structure formation and the yielding process related to the application of a shear stress in MR fluids. Models have been usually applied successfully for very low particle volume fractions [26–28], while for higher volume fractions, simulations are compulsory [15,29,30]. The yielding of MR fluids of iron particles suspended in a low viscosity oil was studied as a function of the particle volume fraction by Segovia-Gutiérrez et al. [23]. In that work, a two-step behavior was observed when they were subjected
J. Rodríguez-López et al. / Ultrasonics 61 (2015) 10–14
to intermediate magnetic fields. This feature was explained as a transition from a strong-link to a weak-link particle regime when particle concentration was increased. Considering the situation on which the magnetic field energy is higher than the thermal energy, the rearrangement of the particles is induced. For low particle volume fraction, the fluid will be in the strong regime: the links between chain aggregates (transversal direction) are stronger than the links which keep particles forming the chain-like structures – longitudinal direction. Consequently, if any breakage under shear stress takes place, the breaking of bonds will occur within a chain-like aggregate. However, when particle volume fraction is high, the chains are thicker and there are lots of transversal connections between them. The chain aggregates are more resilient than the interlinks between different structures so two different breaking mechanisms are found; this is the weak-link regime. The particle volume fraction which gives way from one regime to another is a critical or threshold particle volume fraction which depends on the features of the suspension [23]. Ultrasonic techniques are very convenient to study MR fluids. They are non invasive, universal and give results on real time [31,32]. As ultrasonic propagation is sensitive to the inner structure of the fluid, changes in the mechanical stiffness and density in one direction will be accompanied by changes in the ultrasonic velocity and attenuation along this direction [33–35]. Due to attenuation mechanisms, acoustic filters or active matching layers associated to changes in the velocity have been studied using ultrasound [24,36]. Sedimentation processes, thermal behavior, hysteretic effects, yielding processes and evolution of the inner microstructure of MR fluids have been studied with these techniques [25,37,38]. In the present paper, a through-transmission ultrasonic technique has been used to measure the variations of the velocity of sound as a function of particle volume fraction in static conditions. Three different regions regarding the kind of structure have been found, identifying the particle volume threshold between the strong-link and the weak-link regimes. Pictures of the inner structure at the three regions have been taken.
2. Materials and methods Ten MR fluids of different particle volume fractions – from 0.01 to 0.35 – of carbonyl iron powder, grade CC (BASF), suspended in an epoxy resin with a viscosity of 1000 mPa s have been made. Due to the high viscosity of the host fluid, the suspensions are stable for at least 3 h as can be deduced from the velocity of sound measurement which remains constant. The carbonyl iron powder used has a magnetization saturation of 190 emu/g at 9 kOe at room temperature and a diameter between 3.8 lm and 5.3 lm. This particular grade has been chosen as particles show good ferromagnetic response and they are easily dispersed in the carrier fluid. Two piezoceramics – PZ 27 Ferroperm – with a frequency resonance of 1 MHz were used as the emitter (A) and the receiver (B). An electromagnet with 212-mT magnetic field (C) was used applying the field parallel to the ultrasonic propagation (D) as it is shown in Fig. 1b. The fluids were introduced in a 33 33 6 mm3cell, made of methacrylate, being the ultrasonic path 6 mm. The cell (1) was placed between the two poles of the electromagnet (5). The electromagnet, controlled by a current source (6), provides a very uniform magnetic field which affects the whole measuring cell. The ultrasonic emitter (A) was excited by a function generator – Agilent 33250A – (2). The ultrasonic received signal was acquired by an oscilloscope – Tektronics 1020 – (3) connected to a PC (4). The phase velocity of the ultrasonic waves was obtained from their spectrum phase using a FFT algorithm [39].
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Fig. 1. (a) Experimental set up. (b) Detail of the experimental set up where the cell is shown. The blue arrow (D) indicates the ultrasonic direction of propagation, while the red (C) one shows the direction of the magnetic field applied. Both fields are parallel. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The fluid fabrication procedure consists in mixing thoroughly the particles with the carrier fluid with a stirrer for about 10 min at 100 rpm till the mixture becomes homogenous slurry. After this step, the sample is introduced in a vacuum chamber at 1 kPa for 15 min to remove all the air bubbles. Next, the sample is poured into the cell and kept at 30 °C for 1 h until the sample reaches the thermal and mechanical equilibrium. After this step, the experiments are carried out, maintaining a constant temperature, in order to avoid fluctuations of the velocity induced by thermal variations. 3. Results In Fig. 2, it is shown the longitudinal velocity of sound as a function of the particle volume fraction (c/) when no magnetic field is applied (subscript X0, dark squares) and when the samples are subjected to a 212-mT parallel magnetic field (subscript XB, light circles). When no magnetic field is applied, the typical behavior of a suspension of high dense solid particles in a fluid is obtained. As a general trend, the velocity of sound depends inversely on the square root of the compressibility (j/) and density (q/), c/ = (j/q)1/2. Measuring the velocity and the density, the compressibility j/0 is calculated. As it is shown in Table 1, the higher the particle volume fraction, the lower the compressibility and the higher the density;
Fig. 2. Velocity of sound as a function of the particle volume fraction (c/), when no magnetic field is applied (dark squares) and when the sample is subjected to a 212mT magnetic field parallel to the ultrasonic waves (light circles).
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Table 1 Velocity of sound and its error (c/0 and ec/0 ), compressibility (j/0 ) and density (q/ ) as a function of particle volume fraction (/) when no magnetic field is applied. /
c/0 ec/0 (m/s)
j/0 ± 0.02 1010 (Pa1)
q/ ± 1 (kg/m3)
0 0.01 0.03 0.06 0.09 0.12 0.16 0.2 0.25 0.35
1611 ± 2 1570 ± 2 1516 ± 1 1438 ± 1 1376 ± 2 1310 ± 2 1266 ± 2 1221 ± 2 1187 ± 3 1151 ± 3
3.35 3.34 3.22 3.12 3.02 2.99 2.81 2.70 2.52 2.17
1149 1216 1349 1548 1748 1947 2213 2479 2812 3477
nevertheless, as the change in density is bigger than the change in the compressibility, the velocity of sound decreases with the increase of particle volume fraction. Once the velocities of sound of the MR fluids without magnetic field are obtained, the magnetic field is applied and the velocity of each suspension under this effect – light circles in Fig. 2 – is measured. Fig. 3 shows the measuring procedure used to obtain the velocity of sound (c) in the case of the suspension with 0.06 particle volume fraction as an example. First (I), no magnetic field is applied to check the stability of the suspension for 75 s. After, (II) the 212-mT parallel magnetic field is applied to the sample for 600 s. Finally, (III) the magnetic field is removed. The different sound speed results obtained in I and III highlight a hysteretic behavior. In this work, this method has been used to carry out the measurement of the velocity of sound at a moderate uniform magnetic field – 212 mT – as the changes are more evident. However, as it can be seen in the figure below, Fig. 4, the experimental set up has enough accuracy to measure changes in the velocity of sound at lower magnetic fields, 10 mT. This example corresponds to a MR fluid with 25% of particle volume fraction. The measured velocities of all the samples under the effect of the 212-mT magnetic field are shown in Table 2 with the calculated compressibility. Comparing the results of the velocity of sound when the magnetic field is applied to the case when it is not – Fig. 2, the next results are obtained. When the magnetic field is applied, the inner structures change and the fluid evolves from a homogenous suspension to an ordered material. Nevertheless, from Fig. 2 (light circles), the sound speed trend with the particle volume fraction is very similar to the case of no magnetic field applied. The system behaves closely to a
Fig. 4. Change of the velocity of sound as function of the intensity of magnetic field applied in a MR fluid with 25% particle volume fraction.
Table 2 Velocity of sound and its error (c/B and ec/B ), compressibility (j/B ) and density (q/ ) as a function of the particle volume fraction (/) when a magnetic field is applied. Dc/B ¼ c/B c/0 is the change of the velocity of sound with and without the magnetic field for each volume fraction. eDc/B is the error. /
c/B ec/B (m/s)
j/B ± 0.02 110
q/ ± 1 (kg/m3)
Dc/B eDc/B
1149 1216 1349 1548 1748 1947 2213 2479 2812 3477
0±2 8±2 13 ± 1 15 ± 1 13 ± 1 14 ± 1 14 ± 1 16 ± 1 18 ± 1 25 ± 2
(Pa1) 0 0.01 0.03 0.06 0.09 0.12 0.16 0.2 0.25 0.35
1611 ± 2 1578 ± 2 1529 ± 1 1453 ± 1 1389 ± 2 1324 ± 2 1280 ± 2 1237 ± 2 1204 ± 3 1176 ± 3
3.35 3.30 3.17 3.06 2.96 2.93 2.76 2.64 2.45 2.08
non-completely consolidated material [40]. Due to the high magnetic field applied, there are magnetic interactions among particles which create an inner frame, but the interaction strength is far from that of interparticle interactions in a solid. Nevertheless, the rearrangement of particles due to the magnetic interactions causes always an increase of the velocity of sound and, consequently, a decrease of the compressibility; the sample becomes more rigid. The higher the particle volume fraction, the more noticeable the sound speed changes are. More detailed information can be obtained about the microstructure of MR fluids when, for each volume fraction, the velocity of sound of the MR fluid with magnetic field applied (c/B ) is compared to that with no magnetic field (c/0 ) – Fig. 5(a). Doing so, the effect of the change in the microstructure due to the magnetic field is highlighted. Pictures of the microstructures taken at different volume fractions and with different angles are shown in Fig. 5(b) and (c). Fig. 5 (b) shows the images obtained when the camera is placed parallel to the ultrasonic propagation, and therefore parallel to the magnetic field lines. Fig. 5(c) corresponds to the same structures but the images are taken perpendicularly. Three regions can be distinguished: Three regions can be distinguished:
Fig. 3. Velocity change behavior of the sample having 0.06 particle volume fraction when initially no magnetic field is applied (I), then the magnetic field is switched on (II), and finally it is removed (III).
(a) Low volume fraction region (up to 0.06). There is a linear and strong increase on the change of the velocity of sound. Accordingly to the pictures shown in Fig. 4, in this region, especially at the lowest volume fractions, practically isolated chain-like structures or fibers are formed. As soon as more
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Fig. 5. (a) Change of the velocity of sound, Dc/B , obtained subtracting c/B c/0 , for each volume fraction. Pictures of the microstructure for each volume fraction taken in parallel (b) and in perpendicular (c) to the acoustic propagation are shown, the latter are reprinted with permission from J. Phys. D: Appl. Phys. 46 065001.
particles are added, the structures become thicker and some of them start to be linked transversally. Due to defects in particles and the effect of close neighbors, perpendicular interactions are expected theoretically. This region seems to be related to the strong-link regime shown by SegoviaGutiérrez et al. [23]. If the particle volume fraction is increased up to 0.06, complex fiber structure interconnections start giving way to the second intermediate region. (b) Intermediate volume fraction region (from 0.06 to 0.09). The change in the velocity of sound starts decreasing slightly. Accordingly to the pictures, structures are more interconnected. Big oriented cavities are shown, which become smaller when the particle volume fraction increases. This second region could be considerate as an intermediate between a fiber-like structure and an interconnected like-porous material. Probably, transversal structures linking parallel structures are not enough significant to create stiff joints, but they are important enough to difficult the sound propagation avoiding the clearance of the fluid regions. As a result, the change of velocity decreases. This region can be related to the critical volume fraction shown by SegoviaGutiérrez et al. [23] in which the transition from the strong-link regime to the weak-link regime occurs. (c) High volume fraction region (from 0.09 up to 0.35). A clear increase of the change of velocity of sound as a function of the particle volume fraction is shown, but weaker than that of the low volume fraction region. Pictures show a concentrated, oriented and linked material with small cavities. There is a frame connected by horizontal and transversal structures. In this region, the contribution of the solid part becomes more important when adding more particles, highlighting the interconnected nature of the structures. This part could be related to the weak-link regime [23].
4. Conclusions In this work, ultrasonic techniques are confirmed as an important tool to study MR fluids, not only in dynamic conditions as it is shown in literature, but also in static conditions. Important features regarding the inner microstructure of these smart responsive
materials are found studying its velocity of sound. When no magnetic field is applied, MR fluids behave like suspensions of particles, while if a magnetic field is applied, due to magnetic interactions, particles reordered into an oriented inner frame. The structure formed is less compressible although it is not completely consolidated, being far from becoming a solid. Moreover, from the different trends of the change in the velocity of sound as a function of volume fraction, different microstructural regions that could be correlated with the strong-link and weak-link regimes described in the literature have been shown. A particle volume fraction region which separates both regions has been clearly identified as a transition region where the inner structures pass from a one directional structure to a three dimensional one. The methodology exposed in this work is a very powerful tool to study the structure of MR fluids under the effect of an external magnetic field. According to the accuracy of the experimental technique, this method opens new opportunities to analyze the effect of different factors, such as the viscosity, the intensity of the applied magnetic field or the kind and size of particles used on the microstructure of MR fluids. This work was supported by a CSIC JAE fellowship and funding from Spanish Ministry of Science and Innovation (DPI2013-46915C2-1-R). References [1] J. Rabinow, The magnetic fluid clutch, AIEE Trans. 67 (1948) 1308–1315. [2] G. Bossis, S. Lacis, A. Meunier, O. Volkova, Magnetorheological fluids, J. Magn. Magn. Mater. 252 (2002) 224–228. [3] A.G. Olabi, A. Grunwald, Design and application of magneto-rheological fluid, Mater. Des. 28 (2007) 2658–2664. [4] J. de Vicente, D.J. Klingenberg, R. Hidalgo-Alvarez, Magnetorheological fluids: a review, Soft Matter 7 (2011) 3701–3710. [5] A.J.F. Bombard, I. Joekes, M.R. Alcântara, M. Knobel, Mater. Sci. Forum 416–418 (2003) 753–758. [6] I.B. Jang et al., Role of organic coating on carbonyl iron suspended particles in magnetorheological fluids, J. Appl. Phys. 97 (2005) 10Q912. [7] P.-B. Nguyen et al., Brake performance of core–shell structured carbonyl iron/ silica based magnetorheological suspension, J. Magn. Magn. Mater. 367 (2014) 69–74. [8] M. Mrlik et al., Cholesteryl-coated carbonyl iron particles with improved anticorrosion stability and their viscoelastic behaviour under magnetic field, Colloid Polym. Sci. 292 (2014) 2137–2143. [9] W.L. Zhang, H.J. Choi, Self-assembly of graphene oxide coated soft magnetic carbonyl iron particles and their magnetorheology, J. Appl. Phys. 115 (2014) 17B508.
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