A review on the magnetorheological fluid preparation and stabilization

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A review on the magnetorheological fluid preparation and stabilization M. Ashtiani, S.H. Hashemabadi, A. Ghaffari

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Received date: 2 June 2013 Revised date: 29 May 2014 Accepted date: 4 September 2014 Cite this article as: M. Ashtiani, S.H. Hashemabadi, A. Ghaffari, A review on the magnetorheological fluid preparation and stabilization, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2014.09.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Review on the Magnetorheological Fluid Preparation and Stabilization  M. Ashtiani, S. H. Hashemabadi*, A. Ghaffari Computational Fluid Dynamics (CFD) Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, 16846, Tehran, Iran *

Corresponding author. Fax: +9821 7724 0495.

E-mail: [email protected]

Abstract Magnetorheological fluids (MRFs) are one of the categories of smart materials, whose viscosity increases considerably in the presence of a magnetic field. These fluids are prepared by dispersing magnetizable micro size particles into a carrier fluid with stabilizer additives. The main feature of these fluids is their ability to change from liquid to semi-solid state with controllable yield stress just a few milliseconds after applying a magnetic field. Low magnetorheological effect and instability of MRFs are the most important problems against the extensive application of MRFs technology in modern industries. Various methods have been proposed and used by researchers to improve the magnetorheological effect and also the stability of these fluids. The main focus of this study is to present a comprehensive review on different methods of preparation and stabilization of MR fluids. Furthermore, rheological models and application of MR fluids are discussed briefly in this study.

Highlights • • •

 

MRF synthesis was studied with respect to their stability and MR effect. Stabilization of magnetic suspensions as the main challenge still need to investigate. Carbonyl iron particles are the most appropriate candidates for MRF synthesis.

Keywords Smart fluid, Magnetorheological Fluid (MRF), stabilization, magnetorheological effect, magnetic suspension

1. Introduction Materials whose properties vary considerably in the presence of an external stimulus are known as smart materials. Various smart materials have been identified so far, the most important of which are: magnetic suspensions (magnetorheological fluids and ferrofluids), electrorheological fluids, piezoelectric materials and shape-memory alloys [1]. Magnetic suspensions are complex fluids whose rheological characteristics change significantly in the presence of a magnetic field. These materials are divided into two groups: ferrofluids which are a stable colloidal mixture of ferrimagnetic and/or ferromagnetic nanoparticles and magnetorheological fluids (MRFs) which are suspensions of magnetizable microparticles dispersed in a carrier fluid [2, 3]. Magnetorheological fluids were introduced for the first time in 1948 by Rabinow in the US National Bureau of Standards [4]. Particles in MRFs are magnetically multidomain so that the application of an external magnetic field induces a magnetic dipole in each particle and results in very strong interactions between the particles. This could lead to the formation of a network of particles and/or agglomerates throughout the suspension. As a result, the magnetorheological fluid reversibly changes from liquid state to semi-solid state in the presence of a magnetic field [2]. In the absence of a magnetic field, viscosity of MRFs is a function of carrier oil properties, suspending agents and volume fraction of the particles [5]. Rheological characteristics of magnetorheological fluids such as yield stress and apparent viscosity can quickly be controlled by applying a magnetic field. In the presence of

 

an external magnetic field, the fluid shows non-Newtonian behavior which is characterized by a field-dependent yield stress and an increase in viscosity [1, 6]. Viscosity of the magnetorheological fluid depends on magnitude and direction of the applied magnetic field and also shear rate [7]. Apparent viscosity and other rheological characteristics of MRFs can be controlled by manipulating intensity of the applied magnetic field [8]. In most applications of magnetorheological fluids, response time of MRFs has the greatest importance. This time varies in the range of 10-20 milliseconds depending on magnetic circuit design. Therefore, MRFs are one of the fastest electromechanical interfaces for mechanical applications. A magnetorheological fluid based on its composition and flux density is able to demonstrate dynamic yield stress up to 100 kPa [9]. Some major characteristics and applications of magnetic suspension are summarized in Table 1.

Table 1. Major characteristics of magnetorheological fluids [1 and 5] Property

Typical Value

Initial viscosity

0.2-0.3 [Pa.s] (at 25℃)

Density

3-4 [g/cm3]

Magnetic field strength

150-250 [kA/m]

Maximum yield stress

50-100 [kPa]

Reaction time

Few milliseconds

Typical supply voltage and current intensity

2-25 [V], 1-2 [A]

Work temperature

-50 to 150 [℃]

Since the discovery of magnetorheology, new structures of MRFs, their characteristics and influencing factors have been investigated by researchers. In a relatively comprehensive study, Charles [10] investigated and categorized characteristics of ferrofluids and magnetorheological fluids. In this work, the synthesis methods of magnetizable particles as

 

well as preventive ways of their aggregation were explored. Vicente et al. [11] reviewed recent development in the understanding of MR suspension behavior. They focused on governing rheological model of MRFs and discussed about it in details. Carlson and Jolly [5] have investigated the characteristics of magnetorheological fluids, magnetorheological foams and magnetorheological elastomers, in addition to their applications. Their studies showed that new applications of magnetorheological technology lead to producing new structures of these materials. In an attempt to understand the rheology of magnetorheological fluids, Bossis et al. [12] studied influencing forces on magnetorheological effect. They concluded that magnetic, hydrodynamic and thermal forces are influencing forces which must be considered in studying magnetorheological effect. In our previous works [13-18], the rheological characteristics of Newtonian and non-Newtonian fluids were investigated by the aims of different rheological methods and it’s revealed that rotational rheometery is one of the most common methods in magnetorheology. Li and Zhang [19] assessed the effect of friction on magnetorheological fluids experimentally and theoretically. Their experimental work revealed that MR effect is generally affected by two factors, namely magnetic force and friction force. It was shown that the stress due to friction is significant at small shear deformations, while it reaches up to one-third of the total stress at greater shear deformations. That is why it is impossible to ignore the friction force in most cases. In another research, Yamaguchi et al. [20] focused on rheological characteristics of a magnetorheological fluid in a magnetic field. They noticed that viscosity and elasticity of MRFs are significantly affected by magnetic field and concentration of metallic particles. In recent decades due to the improvement in MR technology, researches on MRFs and/or their effects are growingly increased and also some review papers are published in this area [1, 5, 11 and 21]. In most of these papers, preparation and applications of MRFs have been studied [22-24]. Unfortunately, stabilization methods of these fluids have not been fully

 

covered in the literatures. Taking into account the ever increasing applications of magnetorheological fluids in today’s modern industries, it seems necessary to launch a comprehensive study on the preparation and stabilization of MRFs as well as their magnetorheological effect. The most important problem reported about magnetorheological fluids is associated with sedimentation of heavy iron particles in the carrier fluid. Accordingly, in this study the main focus is to present a comprehensive review on composition of these fluids in addition to introduce and compare the most widely used stabilization techniques of these fluids. Since the required stability and MR effect in each specific application are different from other applications, some of the most important MRF applications are introduced. In this context, for a better understanding of the magnetorheological effect and effective parameters, rheological behavior and the basic models which can be used for describing MRFs behavior are briefly addressed.

2. Rheological Properties of Magnetorheological Fluids Magnetorheological effect is characterized by a reversible increase in the viscosity of a magnetorheological fluid and showing yield stress due to the introduction of a magnetic field which can be explained by the particles chain formation. MR effect can be controlled by magnetic field intensity and rheological characteristics of magnetorheological fluid constituents. In other words, controllable rheological characteristics of an MRF are attributed to polarity of suspended particles induced by magnetic field [25]. In the presence of a magnetic field, each metallic particle is transformed into a dipole and forms a chain with its adjacent particles which can resist failure for a certain shear rate and consequently provides a semi-solid structure. Interactions between these induced dipoles cause the particles to be aligned along the applied field and form a columnar structure. The chain-like structure will then inhibit movement of the fluid and thus increase viscosity of the suspension. Mechanical

 

energy needed to overcome this chain-like structure is increased by reinforcement of the applied magnetic field [5]. When shear rate exceeds an extreme value, the chain structure will break down and the fluid will flow. The stress which MRF sustain at this intense shear rate is called apparent yield stress of the fluid [8]. In other words, yield stress is the maximum stress which can be applied before the MRF continuous flow which is a function of the magnetic field [25] and associated with the enhancement in viscoelasticity behavior [11]. Yield stress as a crucial factor in industrial applications of MRFs change between 10 and 100 kPa in a specific range of magnetic field [26]. This factor depends on shape, size distribution [27-28], volume fraction of the particles, intensity of the applied magnetic field [8], interactions of the particles and formation of agglomerates [29]. To design magnetorheological devices and to predict how they work, one would need to find a specific relation between shear stress and shear rate in the magnetorheological fluid [30]. Behavior of MRFs in the absence of a magnetic field is very similar to the behavioral pattern of carrier fluids, except that the existing metallic particles in these fluids make the liquids a bit more concentrated [25]. Having no claim to cover all the models, some of the most promising models will be introduced in the following paragraphs. One of the most fundamental equations to explain the behavior of magnetorheological fluids is Bingham model [31] which is extensively applied to explain flow curve (shear stress versus shear rate) of magnetorheological fluids. According to Bingham model, the apparent yield stress for each flow curve can be extracted from interpolation at zero shear rate [32-33]. Numerous researchers have used the Bingham plastic model to justify behavior of MRFs in recent years [5, 25 and 34-38]. In one particular instance, Claracq et al. [36] have studied the rheological characteristics of magnetorheological fluids and concluded that they follow Bingham plastic model.

 

The other basic two parameter equation is Casson model [39]. Gabriel and Laun [40] observed that Casson model confirms the experimental results at greater accuracy and is thus a more appropriate model for designing magnetorheological devices. One reason for this observation is that unlike Bingham model, slope of shear stress versus shear rate in Casson model is a function of shear rate; consequently it reveals more consistency with experimental results of the magnetorheological fluid. Herschel-Buckley model is a three parameter model which was developed to describe flow curve and yield stress of viscoplastic fluids and is more general in comparison to previous models [41]. Some researches on the technology of magnetorheological fluids emphasize on a better consistency of this model with the behavior of these fluids [40 and 4244]. Other models such as the power law model have been proposed to explain the behavior of non-Newtonian fluids which are not extensively applied for magnetorheological fluids [45].

3. Magnetorheological Fluid Applications Potential applications of MRFs are in devices which need quick, continuous and reversible change in rheological characteristics [5]. Magnetorheological devices have gained a great interest during the last decades, since a magnetorheological fluid puts mechanical devices in direct contact with an electronic system, thereby enabling continuous determination of mechanical characteristics of the apparatus. Some of these devices which utilize magnetorheological fluids are a new generation of dampers, clutches and brakes. Magnetorheological dampers especially as shock absorbers are the most used devices of this kind [44]. Control valves, power steering pumps, artificial joints, engine mounts, alternators, sound propagation, chemical sensing applications and others are some of these examples [11,

 

46]. Drug delivery and cancer treatment methods in medicine are some modern applications of magnetic suspensions [45]. In a relatively comprehensive study, Bica et al. [23] studied the potential applications of magnetorheological suspensions. They identified important factors affecting the performance of MRFs in any specific function. In another study, Wang and Meng [24] evaluated various characteristics and applications of magnetorheological fluids. Their survey showed that the three main problems against widespread use of MRF technology in many modern devices are their settling stability, cost factors and their durability. Having no claim to cover all the applications, the followings briefly discuss some of the most important applications of magnetorheological fluids. Kciuk and Turczyn [1] studied basic properties of MRFs and their widespread applications in various industries. In another study, Olabi and Grunwald [25] studied characteristics of magnetorheological fluids as well as their applications. According to their survey, promising features of MRF technology like fast response, simple interface between electrical power input and mechanical power output and also precise controllability, make them the next technology choice for many applications. •

Dampers: Damper is a device to reduce shock, in addition, to limit movement, to

neutralize the kinetic force, etc. A magnetorheological damper is a damper which contains magnetorheological fluids and is controlled with the power of magnetic field. Magnetorheological dampers are developed since they are able to apply various damping forces to the suspension and are naturally fail-safe. Whenever a problem occurs in the system, the magnetorheological damper is able to act as a passive damping system with known operational parameters depending on specifications of the magnetorheological fluid in the absence of a magnetic field [44]. MR dampers have applications in vibration control systems, prosthesis control, washing machine, seismic damage mitigation in civil engineering and also control of wind-induced vibrations in cable-stayed bridges [1].

 

•

Valves: When the magnetorheological fluid flows through the valve, a magnetic field

is applied to the fluid and causes viscosity of the fluid to rise. This change in the viscosity creates resistance against flow of fluid through the valve. Thus the input pressure is increased and flow of the fluid slows down or completely stops. Rosenfeld et al. [47] examined structure and performance of magnetorheological and electrorheological valves and finally found that performance of the former significantly depends on driving force and active volume of the fluid. •

Polishing: Optical polishing, which was first introduced by Kordonski et al. [9], is

known as another industrial application of magnetorheological fluids. In the presence of a magnetic field, magnetic particles form a chain-like structure and cause abrasive particles to perform the polishing operation. The abrasive holding power and stiffness of a magnetorheological fluid is significantly dependent on the size of magnetizable particles. Concentration [48-49] and type of the abrasive particles are other parameters which affect quality of the workpiece surface [24]. •

Brakes: Magnetorheological brake is a device which is utilized for torque

transmission. A rotational magnetorheological brake in the presence of a magnetic field is able to change the braking torque quickly [50]. The simplicity and ease of control make MRF brakes a very cost-effective choice for a wide variety of applications [51].

4. Preparation of Magnetorheological Fluid MR suspensions are prepared in a very simple procedure which is mixing all the constituents together. The major issue is type and fraction of ingredients. An MRF usually has three main constituents: base fluid, magnetizable particles, and stabilizer additives. The base fluid behaves like a carrier which contains metallic particles. In fact, this is a media where the metallic particles are suspended in it. Magnetizable particles, with the key role in

 

magnetorheological effect, are dispersed in the base fluid. Stabilizer additives are utilized to overcome sedimentation problem of heavy magnetizable particles. A simple and efficient magnetorheological fluid is obtained from the dispersion of iron particles in silicone oil and using some surfactants such as stearic acid (to avoid irreversible aggregation) [12]. For this purpose, one needs to disperse magnetizable particles into the base fluid in an efficient way so particles don’t aggregate and a homogenized suspension may be attained. One of the most effective ways for obtaining a stable MRF is to use stabilizer additives before adding magnetizable particles to the base fluid. This procedure may lead to efficient MRF preparation. In the following sections, the MRF ingredients have been discussed in details.

4.1. Continuous Phase The base fluid primarily plays the role of a liquid in which magnetic particles of the dispersed phase are suspended [52]. Viscosity of the base fluid must be low in order to achieve the maximum MR effect [25]. The base fluid can be either a polar or non-polar liquid, and is usually selected with respect to its rheological characteristics and temperature stability. Some other factors which must be considered in choosing the base fluid are: compatibility with magnetic particles, chemical stability, surface tension, etc. Some of the most commonly used carrier fluids in MRFs are petroleum-based oils, mineral oils, silicon, polyester, polyether, water, industrial hydrocarbonic oils and many other materials. For any application, an appropriate base fluid can be chosen based on the required properties, stability and magnetorheological effect. For example, considering cost, environmental issues and cooling characteristics of the surface which is to be cooled, water is the best candidate for magnetorheological polishing processes [53]. Viscosity is one of the most important characteristics of the continuous phase in MRFs. Another important feature of the carrier fluid in magnetorheological fluids is its low

 

vapor pressure, as it is not simply vaporized and thus can be used in a wide range of temperatures. An appropriate magnetorheological fluid should contain a carrier fluid with low viscosity. Also it should incorporate other essential characteristics for operational temperatures and redistribution of the particles [5]. In other words, one must pay special attention to determine viscosity of the carrier fluid, since a low viscosity can lead to instability and sedimentation problems, whereas a high viscosity may raise viscosity of the magnetorheological fluid in the absence of a magnetic field which is undesirable. Generally, oils with low viscosity, like silicone oil, are preferred to make magnetorheological fluids [8, 26 and 54]. In section 5.4 effects of the base fluid on MR effect and MRF stability will be discussed.

4.2 Dispersed Phase Many metals, alloys and ceramics compositions can be used as particle phase to prepare a magnetorheological fluid. Some of these particles are ferrite-polymer, iron-cobalt alloy, carbonyl iron, nickel-zinc ferrites [55], iron and its compounds [25, 52] and other magnetizable materials. Among these particles, carbonyl iron and its alloys have the highest saturation magnetization [22]. These particles are magnetically multidomain and demonstrate a low magnetic coercivity (which is called magnetically soft) [9]. Soft materials are temporary magnets, a feature which is very important for reversibility of magnetorheological effect. Since soft magnetic materials are easily magnetized and demagnetized and by applying an external magnetic field, it’s possible to control reversibly rheological characteristics of MRF [56]. In the past, Fe3O4 magnetic particles were extensively used for making magnetorheological suspensions due to their nontoxic nature and availability. However, their application has been limited due to their low magnetic saturation. On the other hand, using

 

ferrites (Fe3O4) in an MRF often incorporates some problems including sedimentation and aggregation [42]. As mentioned before, the most common magnetic particle in MRF preparation is high purity iron powder known as carbonyl iron particles (density: 7.91g/cm3) which is chosen because of its high magnetic saturation (approximately 2.1 Tesla) and proper particle size (average particle size: 4.25 μm) [9, 22, 54 and 57-58]. Nevertheless, due to the very high density of carbonyl iron particles, sedimentation of these particles has limited their widespread uses in commercial applications. Therefore, numerous methods have been proposed to avoid the contact between carbonyl iron particles and to reduce the density for slowing down their sedimentation [54]. Some of the most important of these methods will be discussed in the following sections. Yield stress in MRFs containing carbonyl iron (CI) particles can be adequate for industrial applications. For example, a magnetorheological fluid with 30 vol% spherical particles of carbonyl iron in a magnetic field of H=318.31 kA/m has a yield stress equal to 50 kPa [12]. An alloy of iron and cobalt is known to provide even higher saturation magnetizability (about 2.4 Tesla) and is used in magnetorheological fluids [5]. Many researchers, including Gorodkin et al. [59], investigated rheological characteristics of an MRF containing carbonyl iron particles. They observed that magnetizability of iron particles is a linear function of the average size of the particles. Based on this observation, they concluded that the magnetizability data can be used to evaluate and determine the allowable concentration of carbonyl iron particles in magnetorheological fluids. Park et al. [60] reviewed recent studies on the stabilization of MR suspension by using proper magnetizable particles. They focused on the use of CI and magnetite particles and their composite as dispersed phase. In our previous works [35, 61], a magnetorheological fluid was prepared by using carbonyl iron particles and a low viscosity lubricant oil. It’s observed that yield stress is a function of weight percent of the particles and magnetic field

 

and increasing either of which will improve the yield stress experimentally and numerically. It’s also investigated that by using rotational disk in rheological tests, promising results are obtained for non-Newtonian fluids [17, 61-65]. In another investigation, Kciuk et al. [52] have studied the characteristics of carbonyl iron-based magnetorheological fluids. They used SiO2 coating on carbonyl iron particles and silica additives to improve stability of this fluid. Hua-jin et al. [66] prepared a magnetorheological fluid by dispersing gelatin-carbonyl iron composite in silicon oil. They showed that the MR effect is superior in lower magnetic field intensities and the stability is excellent in comparison with a bare carbonyl iron-based suspension. The amount of metallic particles in an MRF can reach to 50 vol% [25]. In an effort to determine the effect of particle size and particle size distribution, Chiriac et al. [67] dispersed Fe microparticles in mineral oil. They observed that by increasing particle size or reducing the particle size distribution, the MR effect increases and this enhancement is more evident in high magnetic field strengths. At high volume fractions of magnetizable particles, the particles are sufficiently close and affected by the flow field of adjacent particles. Hydrodynamic interactions also play a crucial role in the particle laden flow. Approximately at 50 vol% of the particles, a quick increase in viscosity is significant [22]. Unfortunately, the maximum percentage of magnetizable particles has not attracted much attention among researchers. In one of the few studies in this area, Lopez-Lopez et al. [68] investigated MR effect of highly dense suspensions. Their experimental result revealed that 50 volume percent silica coated iron particles in mineral oil without further additives is the upper limit of particles concentration in such an MRF. To improve the magnetorheological properties of MRFs, Xu-feng et al. [69] introduced a new magnetorheological fluid by dispersing Fe76Cr2Mo2Sn2P10B2C2Si4

 

amorphous alloy particles in silicon oil. Their results indicate that the amorphous-based MRF has better MR effect and sedimentation stability at lower field intensities in comparison with the carbonyl iron-based MRF. Looking at the studies implemented in the context of magnetorheological fluids demonstrates that carbonyl iron particles with average particle size of 4 μm are the most appropriate candidates for the preparation of a magnetorheological fluid due to their great magnetic saturation characteristics and relatively simple preparation process. Volume fraction of these particles varies from 10 to 70 vol% depending on the stress range required as well as the allowed range of sedimentation. As already mentioned, selection of magnetizable particles for the preparation of an efficient and stable MRF must be done carefully. Coercivity and remnant magnetization of particles must be low for the reversibility of MR effect. For a promising MR effect, magnetic saturation must be as high as possible. A typical value of magnetic saturation around 2 Tesla is appropriate. Other influencing factors are size, shape and density of particles. An ideal range is expected to be 0.1-10 μm in choosing particle size. In section 5.2 and 5.3 the effect of particle size and shape have been discussed in details. Low density particles are necessary for preparation of a stable MRF. In addition to choosing particle type, some other parameters like compatibility between particles and carrier fluid and also chemical stability of particles must be considered.

4.3. Additives Various types of additives (stabilizers and surfactants) are added to the magnetic suspension to prevent sedimentation due to gravity, produce a stable suspension, enhance lubrication, and change initial viscosity of the MRF [1]. These materials include suspending as well as thixotropic agents, anti-friction and anti-abrasion/erosion compounds. High

 

viscosity materials such as grease or other thixotropic materials are added to improve stability of particles against sedimentation in magnetorheological fluids. Iron naphthanate or Iron oleate are added to the composition as dispersants, while metallic soaps like lithium stearate and/or sodium stearate are added as thixotropic additives. Additives are necessary to control viscosity of the fluid, sedimentation of particles and interparticle friction, in addition to prevent thickening of the fluid after several cycles of use [25]. Thixotropic agents form a network of particles which creates a weak structure at low shear rates and decreases sedimentation of the particles [10]. In section 5.5, the effects of additives on the stability of magnetorheological fluid will be discussed in details.

5. Challenges in MRF Technology: Stability and MR effect Magnetorheological fluids should attain certain characteristics to survive in the competitive market of engineering applications. Some of these characteristics are: operation in a wide range of temperature, chemical stability, stability against sedimentation, reversible flocculation and high magnetic saturation. Furthermore, an MRF must show high yield stress in the presence of a magnetic field and low apparent viscosity in the absence of a magnetic field [11, 56]. One of the major challenges in manufacturing a magnetorheological fluid is to prevent sedimentation and aggregation of the particles. Sedimentation which occur due to great density mismatch between magnetic particles and carrier fluid, restrict widespread applications of MRFs. Therefore it is necessary to overcome Van Der Waals attraction forces between the particles and also to create a uniform and stable distribution as well [30]. Like other systems which contain suspended particles, stability and redistribution of these suspensions is considered fundamental problems especially at high concentration of solid particles. There are two methods to determine sedimentation rate in magnetorheological fluids: measuring the rate of changes in magnetic

 

permeability of the MRF upper layer and passing laser through a column of the magnetorheological fluid [30]. However there is no promising method to determine the stability of MRFs. Conventional methods for determining stability of other dispersed systems, such as scattering light and determination of turbidity are not applicable in these cases, because scattering light for several times and negligible light transmission make the measurements unreliable. In particle laden fluids, practical methods are based on using X-ray and gamma-ray but they are rather expensive and complicated [6]. When sedimentation occurs, particles must be distributed once again. Since there is no standard and repeatable method in this case, and determination of the number of redistributed particles is so difficult, an appropriate method has not been presented for redistribution of particles so far. However, numerous methods have been developed by various researchers to overcome this problem to some extent by modifying MRF compositions, including: reducing size of the particles, addition of thixotropic materials (such as carbon fibers, silica nanoparticles and organoclays), surfactants, using viscoplastic fluids, or application of an emulsion as the continuous phase [6]. The difference between density of the particles and the carrier fluid, and also the different size of the particles causes the fluid to be prone to sedimentation once not in use. When sedimentation occurs, attraction due to the remnant magnetization makes redistribution of the particles difficult [27]. This phenomenon can often be inhibited to a large extent by using thixotropic materials and surfactants such as xantham gum, silica gel, stearate and carboxylic acids. These stearates form a network of swollen strands which traps particles and collects them. Meanwhile, thin carbon fibers have been used for this purpose [5]. Therefore, a special attention must be devoted to the constituents of these fluids in order to overcome the existing problems in industrial application of magnetorheological fluids.

 

Erosion is another problem caused by interparticle contact friction in fluid flow and has limited the application of magnetorheological fluids. Carbonyl iron particles which are one of the most commonly used particles in MRFs, show an onion-like structure and their structure may be simply changed by friction and/or shock. Erosion leads to irreversible thickening of the suspension and thus decreases in MRFs performance. Much attention has been paid to surface treatment for increasing magnetorheological fluids lifetime [36]. Some of the parameters which considerably affect stability and redistribution of particles in magnetorheological fluids are: concentration and density of particles, particle size and shape distribution, magnetic saturation, coercive field, characteristics of the base fluid, surfactants and anti-abrasion materials, applied magnetic field and temperature [22]. Vekas et al. [29] examined the effect of chemical composition of the fluid and dipole interactions between particles on characteristics of the MRF flow. They observed that flow characteristics are more affected by chemical composition than conditions of the system and environment. The following subsections provide specifications and characteristics of the chemical composition of magnetorheological fluids to solve high sedimentation, low magnetorheological effect and some other problems.

5.1 An Overview on Stabilization Techniques As mentioned in previous sections, some limitations have prevented widespread use of MR technology in industrial applications. In general, sedimentation and low MR effect are the two key challenges in magnetorheology. Some of the suggested methods to improve stability and MR effect are coating magnetizable particles, using nanowires, combining magnetorheological fluids and ferrofluids, using stabilizer additives, using gels or other polymeric liquids as the base fluid. Figure 1shows the conventional stabilization methods which try to improve both MR effect and stability. Among the mentioned techniques, coating

 

magnetizzable particlees, using prooper additivees and nano spherical s parrticles have been b more noticeablle. In the following, som me of these m methods havee been discussed.

Partticle Coaating Naano Wire W Partticles

Nanno Spherrical Particcles

Stabiliization Methhods Addditives

Vario ous Carrrier Flu uid

(Surffactant, Thixootropic Agen nts …)

Othher Meth hods (Porrous Mediia …)

Fig. 1. Ovverview on MRF M Stabilizzation methoods.

5.1.1 Coa ating Magn netizable Particles M Many recent works w on staability of MR RFs have foccused on usiing magneticc particles inn the core-shell structuure in order to t overcome the sedimenntation probllem in magn netic suspensio ons. Compossite particless with core-sshell structurre are widelyy-used materrials which often havve improved physical and chemical ccharacteristiccs. The “sheell” part, e.g.. amorphous silica, cann efficientlyy prevent agggregation of particles and d protect corre materials from externaal degradatiion effects [442]. Coatingg magnetic pparticles with h organic pollymers is onne of the mosst common stabilizationn techniquess of magnetoorheological fluids. In this method, sedimentationn

is mitigated by reducing density difference between magnetic particles and base fluid [26]. Figure 2 shows a schematic image of coating process. As can be seen in this figure, firstly, surface treatment is done to activate particle surface and also enhance compatibility between particles and the monomer. Then, by adding monomer and stabilizer, the polymer coating step can be started on the modified particles. In this context, Liu et al. [34] collected recent researches on the synthesis of polymeric composite magnetic particles as dispersed phase in MRFs. According to their survey, both the surface morphology and mass ratio of the polymeric layer have influence on MR effect and stability of the magnetorheological fluid. In the following paragraphs, some of these researches have been discussed.

Fig. 2. Schematic of particle coating process with polymeric compound; (a) bare particle, (b) activated particle, (c) monomer addition and (d) coated particle (adopted from [28, 34]).

Cho et al. [28] reduced density of the particles by coating carbonyl iron particles with poly methyl methacrylate (PMMA). By this method, sedimentation was diminished considerably at the expense of decreasing yield stress in the presence of a magnetic field. They reported a non-Newtonian behavior for their synthesized MRF in the absence of a magnetic field. In a recent work, Mrlik et al. [70] coated CI particles with cholesteryl chloroformate, and by dispersing these particles in silicon oil, they synthesized a new MRF. According to their observations this magnetorheological fluid has low sedimentation rate due

 

to better compatibility between the particles and silicon oil. Moreover, due to the existence of the coating layer in the oxygen containing atmosphere, they observed better thermal stability for their new MRF. It's important to consider particle oxidation because of its effect on decreasing magnetic saturation and consequently magnetorheological fluid performance. Comparison between the composite-based magnetorheological fluid and bare CI-based ones showed that coated CI particles with cholesteryl chloroformate decreases MR effect slightly but improves thermal and sedimentation stability as well as redistribution of particles in silicon oil. They declared that an MRF containing 80 wt% of the coated CI is satisfactory enough for practical applications. Fang et al. [71] fabricated carbonyl iron/polystyrene (CI/PS) composite magnetic particles and dispersed them in lubricant oil. By adopting this approach, sedimentation stability of this suspension was enhanced significantly and also possessed good MR performance. In order to use polymeric compound for reducing density mismatch between particles and base fluid, Jun et al. [55] fabricated a composite of magnetic particles with a polymeric core and an iron oxide (magnetite) shell. They observed that yield stress of the magnetorheological fluid is intensified at higher content of iron oxide. They also noticed that stability of the magnetic composite is greater than that of the magnetite suspension. In another study, Choi et al. [56] synthesized particles with polymeric core and magnetite shell to solve the stability problem of magnetorheological fluids. Moreover, they argued that magnetorheological fluids have a non-Newtonian behavior in the absence of a magnetic field. They attributed this observation to remnant magnetization of magnetite particles and large volume fraction of magnetite particles in silicon oil. Choi et al. [32] prepared a core-shell structure of magnetic particles from carbonyl iron-PMMA and observed that this core-shell structure leads to a lower density of the particles with no considerable change in magnetorheological effect. Figure 3 depicts morphology of carbonyl iron particles and core-

 

shell structure of carbonyl iron-PMMA. As shown in this figure, the particle size increases with the core-shell structure.

Fig. 3. SEM image of (a) Bare CI and (b) CI-PMMA core-shell structured particles (reprinted with permission from ref. [32]).

To reduce density of carbonyl iron particles through coating, Cheng et al. [53] used N-Glucose EthyleneDiamine Triacetic Acid (GED3A). They reduced the sedimentation rate by preparing an MRF containing water in which particles form a network configuration via hydrogen bonds. In the absence of a magnetic field, their MRF showed a non-Newtonian behavior. Wu et al. [73] used CI particles coated with guar gum and methyl silicon oil to synthesize a stable MRF. Their experimental result demonstrated that coating magnetic particles with guar gum leads to an increase in MR effect and also stability. Liu et al. [72] made a magnetorheological fluid, in which magnetic particles were coated by silica shell using sol-gel technique. They observed that CI-SiO2 particles had lower density and better resistance against corrosion with acid and oxidation, though they detected smaller magnetorheological effects. They also observed that a greater shear stress is achieved by raising viscosity of the carrier fluid.

 

Table 2. Summary of studies that implemented coating on magnetizable particles Particle Type (Average Size)

Coating Material

Carbonyl Iron (2.57 µm)

Poly Methyl MethAcrylate (PMMA) N-Glucose EthyleneDiamine Triacetic Acid (GED3A) Poly Methyl Methacrylate (PMMA) Cholesteryl Chloroformate PolyStyrene (PS)

Carbonyl Iron (1-6 µm)

Carbonyl Iron (2.57 µm) Carbonyl Iron (0.5-2 µm) Carbonyl Iron (4.5-5.2 µm) Carbonyl Iron (2-4 µm) Carbonyl Iron (4.5 µm) Carbonyl Iron (4.5 µm) Iron Oxide (7.68 µm) Iron-Silica (Fe-SiO2)

PolyStyreneAcetoAcetoxyet hel Methacrylate (PS-AAEM) Montmorillonite clay

Coated Particle density (g/cm3) 1.83

Carrier Fluid

Magnetic Flux Range

Yield Stress Range (Pa)

Reference

Mineral Oil

0-3.43¯102 (kA/m)

100-2¯103

Choi et al. [32]

7.70

Water

0-4¯10-1 (T)

100-2¯103

Cheng et al. [53

NA♦

Mineral Oil

0-3.43¯102 (kA/m)

10-104

Cho et al. [28]

NA

Silicon Oil Lubrican t oil Methyl silicon oil Silicon Oil Lubrican t oil

0-3¯10-1 (T) 0-3.43¯102 (kA/m) 0-4.5¯10-1 (T)

101-104

Mrlik et al. [70] Fang et al. [71] Wu et al. [73]

0-2.57¯102 (kA/m) 0-3.43¯102 (kA/m)

101-3¯103

Silicon Oil Poly Ethylene Glycol 400 Silicon Oil

0-1.3¯101 (T) 0-3¯101 (T)

100-2¯102

0-3.43¯102 (kA/m)

101-2¯102

Choi et al. [56]

Water

0-2.82¯102 (kA/m)

0-2¯103

Gonzalez et al. [74]

2.51

Guar gum

NA

Silica

6.11

Multi-Walled Carbon Nanotubes Polymer composite PolyEthylene Glycol (PEG)

5.676.55 NA NA

Magnetite

1.3

Magnetite

NA

* Values in the table are approximate, ♦ Not Available  

 

100-104 0-5¯104

100-104

10-1-101

Liu et al. [72] Fang et al. [75] Jun et al. [55] Gu et al. [42]

Some details of the studies that implemented coating the particles are tabulated in Table 2. Galindo-Gonzalez et al. [74] examined rheological characteristics of a specific type of MRF which was prepared from suspending montmorillonite clay particles coated by magnetite nanoparticles in an aqueous solution. They found stability of such an MRF much higher than that of a conventional magnetorheological fluid. They also expressed that in the absence of a magnetic field, suspension of clay particles coated with magnetite demonstrates a rather poor yield stress due to aggregation. On the other hand, yield stress, which is an indicative of MR effect, enhances significantly in the presence of a magnetic field. A survey on the researches conducted so far in the field of coating heavy magnetizable particles reveals that this method can reduce density of the particles and increase stability of magnetorheological fluids. At the same time, due to low magnetic susceptibility (or no magnetic susceptibility at all) of coating materials, the MR effect is reduced; therefore, this method should be applied cautiously. Since coating with multi-walled carbon nanotubes is a simpler approach to reduce MRF instability in comparison with coating with polymeric shell, Fang et al. [75] used the mentioned coated carbonyl iron particles. They observed that yield stress and elastic properties of the composite-based MRF was reduced. However, its stability improved in comparison with the magnetorheological fluid which contains bare carbonyl iron.

 

(a)

Shear stress [Pa]

1000 100 10 1 0.1 0.001 1

0.01

0.1

1 10 Shear ratte [s‐1]

100

100 00

(b)

Shear stress [Pa]

10000 1000 100 10 1 01 0.00

0.01

0.1

1 10 Shear rrate [s‐1]

100

1 1000

(c)

Shear stress [Pa]

100000 10000 1000 100 10 001 0.0

0.01

0.1

1 10 ‐1 Shearr rate [s ] 

100

1000

Fig. 4. A survey of exxperimental sttudies on the eeffect of coating magnetizable particless on MR effecct in differennt magnetic fiield strength; (a) H=0, (b) H=86 and (c)) H~342 kA/m m.

Figure 4 shows some of the studies implemented on the field of coating magnetizable particles. As can be seen in the figure, PMMA [28] and multi-walled carbon nanotubes [75] are the most promising materials for coating magnetizable particles to enhance shear stress and also yield stress in the presence of magnetic field. It seems that among these materials, PMMA is more accepted due to the fact that this material improves MR effect without significant increase in the off state viscosity. On the whole, coating carbonyl iron with polymeric compounds seems to be promising both for improving stability and having adequate MR effect.

5.1.2. Spherical Nanoparticles Properties of nanoparticles are controlled by two interrelated factors: size and surface characteristics. When metallic particle size decreases to the nanometer length scale, the surface to volume ratio increases [76]. Nanoparticles are added to the magnetorheological fluid as solid additives in order to prevent sedimentation phenomenon which occurs due to density mismatch between magnetic particles and the liquid carrier. Some of these particles are iron and its compounds, fumed silica, organoclay, graphite fibers, carbon nanotubes, etc. Since these particles have relatively low density and large specific surface area, they can efficiently prevent sedimentation of magnetic particles [72]. Enhancement in MR effect by using nanoparticles may be attributed to the fact that smaller particles tend to break up aggregates of micro particles and produce more anisotropic structures [11]. Nanoparticles are single domain; they are always a magnetic dipole and preserve their magnetic property even in the absence of a magnetic field [10]. A schematic image of MR effect is represented in figure 5 with and without adding nanoparticles. As can be seen in the figure, nanoparticles which are added to an MRF dispersed in the voids between the

 

microparticles. This combination forms more regular chains of particles in the magnetic suspension fluid and increases the yield stress considerably.

Fig. 5. Schematic of MR effect; (a) microparticles and (b) bidisperse particles ( microparticles and

represent

spherical nanoparticles).

Recent studies on MRF preparation have concentrated on a very useful method which is using ferrofluids as the continuous phase. In most of these investigations, the yield stress is reported to be up to four times greater than that of conventional magnetorheological fluids. This phenomenon is assigned to the greater magnetic interaction between particles in the media with higher magnetic permeability. Furthermore, these magnetorheological fluids show higher stability against aggregation and sedimentation. This is because every iron microparticle is surrounded by a cloud of nanoparticles [6]. Nanoparticles which are added to an MRF, disperse in the voids between the microparticles and form more regular chains of particles in the magnetic suspension fluid and increase the yield stress considerably [30]. Preparation of ferrofluid-based MRFs are mostly consisted of dispersing magnetic microparticles in a nanoparticle fluid [2, 6, 27 and 50]; but in some cases, magnetic particles  

coated with non-magnetic particles and dispersed in a ferrofluid to prepare a more stable MRF which is the main subject of the previous section. It seems that addition of nanoparticles to an MRF, leads ascending and descending behavior in the yield stress which has been discussed in the following paragraphs in details. An MRF with two dispersed phases can be obtained if a small amount of nanoparticles are added to the suspension of microparticles. Instability of such a fluid is much lower than that of common magnetorheological fluids, but yield stress decreases significantly while viscosity of the fluid increases considerably in the absence of a magnetic field which is an undesirable effect in magnetorheology [53]. For enhancing MR effect, Park et al. [77] dispersed carbonyl iron micro and nanoparticles in lubricant oil without further additives to prepare a magnetorheological fluid. According to their findings, yield stress of the MRF slightly increases by adding 1 wt% CI nanoparticles to the suspension. In another study, Song et al. [78] examined the effect of adding CI nanoparticles to a suspension of CI microparticles and lubricant oil. Their results showed that adding 1 wt% nanoparticles improve both stability and MR effect. Chin et al. [79] prepared a suspension of magnetite and carbonyl iron particles (nano and micron-sized), silicon oil as well as Co-γ-Fe2O3 and CrO2 surfactants to introduce a proper composition of an MRF with desirable stabilization and improved magnetorheological effect. In order to improve the stability of magnetorheological fluids, Rosenfeld et al. [27] also used iron nanoparticles. They observed when a magnetic field is applied, fluids with microparticles and nanoparticles show the maximum and the minimum yield stresses, respectively. Shimada et al. [80] investigated the effect of adding magnetite nanoparticles to an iron-based magnetorheological fluid. They noticed that at a constant volume fraction of microparticles (30 vol%), addition of nanoparticles (18 to 26 vol%) at a given field intensity

 

(400 Gauss) and shear rate (20 s-1) should considerably reduce the shear stress (25% approx.). They also observed that the addition of nanoparticles to a magnetorheological fluid up to 40 vol% at constant field intensity and shear rate increases the shear stress to a large extent. Genc and Phule [9] discovered that MRF yield stress decreases by increasing the content of nanoparticles. They attributed this lower yield stress to the presence of smaller particles and reduction of magnetic saturation. In another research on using ferrofluids in MRFs synthesis, Patel [51] used magnetite microparticles dispersion in a ferrofluid. This ferrofluid consists of magnetite nanoparticles coated with oleic acid dispersed in kerosene. Their result showed that ferrofluid-based MRFs are more stable than conventional ones. As can be seen in figure 6, micro-cavities in the ferrofluid-based magnetorheological fluid are filled with the small particles.

Fig. 6. Stabilization mechanism of a ferrofluid-based MRF; (A) Magnetically induced chain formation in a conventional MRF, (B) Filling the microcavities by nanoparticles in a bidispersed MRF [51].

Some researchers tried to find a limit value for adding nanoparticles. In this regard, Shimada and Oka [81] studied the effect of volume fraction of carbonyl iron microparticles

 

and magnetite nanoparticles on magnetic saturation. Based on their observation, at constant volume fraction of nanoparticles, raising volume fraction of carbonyl iron microparticles lead to considerable increase in the magnetic saturation. However, growing up the volume fraction of nanoparticles at constant volume fraction of microparticles cannot lead to much rise in magnetic saturation. In another study, Wereley et al. [30] reported that the addition of nanoparticles up to 15-20 vol% would enhance yield stress of the magnetorheological fluid, but further increase could possibly incur an adverse effect and rather decrease the yield stress. According to their observation, replacement of 20 vol% of microparticles with nanoparticles reduces the rate of sedimentation considerably. As a result, the dynamic yield stress can be improved significantly (for more than 15% at high magnetic fields) in comparison with a conventional magnetorheological fluid. They also expressed that nanoparticles have no considerable effect on MR effect in low flow rates. Iglesias et al. [6] investigated dynamic characteristics of a magnetorheological fluid prepared with the distribution of iron particles in a ferrofluid. They used no viscosity stabilizer and/or modifier material. Their results showed that increasing volume fraction of magnetite nanoparticles for 3 vol% to a suspension of iron microparticles with a high content of the particles (32 vol%) develop a stable magnetorheological fluid. Furthermore, this method avoids the formation of a sediment cake during storage and leads to the improvement of magnetorheological characteristics. It was shown that the magnetorheological effect is reduced for volume fraction of more than 7 vol%. This is justified by the formation of a thick halo of magnetite nanoparticles around iron spheres which weakens magnetic interactions. Lopez-Lopez et al. [2] examined rheological characteristics of a suspension of magnetic microparticles which were dispersed in a magnetite-based ferrofluid. They reported an enhancement in Bingham yield stress by increasing nanoparticles content to more than 21 vol% in the ferrofluid. They finally concluded that this enhancement is probably due to the

 

formation of heterogeneous iron-magnetite structure which avoids irreversible aggregation of iron particles and facilitates formation of an induced chain by the applied magnetic field. Unfortunately, they have failed to address the allowed limit for adding magnetizable nanoparticles. Moreover, other studies have not confirmed the enhancement of yield stress to this volume fraction of nanoparticles. In a similar investigation, Viota et al. [82] studied magnetic characteristics of a suspension of magnetite microparticles and nanoparticles in an aqueous solution. Based on their results, it can be concluded that addition of even a small amount of nanoparticles leads to the improvement of magnetic characteristics of the magnetite suspension. They also observed that magnetic saturation is strengthened by increasing volume fraction of magnetite nanoparticles which is consistent with the results of other researchers. In another research, Cao et al. [33] prepared a composite of Fe3O4 particles and observed that this one demonstrates greater superparamagnetism, magnetic saturation and yield stress as compared to those of a magnetorheological fluid containing carbonyl iron and/or magnetite particles. Their results also showed more stability for the MRF with Fe3O4/PMMA particles while carbonyl iron particles show the minimum stability. Fang et al. [54] have used magnetite nanoparticles to reduce sedimentation of magnetorheological fluids which has a lower density in comparison with carbonyl iron (approx. 4.32 g/cm3) as well as proper magnetic behavior. They utilized polyStyrene (PS) microspheres to avoid aggregation of magnetite nanoparticles, which were coated with amorphous silica, as the core of magnetite particles. They observed that using the PS/Fe3O4 structure leads to the reduction of magnetic saturation as compared to the single Fe3O4. On the other hand, the amount of sedimentation is decreased due to the reduction of density mismatch between dispersed particles and the carrier fluid. In a similar study, Susan-Resiga et al. [43] compared the rheological behavior of a common MRF with the one containing iron

 

microparticles and magnetite nanoparticles. They understood that magnetite nanoparticles raise the relative viscosity of the magnetorheological fluid significantly. Tan et al. [83] introduced a new kind of magnetic composite particles by reduction of Fe2+ with poly acrylamide-co-methylacrylic acid microgel. Their synthesized magnetorheological fluid (produced by dispersing composite particles and carbonyl iron in water) showed that encapsulating by microgels reduced the apparent density and subsequently decreased sedimentation rate. They also reported that the saturation magnetization of the composite particles increased with increasing content of iron nanoparticles in the composite. Table 3 presents a brief history of the most important researches done so far on using nanoparticles in magnetorheological fluids. According to the mentioned researches, it can be concluded that to achieve the improving influence of adding nanoparticles, volume fraction of nanoparticles must be greater than a specified value. Further studies are required in this area to determine this value for each particle and MR suspension. A comparison between Tables 2 and 3 proves that using nanoparticles instead of coated magnetizable particles not only improves the stability, but also provides a higher yield stress and magnetorheological effect. As previously mentioned a survey on the researches about using nanoparticles in MRFs shows that nanoparticles with high magnetic saturation such as iron and magnetite are promising particles for using in ferrofluid-based MRFs.

 

Table 3. Summary of studies that utilized nanoparticle in MRF Micro Particle

Nano Particle

Nano

Micro

Magnetic

Yield Stress

(Average Size)

(Average

Particle

Particle

Flux Range

Range (Pa)

Size)

Fraction

Fraction

(%)

(%) 0-2¯104

Carbonyl Iron

Magnetite

0-8

32

0-2.57¯102

(2 µm)

(20 nm)

(vol%)

(vol%)

(kA/m)

Iron

2-40

60

0-3¯10

(30 µm)

(28 nm)

(wt%)

(wt%)

(T)

4

0-1.2¯10

Carbonyl Iron

1

75

0-3.43¯10

(7 µm)

(4 nm)

(wt%)

(wt%)

(kA/m)

(4.5-5.2 µm)

Wereley et al. [30]

Carbonyl Iron

Magnetite

Iglesias et al. [6]

-1

Carbonyl Iron

Carbonyl Iron

Reference

2

2

4

10 -10

Park et al. [77]

1

0-10

40

0-5.1¯10

(vol%)

(vol%)

(kA/m)

3

1-10

Chin et al. [79]

Iron

Magnetite

21

10

0-3.43¯10

(930±330 nm)

(7.8±0.3 nm)

(vol%)

(vol%)

(kA/m)

2

0-1.2×104

LopezLopez et al. [2]

-1

Iron

Magnetite

20

20

0-5¯10

(2.1 µm)

(7 nm)

(wt%)

(wt%)

(T) NA♦

1

0-10

SusanResiga et al. [43]

Iron

Iron

30

30

(45 µm)

(15-25 nm)

(wt%)

(wt%)

4

0-2.3¯10

Rosenfeld et al. [27]

-2

Iron

Magnetite

26-40

2-30

0-5.7¯10

(1.2-1.6 µm)

(10nm)

(vol%)

(vol%)

(T)

Iron

Magnetite

(5-26)

(5-31)

0-1.6¯101

(1.2 µm)

(10 nm)

(vol%)

(vol%)

(kA/m)

2

0-3.5¯10

Shimada et al. [80]

NA

Shimada and Oka [81]

Magnetite

Poly methyl

Composite:

methacrylate

37-56-74

(PMMA)

2

0-1.7¯10

Cao et al.

4

5¯10 -

(kA/m)

1¯10

[33]

NA

Viota et

(wt%)

Magnetite

Magnetite

0.5-7

1-10

0-1.4¯101

(1.45 µm)

(9 nm)

(vol%)

(vol%)

(kA/m)

Poly (acrylamide-

Iron

8.9-16.4

NA

0-1.2¯10-1

co-methylacrylic

(5-20 nm)

(wt%)

acid) (9-30 µm)

* Values in the table are approximate, ♦ Not Available  

2

(T)

al. [82] NA

Tan et al. [83]

5.1.3. Wire Particles MRFs based on utilizing spherical nanoparticles, suffered from broad size distribution and poor consistency of particle shape. Nevertheless, fluids containing elongated particles demonstrate less sedimentation [84]. Some of the researchers have mentioned that using nanowire enhances yield stress as well as sedimentation stability [84-86]. However, due to the high wetted area and wire-to-wire interactions among the wires, the iron loading is not as high as that of the spherical particles. As a result, the yield stress of wire-based MRFs is smaller than that of spherical-based ones. Therefore, a combination of spherical and nanowire particles may lead to a stable magnetorheological fluid with promising MR effect [86]. The rheology of magnetic fiber suspensions has attracted little attention in comparison with spherical ones. Fibers due to their shape anisotropy and the existence of more friction, enhance MR effect significantly (two or three times more in comparison with the spherical suspensions) [87]. Bell et al. [84] made a magnetorheological fluid by dispersing iron microwires in silicon oil. According to their findings, volume fraction of iron particles is limited due to the large wet area of tubular microwires and interaction of the wires on each other. Therefore, the yield stress of these fluids is lower than that of conventional MRFs which are made with a greater content of spherical particles. But it should be noted that if the same percent of particles are considered, the microwire-based magnetorheological fluid show higher yield stress and lower sedimentation in comparison with spherical-based ones. Thus, a combination of carbonyl iron spherical particles and iron nanowires was applied to increase yield stress and solve the sedimentation problem. In order to compare microspheres and microwires, Chiriac et al. [88] used FeSiB microwires and mineral oil to prepare a new MRF and Fe microspheres and mineral oil to prepare a commercial MRF. Their findings showed that the

 

particle oxidation and particle size distribution can influence the magnetorheological effect. In another study, Lopez-Lopez et al. [85, 87] developed an MRF based on cobalt fiber which significantly inhibited sedimentation and at the same time, improved yield stress as compared to conventional magnetorheological fluids with cobalt spherical particles. Jiang et al. [26] produced an amorphous magnetorheological fluid consisted of carbonyl iron and iron nanowires. They observed with growing up the weight percent of iron nanowires at constant shear rate and magnetic field intensity, the yield stress and stability of fluid are increased. Ngatu et al. [86] prepared a magnetorheological fluid by dispersing spherical iron microparticles and iron nanowires in silicon oil. They investigated and quantified the changes in MR effect, sedimentation and also off-state viscosity of MRF. Their results showed that by substitution of a quantity of spherical particles by nanowire particles in a dimorphic MRF, the rate of particle settling is reduced significantly and also suggested easier particle redispersion but MR effect did not increase considerably. Another modern method to support promising stability of magnetorheological fluids, which has been considered by many researchers, is using carbon nanowires. In this regard, Fang et al. [57] used single-walled nanotubes (SWNT) as a gap-filler in a suspension of carbonyl iron particles to control instability of the MRF. They compared rheological characteristics of the carbonyl iron-based MRF with and without single-walled nanotubes and finally observed that the addition of carbon nanotubes (CNT) leads to the improvement of elastic characteristics of the magnetorheological fluid in the presence of a magnetic field. They also discovered that using CNT improve stability of the carbonyl iron-containing magnetorheological fluid. Using nanowires in MRFs is a good way for enhancing their stability, but difficulty of producing nanowire and their relatively high price as well as their influence on reducing MR effect, restrict their widespread usages.

 

5.1.4. Carrier Fluid Taking into account the significant effect of nonmagnetic media on rheological characteristics of the magnetorheological fluid, the carrier fluid must be examined carefully [89]. One solution which has been proposed to overcome sedimentation issues of MRFs is to use a more viscose carrier fluid such as a lubricant or a gel. A magnetorheological gel, which is a suspension of magnetic particles in a gel, is actually a new generation of magnetic suspensions with widespread applications in vibration controllers and dampers. These fluids can properly reduce sedimentation of magnetic particles. Stability of the system is improved by deposition of the polymeric gel on the surface of iron particles [90]. But MR gels suffer from increased viscosity in the absence of a magnetic field which restricts their use. Polyethylene oxide (PEO) is a widely used polymer in the production of polymeric solutions. This is a linear polymer which is soluble in organic media and is made from oligomer in the form of macro molecules in industry. It has promising gelation capability and desirable low toxicity. In this regard, Kim et al. [89] developed a magnetorheological fluid which was obtained from the distribution of carbonyl iron in a polymeric solution. They prepared the polymeric solution by solving PEO in distilled water. Viscosity of PEO is greater than that of the conventional oils. They studied behavior of the magnetorheological fluid in different magnetic field intensities. It was understood that at a specific magnetic field intensity, the yield stress of the PEO-containing suspension is much greater than that of the conventional MRF while its sedimentation rate was smaller. Like many other studies that have been conducted on using polymeric compounds in magnetorheological fluids preparation, Gu et al. [42] used the distribution of Fe-SiO2 composite particles in poly ethylene glycol (PEG) 400 in order to make an MRF with a relatively high magnetoviscous effect at low shear rates. They also noticed that yield stress of this fluid is improved by

 

increasing the magnetic density. Their finding was in agreement with the observations of other researchers. Wang and Gordaninejad [7] produced an MR polymeric gel based on polyalphaolefin (PAO) and compared the rheological characteristics of this magnetorheological fluid with those of the MRF based on hydrocarbonic oil as well as ferrofluid-based MRF. They observed that at a given shear rate and density of magnetic field, the apparent viscosity of the polymeric gel and ferrofluid-based MRFs are greater than that of the conventional ones. They also found that the ferrofluid-based MRF has greater yield stress in comparison with the gel magnetorheological fluid and the hydrocarbon oil-based MRF. Unfortunately, such a yield stress for the ferrofluid-based MRF is not consistent with observations of other researchers. On the grounds of using gel in MRFs, Wei et al. [90] developed a magnetorheological gel using poly urethane (PU). They expressed the viscosity of the magnetorheological fluid is increased by rising the molecular mass of poly propylene glycol, weight percent of carbonyl iron particles and the applied magnetic field. In a promising study, Kim and Choi [91] examined the effect of using polymeric compounds as a carrier fluid in shear stress and stability. Their experimental results showed that dispersing CI particles in a mixture of Polyisobutylene/Polybutene (PIB/PB) instead of mineral oil make MRF more stable and enhance MR effect considerably but unfortunately, this mixture increases off state viscosity undesirably. Some researchers have shown that using aqueous medium for MRFs preparation will be promising in some cases. In this regard, Park et al. [46] examined rheological and stability characteristics of a magnetorheological fluid containing carbonyl iron particles which were dispersed in a water-oil emulsion for the first time. Their results showed the magnetorheological fluid stability is strengthened by increasing the water content. They also found that the magnetic saturation of carbonyl iron particles is met at a higher power of

 

magnetic field in comparison with that of magnetite particles. They concluded, consistent with the results of other researchers that because of higher magnetic saturation of carbonyl iron particles, the magnetorheological fluid with carbonyl iron particles has a stronger MR effect compared with the MRF with magnetite. Mazlan et al. [92] used a water-based and hydrocarbon-based MRF for investigating the effect of carrier fluid. Their findings showed that in the same conditions, shear stress of the water-based MRF was more than that of the hydrocarbon-based MRF. In another study, Viota et al. [93] presented the effect of polymer adsorption on sedimentation rate of an aqueous suspension of magnetite particles. They observed that using polymers with high molecular weight such as poly acrylic acid leads to the reduction of system stability since hydrophilic adsorption acts against spatial repulsion. They found that the stabilized suspension is supported by using a correct concentration of the polymer with low molecular weight. Ionic Liquids (ILs) which are largely made from ions are used as a new base fluid for MRFs. Unlike the conventional magnetorheological fluids, characteristics of ionic liquids (e.g. viscosity, solubility, electrical conductivity, melting point, etc.) can be simply altered considering composition of its ions. Furthermore, ionic liquids are very stable and at the same time, environmentally friendly due to their liquid state and negligible vapor pressure and flammability [46]. Using ionic liquids as carriers in an MRF promote the fluid performance, especially in ultra-high vacuum applications, in which organic solvents would easily evaporated. The improvement of MRF stability when using ionic liquids as carriers is related to physical adsorption of the IL ions on particle surfaces that gave rise to steric repulsion between the dispersed particles and decreased particle aggregation [94]. In this regard, Guerrero-Sanchez et al. [45] produced an ionic liquid-based MRF. They used magnetite nanoparticles and microparticles for the dispersed phase, and utilized eight different ionic liquids to investigate the effect of ion type on the magnetorheological fluid (for a complete

 

explanation see ref. [45]). Their results demonstrated that using ionic liquids with no stabilizer additive will significantly reduce sedimentation rate of the MRF without any significant undesirable changes in MR effect. By changing the ionic liquid used in the magnetorheological fluid they concluded that the sedimentation rate of the magnetorheological fluid depends on type of the used ions and concentration of magnetic particles. In a relatively comprehensive study, Rodriguez-Arco et al. [94] provided an overview of magnetic suspensions and their applications, and made a detailed study of ionic liquids for using in MRFs. According to their survey, there are still lots of challenges in the field of using ILs in MRFs such as determining compatible dispersed phase and its concentration, analysis of the relation between particle surface and constituent ions and so on. In the context of producing new MRFs, Zhang et al. [95] synthesized Magnetorheological Shear Thickening Fluids (MRSTF) by using shear-thickening fluids. They observed that MRSTF demonstrates a significant thixothropic behavior when the weight percent of base shear thickening fluid is greater than an extreme value (about 15 wt%). This modification reduces the sedimentation of MRFs to a large extent and as a result, stability of these fluids improves considerably. At the same time, the MR effect still remains acceptable. Since their synthesized magnetic suspension was highly viscous even without magnetic field, it is suitable for fail-safe systems. They declared that such a structure will be able to control sedimentation of the magnetorheological fluid completely without any reduction in magnetorheological effect. Therefore, it seems promising for MR dampers, though not suitable for clutches where friction must be minimized. Bednarek [96] used magnetic silicon steel particles and cedar wood oil to prepare a magnetorheological fluid. Graphite microparticles were added to decrease the resistivity of the base phase by filling free spaces between the steel particles and increasing their area contact. It has been declared that this MRF was stable for more than six weeks which is promising.

 

For obtaining well-dispersed magnetorheological fluids, Lopez-Lopez et al. [97] used iron microparticles and different carrier liquids as well as surfactants (for a complete explanation see ref. [97]). They found that the dispersion of bare iron particles in nonpolar carriers is very poor. They showed that the change of viscosity is up to four times larger when particles are well dispersed. Among all the materials, using polymeric gels in magnetorheological fluid preparation has been further extended. In spite of the researches which have been conducted so far to determine the effect of carrier fluid on the MR effect and stability of these fluids, no material or composition has been found satisfactory enough for industrial applications. Therefore, further studies are needed in this context and silicon oil is still used in most applications of magnetorheological fluids as the carrier fluid.

5.1.5. Stabilization Treatments and Additives Only those additives are used in magnetorheological fluids which affect structural stability of the suspension against sedimentation. In a successful research, Lim et al. [98] prepared an MRF from carbonyl iron particles. They studied the effect of adding fumed silica particles on the stabilization of the magnetorheological fluid and found adding a specific amount of fumed silica to the magnetorheological fluid considerably successful in preventing sedimentation of carbonyl iron particles. Bica et al. [99] investigated the effect of different surfactants on the stability of a water-based MRF and saw that coating a double layer of magnetite nanoparticles with lauric acid and/or myristic acid leads to the production of a magnetorheological fluid with appropriate stability. Some researchers, like Viota et al. [100], have focused on using polymeric compounds as stabilizer additives. They examined rheological behavior of an aqueous solution of magnetite particles which were previously stabilized with poly acrylic acid (PAA). They also observed that stability of the magnetorheological fluid enhances by

 

increasing the amount of poly acrylic acid. Lebedev and Lysenko [101] investigated rheological and magnetic characteristics of some magnetorheological fluids which were stabilized with poly propylene glycol (PPG). PPG is a stabilizer which can easily form an emulsion in aqueous solutions. They noticed that the magnetic characteristics of the colloidal fluid are significantly affected by molecular weight of PPG stabilizer. Regarding their conclusions, this might be due to the fact that molecular weight of PPG determines thickness of the shell which surrounds the particles as well as their upper limit of magnetizability. They also found that alcohols are the best media for particles coated with PPG. In another study, Hato et al. [58] studied the effect of adding submicron organoclays to an MRF based on carbonyl iron particles and noticed that the addition of up to 1 wt% organoclays improves redistribution and stability of the suspension. In a similar research, Lim et al. [102] employed organoclays with sub-micron size in order to stabilize an MRF containing carbonyl iron particles. They observed that addition of organoclay without much change in MR effect can improve stability of the magnetorheological fluid. They reported a descending trend for yield stress at first which was later changed to an ascending trend by increasing the magnetic field. This was not consistent with the result of other researchers. Sedlacik et al. [103] exposed carbonyl iron particles to argon and octafluorocyclobutane plasma in order to create fluorine bonds on the surface of carbonyl iron particles. Their results showed that an MRF with plasma modified carbonyl iron particles provides a better stability in comparison with the conventional magnetorheological fluid with carbonyl iron particles. They attributed this phenomenon to the attractive force of a fluorine bond on the surface of carbonyl iron and also of a methyl group which exists in silicon oil. Premalatha et al. [8] studied the effect of additives fraction (grease) on the stability of a magnetorheological fluid. Based on their examination, the sedimentation of the fluid will be less significant at higher percents of the grease. They also observed that the

 

magnetorheological fluid has non-Newtonian behavior in the absence of a magnetic field. Rankin et al. [104] dispersed carbonyl iron particles in mineral oil and used two types of grease as additive. Their results showed that using grease improves stability considerably without significant effect on the MR effect. Shetty et al. [21] used carbonyl iron as dispersion phase and Honge oil as continuous phase. For stabilizing the MR suspension, they used grease and observed that stabilization improved considerably. Their results showed the effect of particles percentage on the increasing yield stress is more promising at stronger magnetic fields. Lopez-Lopez et al. [105-106] examined the effect of adding aluminum stearate to a suspension of iron and kerosene and observed this surfactant improve MR effect and redispersibility of particles which is consistent with the claim of other researchers [5, 25] but reduced stability. As mentioned before, enhancement of yield stress is one of the most important issues in the context of magnetorheological fluids which has been investigated in several recent studies. One of the methods which have been recently used by many researchers is the enhancement of yield stress by passing an MRF through a porous media. In this regard, investigation of the magnetorheological fluid flow through the porous media, Kuzhir et al. [107] observed that yield stress in packed beds is greater than that in straight beds. They also noted that the magnetorheological fluid flow through magnetizable particles will make a greater pressure drop in comparison with passing the fluid from a bed of non-magnetizable particles, so that they will have a higher average yield stress. Based on their observations, the pressure drop in a bed of spherical particles due to larger voids of the bed is much greater than that in the bed with cylindrical particles. Liu et al. [37] studied the effect of metal foam structures on the performance of magnetorheological fluids and derived a relation between permeability and porosity of metal foams. They noticed that the performance of magnetorheological fluids changes a little by passing them through metal foams. Since metal

 

foams show a greater strength respect to sponge-like structures, they are good choices for designing new and low cost dampers. Studies in this field show that using polymeric compositions is significantly effective in the stability of magnetorheological suspensions though an appropriate stabilizer is not introduced yet, which can reinforce the magnetorheological effect besides improving stability. Among the existing methods, it seems using magnetizable nanoparticles and stabilizing additives such as fumed silica in MRF preparation has satisfaction results, such that enhance the magnetorheological effect as well as its stability.

6. Conclusions Nowadays application of magnetorheological fluids with adequate controllability is of great importance in many industries. Magnetorheological fluids which are suspensions of magnetizable particles in a base fluid with stabilizer additives, demonstrate behavior of a semi-solid material in the presence of a magnetic field. Obtaining great yield stress (strong magnetorheological effect) and reducing sedimentation of magnetizable particles, due to gravity, are two important challenges in the context of MRFs. These problems, which mainly depend on type, shape, size and volume fraction of the magnetizable particles, considerably affect rheological properties of these fluids. Choosing magnetizable particles depends on various factors like compatibility with carrier fluid, stability considerations, demanded MR effect and so on. However, carbonyl iron microparticles are the most promising particles for dispersed phase in MRFs. Selection of carbonyl iron particles contributes to high saturation magnetization, low coercivity, relatively low cost and their widespread availability. Up to now, lots of methods have been introduced to improve stability as well as MR effect. The most important of these methods are: decreasing the density of magnetizable particle through coating them, increasing the viscosity of carrier fluid by using high viscose

 

liquids and using nanostructure materials and modifying particle surface by adding stabilizer surfactant. Coating the magnetizable particles is one possible method to reduce interactions between the particles as well as particle density. By using this method stability can be enhanced considerably, although the MR effect will be reduced undesirably. Among all the materials used in this method, coated carbonyl iron particles with polymeric materials have been used more. Another stabilization method that is one of the most promising ones, which also enhances magnetorheological effect, is associated with a new structure of magnetorheological fluids obtained from suspending magnetizable microparticles in a ferrofluid. Using a combination of nanoparticles and microparticles in MRFs creates bonds between these particles which consequently enhance the yield stress. Moreover, existence of nanoparticles in the base fluid increases viscosity of the base fluid and thus reduces sedimentation. Furthermore, some studies indicate that the carrier fluid has a key role in magnetorheology. Using fluids with higher density contributes to minimize the density difference between the base fluid and metallic particles, as the main effective factor in sedimentation and instability of these suspensions. Unfortunately, the viscosity of these fluids increased in the absence of magnetic field which is not desirable. However, silicon oil is still known as the first choice for the magnetorheological fluids preparation due to its availability, low cost, low viscosity and so on. Some researches focused on using proper additives that can improve both stability and MR effect. So far, materials that can provide these requirements have not been identified. Nevertheless, it seems that among all additives and surfactants, stearic acid, organoclay and fumed silica are suitable materials. Unfortunately, many of these stabilizing methods reduce yield stress and thus magnetorheological effect, and additional research in this context seems to be necessary.

 

Despite numerous studies conducted in this area, since the discovery of magnetorheological fluids, it still seems necessary to find a new composition of these suspensions which can be more stable and also more cost effective. Development of magnetorheological fluids applications, need additional research work for improving MR effect as well as stabilization of these fluids. Among all discussed methods, using a combination of ferrofluids and magnetorheological fluids, using nanowires and also adding proper stabilizers may be accounted as appropriate stabilization methods which can improve both stability and magnetorheological effect. Determination of the allowed limit for addition of nanoparticles and nanowires to magnetorheological fluids, and using proper stabilizers needs further experiments which can be the subject of many recent researches.

Acknowledgment Financial Support from the Iran National Science Foundation (INSF) gratefully acknowledged.

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Figure Captions Fig. 1. Overview on MRF Stabilization methods. Fig. 2. Schematic image of particle coating process with polymeric compound; (a) bare particle, (b) activated particle, (c) monomer addition and (d) coated particle (adopted from [28, 34]). Fig. 3. SEM image of (a) Bare CI and (b) CI-PMMA core-shell structured particles (reprinted with permission from ref. [32]). Fig. 4. A survey of experimental studies on the effect of coating of magnetizable particles on MR effect in different magnetic field strength; (a) H=0, (b) H=86 and (c) H~342 kA/m. Fig. 5. Schematic image of MR effect; (a) microparticles and (b) bidisperse particles ( represent microparticles and

spherical nanoparticles).

Fig. 6. Stabilization mechanism of a ferrofluid-based MRF; (A) Magnetically induced chain formation in a conventional MRF, (B) Filling the microcavities by nanoparticles in a bidispersed MRF [51].

Table Captions Table 1. Major characteristics of magnetorheological fluids [1, 5] Table 2. Summary of studies that implemented coating on magnetizable particles Table 3. Summary of studies that utilized nanoparticles in MRF