A review on the magnetorheological materials and applications

Abstract

Magnetorheological materials refer to field-response smart materials whose properties are controllable with a magnetic field, including fluid, grease, elastomer, and gel. The unique magnetorheological effect exhibited by these smart materials is a physical phenomenon where physics and engineering intersect and has extensive application prospects in modern machinery. In electro-mechanical systems, magnetorheological materials offer a superior design method for mechanical devices used in the fields of transmission, damping, and braking. It is important to control the magnetorheological materials for advancing the design philosophy of modern electro-mechanical devices. Hence, this paper presents a recent progressive review on the fundamentals of magnetorheological materials and numerous applications. Firstly, an introduction to the magnetorheological effect and different types of magnetorheological materials are presented in this review. Then, the individual and coupled effects of sedimentation, temperature, and magnetic field on magnetorheological materials are discussed. Finally, magnetorheological materials-based devices have been extensively reviewed, including actuator, clutch, damper, brake, pump, valve, and robot, thus aiming to provide useful information for facilitating the design of complex electro-mechanical systems.

Keywords

Magnetorheological materials magnetorheological effect stability reliability

1. Introduction

Magnetorheological (MR) materials are field-response intelligent materials that refer to the composite fluid/polymer prepared by homogeneously dispersing the magnetizable medium into a non-magnetic matrix. These smart materials exhibit unique properties and functionalities similar to mechanical flexible materials, which can not be realized by conventional materials [1–3]. The magnetizable medium is arranged orderly under the traction of the magnetic field, which makes MR materials look like “solidification”. This phenomenon is known as the MR effect and is affected by multiple factors such as force, temperature, and magnetic field [4–7]. The desired magnetic field can be generated by adjusting the input current to the excitation coil, thereby achieving wide-range independent control of the composite colloidal systems [8,9].

The magnetic field is the primary driving force behind the adsorption and orientation of MR materials, which leads to changes in the rheological properties and physical states [10–12]. This process is also influenced by external factors such as composition [13–17] and temperature [18–23]. MR effect is an intriguing changing process where physics and engineering intersect and has extensive application prospects in modern machinery [24–27]. The unique rheological properties of MR materials allow for fast, continuous, and reversible responses [28–31]. Specifically, MR materials have evolved into various types suitable for different engineering applications, mainly including MR fluid [32,33], MR grease [34,35], MR gel [36,37], MR elastomer [38–40], and MR foam [41,42]. In essence, the MR effect allows MR materials to replace traditional materials in the engineering fields of lubrication, transmission, and damping. In view of this, MR materials have the feasibility of realizing practical applications [43–51].

With this background, research on MR materials and their applications have experienced explosive growth during the past decade, which is expected to continue to improve [52–60]. To the best of our knowledge, temperature is one of the critical factors affecting the properties of composite polymers [61]. Furthermore, we should pay attention to the effect of temperature on the properties of MR materials. Whether MR materials are used for interfacial lubrication and transmission, the heat of friction and energy dissipation will lead to a rise in temperature. Especially under high power and high slip conditions, the sharp temperature rise will directly affect the stability and reliability of MR devices [62–64]. On the other hand, the wide temperature range over which aerospace devices operate also challenges the application of MR materials [65]. The MR effect possessed by MR materials is essential in advancing the application of modern complex electro-mechanical systems. Researchers are constantly trying to maximize the rheological properties of MR materials to maintain the stability and reliability of MR devices. In order to achieve this goal, the mechanism of the MR effect needs to be explored to facilitate the development of high performance MR materials, the optimization of device structures, the enhancement of magnetic circuits, and the development of efficient control strategies.

Numerous research works have been conducted on the development and application of MR materials, and significant progress has been achieved in this field. The purpose of this paper is to present a review on the MR materials and their applications. In this review, we briefly introduce the properties of MR materials and provide an overview of the effects of sedimentation, temperature, and magnetic field. Then, the recent progress of MR devices has been reviewed and comprehensively discussed. It is believed that there is a growing demand to present a comprehensive overview of MR materials and applications, which can help to provide useful information for facilitating advances in intelligent complex electro-mechanical systems.

2. Fundamentals

MR materials have attracted the attention of researchers and the related research has experienced explosive growth during the past three decades with the MR fluid being the highest area of concentration, as summarized in Fig. 1. At present, research on MR materials is mostly concerned with using the MR effect to enhance the performance of base carrier liquid or achieve controllability and stability. For instance, MR fluid can change from a fluid state to a solid-like state in milliseconds and its apparent viscosity will vary by several orders of magnitude by varying the magnetic field strength, which is undoubtedly an excellent property compared to conventional materials such as hydraulic fluid [66,67].

Fig. 1.

Related investigations on MR materials since 1994, Engineering Village^© as per June 15, 2023. All Scopus archived documents (Journal articles, Conference proceedings, and Books) are considered. The keywords “magnetorheological fluid/grease/ elastomer/gel” are used to filter the research works that are focused on MR Materials.

2.1. Constituents of MR materials

The composition of MR materials includes magnetizable medium, base carrier liquid, and additives [68]. The magnetizable medium is usually selected from carbonyl iron powders (CIPs) with high magnetic permeability and saturation magnetization strength [69]. The microscopic morphology of CIPs [70] is presented in Fig. 2. It can be seen from this figure that CIPs are essentially monodisperse, which is beneficial for uniform dispersion in the base carrier liquid. The MR effect causes the magnetizable medium to follow a directional arrangement along the magnetic field to form a magnetic chain, manifested by a change in the physical state and apparent viscosity. The magnetic chains are the main part of realizing the regulation of the structural system, and the magnetic field contributes significantly to the modulation of magneto-static stress [13].

2.1.1. MR fluid

The base carrier liquid for MR fluid is typically silicone oil, mineral oil, or water, which makes them exhibit great fluidity in the absence of a magnetic field. MR fluid responds immediately and reversibly to a magnetic field by converting from a free-flowing Newtonian liquid to a non-Newtonian liquid with solid or solid-like properties. Vicente et al. [71] and Sun et al. [72] investigated the effect of the shape, size, and content of CIPs on the performance of MR fluid. This study has shown that MR fluid containing non-spherical particles exhibited superior initial properties, but this difference decreased with increasing CIPs content and magnetic field strength. Specifically, increasing the content of magnetizable medium can significantly improve the stiffness of MR fluid.

2.1.2. MR grease

The introduction of a magnetizable medium slightly reduces the initial viscosity of the grease, but most MR grease is in a solid-like state at room temperature. Due to the unique composition and microstructure of the grease, MR grease does not require additional additives and exhibits superior MR effect [78]. The soap fibers of the grease can strengthen the structural intensity of the MR grease, but the temperature negatively affects the entanglement degree of soap fibers. For MR grease, the contents of the magnetizable medium increase from 0 to 70 wt%, with a significant enhancement of the MR effect. Moreover, researchers have further confirmed that the MR effect can be increased by up to 950 % by utilizing 70 wt% of CIPs [35]. The MR grease containing non-spherical particles presents more excellent rheological properties [79], and this study confirms that the magnetizable medium shape also contribute differently.

2.1.3. MR elastomer

Elastomer is a composite material that is widely implemented in sensors, wearable devices, and robots [83–85]. MR elastomer is a kind of rubber-like material without sedimentation of the magnetizable medium, which means that the main role of additives is to enhance the MR effect. MR elastomer has been confirmed that small-sized particles as fillers can enhance the performance of polymer elastomer, and the interfacial interaction between magnetic particles and polymer elastomer can improve the MR effect [86,87]. Wang et al. [88] prepared a novel MR elastomer (8.5 vol%) by dispersing CIPs into the shear-stiffening elastomer and indicated that the MR elastomer exhibited a great self-healing ability due to the reversible interaction of B-O bands.

2.1.4. MR gel

MR gel is a kind of composite gel with a magnetizable medium suspended in a polymer gel matrix and can be classified as liquid-like (the solvent content is less than 10 wt%) or solid-like (the solvent content exceeds 25 wt%). MR gel is an intermediate between MR fluid and MR elastomer, the rheological properties of MR gel also show a significant dependence on the magnetic field, with excellent controllability and sedimentation stability [92–94].

Fig. 2.

Scanning electron microscope image of CIPs [70].

The above-mentioned MR materials with their specific composition are summarized in Table 1. It can be seen from Table 1 that CIPs are the most widely used magnetizable medium, with sizes ranging from 1 to 10 μm. Liquid- like MR materials have different rheological properties, and their base carrier liquids include silicone oil, deionized water, and polyurethane matrix/gel. Moreover, the contributions of additives should not be underestimated, and all of them are improving the properties of MR materials in different ways. For example, carbon- and chromium-based additives enhance the dispersion and stability of CIPs in the base carrier liquid, respectively. In order to improve the mechanical and magnetic properties of MR materials [97], Zhang et al. [98] investigated the effect of nano-silica particle additives on MR behavior. Nano-silica particle additives are adsorbed on the surface of the magnetizable medium, which can enhance the friction behavior between the suspended particles. Moreover, Table 1 shows that MR materials are prepared with or without additives, which means additives are beneficial but not an indispensable component for MR materials.

Table 1

Summary of the MR materials from different researches

MR materials	References	Magnetizable medium			Base carrier liquid	Additives
		Type	Size	Content
MR fluid	Vicente et al. [71]	Iron particles	658±10 nm	0.5/1/1/5 vol%	Ga-In-Sn eutectic liquid metal	-
	Sun et al. [72]	Fe	30/50/75/100 μm	10/20/30/40/50 mol%	Deionized water	-
	Bai et al. [73]		50/100/500/1000 nm		Synthetic oil (PAO 10)	-
	Ji et al. [74]	CIPs/Fe-Co-Ni	2 μm/100 nm	40 wt%	100 cSt dimethyl silicone oil	Sodium lauryl phosphate/MES
	Seo et al. [75]	CIPs	4.5 μm	20 vol%	Silicone oil	-
	Zhang et al. [76]	CIPs	4.5 μm	50 wt%	Silicone oil	ZnFe2O4
	Kwon et al. [77]	Fe-Ga alloy	50 μm	30 wt%	Silicone oil	NHT particles/composite particles
MR grease	Pan et al. [21]	CIPs	8.35 μm	20 vol%	Lithium grease	-
	Mohamad et al. [35]	CIPs	5 μm	10/30/50/70 wt%	NPC Highrex HD-3 Grease	-
	Mohamad et al. [78]	CIPs	3.9∼5.2 μm	30/40/50/60/70 wt%	Lithium grease	-
	Mohamad et al. [79]	CIPs	3.9∼5.2 μm	70 wt%	NPC Highrex HD-3 Grease	-
	Mohamad et al. [80]	CIPs	5 μm	30/50/70 wt%	NPC Highrex HD-3 Grease	-
	Kadir et al. [81]	CIPs	5 μm	70 wt%	NPC Highrex HD-3 Grease	Kerosene/ hydraulic
	Kim et al. [82]	CIPs	7 μm	50 wt%	Multiservice grease E	Kerosene
MR elastomer	Wu et al. [86]	Fumed silica	15 nm	33 wt%	Polypropylene homopolymer	Styrene/ethyl acrylate
	Yu et al. [87]	CIPs	1∼8 μm	60 wt%	PU matrix	2,4,6-Triphenol/Di-butyl phthalate
	Wang et al. [88]	CIPs	6 μm	3.4/5.6/8.5/12.2/17.2 vol%	Hydroxyl silicone oil/methly vinyl silicone rubber	Benzoyl peroxide
	Berasategi et al. [89]	CIPs	1.25±0.55 μm	0∼30 vol%	Ribbed smoke sheet	Stearic acid/ZnO/antiagers poly
	Yaghoobi et al. [90]	CIPs	5 μm	70 wt%	Polyurethane/epoxy resin IPNs matrix	-
	Cao et al. [91]	CIPs	3.2 μm	20/40/60 wt%	Ribbed smoke sheet	-

Table 1 (Continued)

MR materials	References	Magnetizable medium			Base carrier liquid	Additives
		Type	Size	Content
MR gel	Xu et al. [12]	CIPs	6 μm	70 wt%	Toluene diisocyanate/polypropylene glycol	Dipropylene glycol
	Wei et al. [92]	CIPs	3.5∼21 μm	60/70/80 wt%	Polyurethane glycol	TDI/1,4-butanediol/tin(II) octoate/acetone
	Yu et al. [93]	CIPs	5∼8 μm	50/60/70 wt%	Diphenylmethane diisocyanate/castor oil	Stannous octoate
	Zhang et al. [94]	CIPs	3 μm	60 vol%	Polyurethane matrix	1,4-butanediol/dibutyltin dilaurate/toluene
	Maurya et al. [95]	CIPs	6∼10 μm	65 wt%	Deionized and filtered water	OPTIGEL-WX
	Pang et al. [96]	CIPs	6 μm	15/30 wt%	Polyurethane gel	-

2.2. Basic research

2.2.1. MR effect

The magnetorheology has seen indelible contributions from researchers around the world. In 1948, Rainbow [4] reported a magnetic fluid and a novel electromagnetic fluid clutch. Preliminary results indicated that the clutch requires very small amounts of electric power, and the control is easy and extremely smooth. Scholars have further confirmed that MR materials are easily manipulated with an external magnetic field [99,100]. In addition, various types of influence factors acting individually or as a couple need to be considered for their contribution to the MR effect. For example, the thickness of the MR fluid layer directly affects torque transfer capability [101], and the wall- slip mainly affects the yield stress of the MR fluid, which becomes more pronounced with increasing volume fraction of magnetizable medium [102]. Reasonable regulation of magnetic field can weakens the wall-slip effect and compensates for the effect of high temperature [21].

Naturally, MR materials can be regarded as a system in which the liquid and the solid phases coexist. Figure 3 illustrates the effect of a magnetic field on MR materials. As plotted in Fig. 3A–B, MR fluid is initially in a fluid state that can flow and disperse freely. The magnetizable medium dispersed inside the base carrier liquid is directionally arranged under the traction of the magnetic field. Figure 3A–B reflects the different states of the MR fluid in the presence or absence of a magnetic field. In this process, the movement of the magnetizable medium in the base carrier liquid is influenced by the magnetic field, viscoelastic resistance, Van der Waals force, and Brownian forces. It should be noted that the base carrier liquid has a limited carrying capacity, so the magnetizable medium (micron or nano-sized particles) dispersed inside MR fluid will partially settle under the effect of gravity, which will make MR fluid not completely homogeneous [103]. In addition, MR grease with a mass fraction of 30% was prepared in our laboratory by selecting CIPs and lithium grease. We observe that the MR grease forms a column-like structure along the direction of the magnetic induction line, as shown in Fig. 3C.

Moreover, constitutive modeling is a practical tool for characterizing the rheological properties of MR materials and contributing to the optimization of MR devices. The rheological characteristics of the MR fluid can be effectively described by the Bingham model [104,105], and the MR effect becomes obvious when the magnetic field increases to a certain strength or reaches saturation [106,107]. For grease, shear yield is followed by a viscous flow area and the classical rheological model can quantitatively describe the flow law of this area [108]. The Herschel–Bulkley model can describe the viscous flow of grease and the rheological behavior of MR grease [109–111]. The MR effect is nonlinear and the basic evolution mechanism of the MR grease is illustrated in Fig. 3D [112,113]. In addition, researchers have confirmed that the rheological behavior of the liquid-like MR gel can be described well by the Bingham model [114,115]. Xin et al. [116] investigated the dynamic viscoelastic model of the MR elastomer that is dependent on the frequency, amplitude, and magnetic field. Chen et al. [117] presented a rheological model considering the interfacial slippage between matrix and particles. In [118,119], Chen et al. developed a continuum theory for a mixture consisting of a fluid and solid continuum. They regarded the structure composed of the magnetizable medium in MR fluid as a solid continuum and modeled the MR fluid as a mixture that consists of a macroscopic fluid continuum and mesoscopic multi-solid continua. In other words, MR materials include liquid- phase, solid-phase, and solid-liquid mixed. The mixture method treats MR materials as a mixed system consisting of a fluid and equivalent solid continuum [120]. However, it is difficult to conduct research on mixture models, but mixture theory and models are valuable in investigating the elastoviscoelastic behavior of fluids [121].

Fig. 3.

Effect of magnetic field on the MR materials. (A) MR fluid in the absence of a magnetic field (adapted from [2]). (B) MR fluid in the presence of a magnetic field (adapted from [2]). (C) MR grease forming ordered columns along the magnetic induction line in the presence of a magnetic field. (D) Evolution mechanism of MR materials under magnetic field (adapted from [113]).

2.2.2. Sedimentation of magnetizable medium

There is a large density mismatch between the magnetizable medium and the base carrier liquid, which indicates that the magnetizable medium will settle under the influence of gravity if the base carrier liquid cannot support it. This phenomenon has a great negative impact and the settling stability is consistently a critical factor limiting the development of MR materials [122,123], especially in the case of the magnetizable medium that precipitates when left unused for a long time.

With the development of technology, scholars have proposed many solutions including additives, changing particle sizes, and coating particles. As described in Table 1, the magnetizable medium size is in the range of 1 to 10 μm to ensure the yield strength while avoiding the sedimentation caused by a too-small size. Ulicny et al. [124] found that non-magnetizable particles can enhance the MR effect and Hato et al. [125] experimentally found that adding organic clay particles can improve the dispersion stability of the MR fluid. Additives have been reported to prevent the physical contact of magnetizable medium [126,127]. Unfortunately, all of these approaches have more or less negative impacts on the rheological performance while increasing stability. Wang et al. [128] fabricated a water- based magnetic fluid with long-term stability and the experiment results show no aggregation and deposition of magnetic particles. Surfactants such as stearic acid can avoid irreversible aggregation of magnetizable medium and prevent settling after aggregation. For instance, the nanoscale oleic acid-coated Fe₃O₄ particles had better dispersion stability due to Brownian motion and van der Waals forces.

The fluidity of the MR grease/elastomer/gel is not very excellent, but these smart materials present superior settling stability compared to the MR fluid. Due to the special characteristics of the base carrier liquid, the sedimentation of the magnetizable medium can be avoided. As shown in Fig. 4, the special soap fiber structure of grease can overcome gravity to keep the magnetizable medium in an equilibrium state [129–131]. Consequently, appropriate approaches have contributed to overcoming the existing problems in the applications of MR materials.

Fig. 4.

Soap fibrous structure of grease tested by Pan et al. [129].

2.2.3. Temperature

The heat generation during the operation of MR devices includes frictional heat and coil heating, which is characterized by an increase in internal temperature. MR materials are composite materials whose properties vary with temperature. MR materials used as damping or transmission medium are typically exposed to high operating temperatures, especially in high-power MR devices. For instance, the mechanical properties of the MR fluid are temperature dependent and the shear viscosity is significantly reduced by prolonged exposure to high temperature [132]. The high temperature enhances the Brownian motion of the magnetizable medium and facilitates the formation of particle chains in the presence of the magnetic field.

Wang et al. [133] measured the magnetic hysteresis loops of CIPs as presented in Fig. 5. It can be seen from this figure that high temperatures degrade the magnetization properties of CIPs, and the rate of degradation is approximately synchronously with temperature. On the other hand, Pisuwala et al. [134] found that the shape effect causes changes in the thermal conductivity of the MR fluid. The flake-shaped particles have higher interactions, which can reduce thermal resistance. Furthermore, mutual friction among CIPs in the slip condition also leads to temperature rise, which may cause the base carrier liquid to thin out or even evaporate, this process will increase the wear of the magnetizable medium and may even result in a complete invalidity of MR materials. Leal et al. [135] proposed incorporating a fibre Bragg grating array into the MR fluid to evaluate the magnetic field strength and position. This work found that temperature variations could lead to errors in the magnetic field assessment. The thermal stability of MR materials dramatically influences the accuracy of MR devices, so the high temperature will cause an adverse effect on the performance of the developed devices.

Fig. 5.

Magnetic hysteresis loops of CIPs with various temperatures tested by Wang et al. [133].

The base grease of MR grease has obvious viscosity-temperature properties, and its fluidity can be improved at high temperature [136,137]. Wang et al. [138] point out that temperature variations affect the nonlinear MR effect of MR grease. Similarly, the properties of MR elastomer vary with operating temperature [139,140]. For instance, the energy storage modulus of MR elastomer decreases with increasing temperature and eventually stabilizes. The shear stress of MR gel is negatively correlated with temperature, and the maximum yield stress decreases with increasing temperature [141]. In addition, Pan et al. [142] conducted a high temperature thermal aging experiments on MR grease with a 30 % mass fraction of CIPs and the the results of shear stress test are presented in Fig. 6. They found that the sustained thermal effect would destroy the micro-structure of the MR grease and degrade its performance. Therefore, it is necessary to pay attention to MR materials with excellent thermal stability. Thakur et al. [143] found that graphite flakes can enhance the strength of MR fluid at high temperature, and this work provides a creative additive ideal for high temperature resistant MR fluid. Li et al. [18] developed a novel high temperature MR fluid with stable shear yield stress suitable for high-power MR devices.

As described above, the effect of temperature on MR materials cannot be negligible. Furthermore, temperature is also closely related to the performance of MR devices. In other words, we should fully consider the influencing factors in the design process of MR devices and the influencing mechanism needs to be further explored.

Fig. 6.

Magnetic scan curves of MR grease treated with different thermal aging time tested by Pan et al. [142].

2.2.4. Magnetic field

Magnetic permeability [144] is the performance evaluation index of MR materials, which is associated with investigating the mechanism of magnetically induced yield stress. The essence of the MR effect is that the magnetic field pulls magnetizable medium such as CIPs to form magnetic chains along the direction of the magnetic induction line. The number, length, and structural strength of magnetic chains are directly proportional to the magnetic field strength. Wang et al. [145] discovered that the magnetic chain length of the MR fluid (30 wt%) reaches more than 10 mm when the magnetic field strength is 12 mT. This proves that Young’s modulus can be changed by controlling the magnetic field strength to improve the stiffness of the MR fluid, which has an important research implication in the fields of variable stiffness such as soft robots [72]. Li et al. [13] discovered that magnetic-base carrier liquid would divert more magnetic flux density, reduce the magnetization strength of magnetizable medium, and decrease magnetically induced yield stress. At most times, the MR fluid operates in typical shear mode and the magnetic flux is applied in a horizontal direction, which causes the shear deformation of the MR fluid when the magnetic field strength reaches a certain value [146,147].

Shan et al. [53] investigated the rheological properties of the MR fluid (25 wt%) in shear rate ramp mode and indicated that the shear stress of MR fluid increases with magnetic field strength at the same shear rate, as shown in Fig. 7A. In [21], the shear stress of MR grease (30 wt%) also increases with magnetic field strength and Fig. 7B presents the variation curve of shear stress under thermal-magnetic coupling conditions. Apart from shear stress, the flow pressure drop in the medium channel is also affected by the magnetic field strength in MR devices [148]. In addition, Xu et al. [149] found that the MR fluid can blocks noise propagation when the magnetic field direction is parallel to the noise propagation direction. In terms of the finishing effect of MR fluid, it was effectively to reduce the surface roughness of steel which decreased with increasing magnetic field [150].

MR grease exhibits strong solid-like characteristics in a magnetic field [151], the dynamic yield stress of which is also correlated with the magnetic field strength, and the storage and loss modulus are constant for a stable magnetic field. In the same case, the MR elastomer swells with increasing magnetic field strength [152]. The creep behavior of MR gel was studied by Meharthaj et al. [153] and they confirmed that the creep strain value decreases with increasing magnetic field strength.

Fig. 7.

Shear stress curves of (A) MR fluid (25 wt%) tested by Shan et al. [53] and (B) MR grease (30 wt%) tested by Pan et al. [21].

2.2.5. Thermal-magnetic coupling

Both temperature and magnetic field influence the rheological properties of MR materials by altering the internal microstructure. Pan et al. [21] revealed the thermal-magnetic coupling evolution mechanism of MR grease according to rheological experiments, as shown in Fig. 8. The variation in the properties of MR materials under thermal- magnetic coupling will directly affect MR devices. Typically, the base carrier liquid will thin out or evaporate with increasing temperature. Weiss et al. [154] found that the plastic viscosity and dynamic yield stress of MR fluid (model MR-100) decreased by 95 % and 10%, respectively, as the temperature rose from −40 to 150 °C. In addition, solid-state grease also changes to a free-flowing state at high temperature [155].

Firstly, the magnetizable medium forms magnetic chains in the magnetic field. Then, the structural skeleton of MR materials transforms into a composite system composed of magnetic chains and molecular chains or soap fiber. The molecular chains and soap fiber link the magnetic chains and contribute to the structural strength of the composite system. The viscosity of lubricating oil and grease decreases significantly at high temperatures [155,156] that will weaken the connections between magnetic chains. The settlement and shear stability of MR materials may also become unsatisfactory. As shown in Fig. 8, the quantities of magnetic chains varies synchronously with the magnetic field. Temperature and magnetic field do not always play a positive role under thermal-magnetic coupling conditions. For instance, the magnetic field strength needs to be increased appropriately to attenuate the adverse effects of high temperature.

In addition, influence factors such as rotational speed, pressure, transmission interface, and wear also have some contributions. For example, in MR transmission, the centrifugal force induced by high rotational speed leads to an uneven distribution of the magnetic medium, which ultimately leads to fluctuations in the transmitted torque [157]. For this situation, it is necessary to increase the magnetic field strength to counteract the centrifugal force. Hu et al. [158] pointed out that magnetizable medium is subject to wear and the magnetic field can effectively slow down this phenomenon. Therefore, it is beneficial to investigate the intermediate mechanism to improve the performance of MR devices in the research and design process.

Fig. 8.

Thermal-magnetic coupling evolution mechanism of MR grease originated from Pan et al. [21].

3. Applications

MR material-based devices have wide applications in the field of mechanical engineering which is so-called MR devices. It is well known that MR devices are able to meeting the requirements of practical applications. There are various types of MR devices such as actuator, clutch, damper, brake, pump, valve, and robot.

3.1. Actuator

Compared with traditional devices, the MR actuator is a torque transfer device that uses MR materials as the transfer medium and the stepless control of torque transmission can be realized by regulating the magnetic field. Wang et al. [159] designed a novel multi-disc MR actuator driven by the shear stress of the MR fluid, the simulation and experimental results indicated that the multi-disc MR actuator has higher torque output capability and is suitable for high torque applications. Diep et al. [160] proposed a bidirectional MR actuator for tactile systems with an output torque response time of about 55 ms. With the demands for technological development, the miniaturization of the actuator has gradually become the focus of research. According to the recent research from Liu et al. [161], the comprehensive performance of the miniature MR actuator was enhanced by adopting a laminar excitation structure consisting of induction coils and ring-shaped iron. The laminar excitation structure can optimize magnetic field distribution and the speed reduction per unit power can be increased by 500 %. The achievements have great significance for the miniaturization of MR devices and promoting the application of MR actuator.

It should noted that the MR fluid still suffers from shear thinning in practical applications. In order to solve this problem, Zhu et al. [162] developed a planetary MR transmission device with a special structure to keep the MR fluid in the transmission regions with low shear rates and high magnetic fields to avoid shear thinning as much as possible. Further, the excitation coil generates heat during operation and frictional heat is generated during transmission, particularly in high power devices. It is also mentioned in [159] that temperature has a significant influence on the performance of MR materials, the torque transfer capability and the dynamic response speed of the MR actuator will decrease with increasing temperature. Therefore, heat dissipation is also an essential consideration in the design process of high power MR actuator, and introducing heat dissipation structures is a common solution to this problem. Chen et al. [163] proposed a magnetically conductive arc water cooling structure with excellent heat dissipation capacity, which can reduce the temperature of the MR fluid from 70 °C to 28 °C with a reduction of 60%. The combination of heat dissipation structure and magnetic circuit design can provide a reference for high power MR devices.

The MR actuator has received wide attention for its excellent performance, and various methods have been proposed to improve its stability under high power or extreme operating conditions, including structural optimization, magnetic circuit design, and incorporation of heat dissipation structure. With the development of technology, the MR actuator is progressing towards higher torque transmission and lightweight.

3.2. Clutch

The clutch is widely used in mechanical transmission systems to separates or engages the power train. To our knowledge, the vehicle transmission systems separates and engages frequently, meaning that the clutch operates at different slip rates. Moreover, the slip conditions are more prone to heat generation and the temperature has been proven to affect clutch stability. Wang et al. [164] investigated the thermal characteristics of a liquid-cooled MR clutch and they confirmed that high temperature decreases the output torque and the liquid-cooled approach can effectively assist in heat dissipation. Moghani et al. [165] designed a hybrid MR clutch using electromagnetic coils and permanent magnets, as shown in Fig. 9. The MR clutch achieves lightweight goals utilizing hybrid magnetic sources, and the torque density is twice of the clutch without permanent magnets. This kind of lightweight MR clutch offers significant advantages in terms of fast response and precise torque control, especially in the field of robots, where it can provide smooth and safe behavior for articulated robots. There also have some studies combining electromagnetic coils and permanent magnets. Wu et al. [166] placed 12 excitation coils in the MR brake, which were independently powered for more flexible braking torque. In their latest research, they proposed a novel multi-stage MR clutch with 6 permanent magnets and 6 electromagnetic coils to achieve a hybrid magnetization, which also aims to reduce the total mass of MR devices [167].

Fig. 9.

The hybrid MR clutch proposed by Moghani et al. [165].

In order to enhance the feasibility of the MR clutch for practical applications, cylindrical MR clutch [168] and wedge-shaped MR clutch [169] have been proposed to achieve high torque density by expanding the geometric size and changing the shape of the internal boundary. Wang et al. [170] have proposed a novel two-layer multi-plate MR clutch and the transmission form is shown in Fig. 10. The experimental results indicated that the clutch can produce a maximum output torque of 1 545 $\text{N}\cdot \text{m}$ and a high steady slip power of up to 35 kW. As can be seen, this form can produce high output torque and effective dissipation of frictional heat. A hybrid multi-disc MR clutch with both hydrodynamic coupling and mechanical friction form is presented in [171]. Park et al. proposed to use hydraulic coupling mode at the beginning of the clutch operation to reduce the shock when the discs come into contact with each other. In subsequent research, they developed an MR clutch with mechanical friction mode by combining the advantages of both dry and wet clutch, which offers excellent transmission performance and does not exhibit obvious vibration or noise during mode switching [172].

Fig. 10.

Two-layer multiplate transmission form of the MR clutch proposed by Wang et al. [170].

In view of the rotational shear mode of the disc-type clutch, there is frictional wear between the magnetizable medium and the clutch disc, which is similar to the polishing process. In addition, the thinner disc may deform as the temperature rises. The surface morphology of the disc also affects the performance of the clutch, and existing research has confirmed that machining grooves in the disc can effectively reduce the drag torque. Thakur et al. [173] investigated torque transmission characteristics of different grooved discs and found that there is a correlation between the layout and orientation of the grooves in the torque transmission. In the simulation, the radial grooved discs have a positive effect on torque transmission from 250 to 750 rpm and the larger surface area of the grooved discs is conducive to heat dissipation. In the subsequent research, they added graphite flake to the MR fluid, which acted as a lubricant to reduce the wear on the clutch disc surface [174].

It has been confirmed that machining the texture on the disc surface, adjusting the layout of magnetic sources and structural optimization can enhance the performance of the MR clutch. On the other hand, mixing operating modes provide a superior idea for smooth and efficient operation. As mentioned above, proper optimizations and improvements have great significance in improving the torque transmission capacity of the MR clutch.

3.3. Damper

The damper is commonly used in aerospace, vehicle, and architectural applications for vibration and energy dissipation. The MR damper has incomparable advantages of a wide adjustable damping range and rapid response, especially suitable for semi-active control [175]. At present, MR damper is implemented in suspension systems [176,177], landing gear [178,179], and cable-stayed bridges [180]. Despite the many advantages and application prospects of the MR damper, its dynamic model has complex nonlinear hysteresis characteristics and is related to factors such as temperature, magnetic field, fluid motion, and solid mechanics. In view of this, the performance of the MR damper is usually explored by establishing mathematical and simulation models [181,182]. Based on the sigmoid function, Lu et al. [183] proposed a high-precision dynamic model of the MR damper, which can realize the application in real-time control. Bui et al. [184] investigated a novel model based on the magic formula and the Pan’s model to predict the inherent nonlinear hysteresis behavior of the MR damper. The reasonable model is more compatible with different operating conditions and is useful to simulate the behavior of the MR dampers in extreme operating conditions.

Temperature rises are present in all three operating modes of the damper, including flow mode, shear mode, and squeeze mode. The viscosity of MR materials is related to the operating temperature and directly affects the damping force of the MR damper, this is manifested in the damping of the MR damper loss at low temperature and viscous damping reduction at high temperature [185]. Du et al. [186] considered a low thermal conductivity insulating material between the excitation coils and the damper cavity, this part is able to provide thermal insulation and the maximum damping force is higher than that without insulation. Verasci et al. [187] investigated the asymmetric MR damper using the finite element method with different external temperatures. It reveals the areas of MR devices with the most intensive magnetic stress and the temperature analysis contributes to the effect of temperature on the magnetization of MR materials. In [188], Wei et al. employed a response surface method to optimize the damping force and dynamically adjustable coefficient for a typical MR damper, which can effectively improve the damping performance of the MR damper in the design stage. Moreover, the higher damping performance is the goal of the design of the MR damper, the optimized structures include a serpentine channel [189], parallel triple channel [190], annular radial gap [191], and double annular gap [192], all of which are proposed to make the damping performance better than the conventional damper. Xi et al. [193] presented a novel MR damper with an axially variable damping gap, which achieves a constant output of the damping force in the stroke and improves the cushioning efficiency. Lee et al. [194] simplify the complex structure of the MR damper by optimizing the magnetic circuits and evaluated it experimentally. The evaluation results indicated that this MR damper can provide greater damping force. Wang et al. [195] proposed an integral MR damper that significantly reduces the nonlinear dynamic response when used in a flexible rotor system, the schematic views is shown in Fig. 11. Uniformly distributed excitation coils inside the MR damper can independently adjust the oil film pressure in different areas,thus controlling the magnitude and direction of the oil film pressure. Jenis et al. [196] added permanent magnets to the original magnetic circuits to enhance the fault protection capability. They found that the fail-safe damping force is about 1/3 of the maximum value and number of magnetization still influences the accuracy. The combination of excitation coils and permanent magnets promises the design of the MR damper with smaller size, lighter total mass, and higher output damping force, which has been demonstrated in the MR clutch.

Fig. 11.

Schematic views of the integral MR damper developed by Wang et al. [195].

MR damper can simulate the force track of a linear quadratic regulator when applied in vehicle suspension systems [197]. Deng et al. [198] designed an MR damper with variable stiffness and variable damping force for vehicle seats to reduce vibration, the effective stiffness of the seat suspension increased from 1 468.69 to 7 844.47 N⋅s/m with the coil current increased from 0 to 2 A. Moreover, the MR damper can also effectively restrain the lateral vibration of high-speed trains [199]. Yoon et al. [200] developed an MR damper with a fast response, and a robust sliding film controller was used to achieve effective vibration control of the whole system. Random road experiments indicated that this damper could improve the smoothness and handling stability of the vehicle.

In the field of parts machining, the MR damper can improve the damping characteristics, dynamic stiffness, and stability of the machining process. Saleh et al. [201] installed a sponge-type MR damper on the drill pipe of the machining center, which can improve the stability of digging at higher cutting depths. Chen et al. [202] used a semi-active MR damper for milling vibration control of thin-walled parts. As we know, the MR damper has been proven capable of improving the mechanical properties of buildings and has been used in civil engineering for two decades. Compared with the applications in mechanical and other fields, large-scale MR damper is generally applied in buildings and the main benefits include improving the mechanical properties of shear walls without affecting the self-centering capacity [203]. In [204], Abdeddaim et al. placed an MR damper between two structures to improve the performance of foundation isolated buildings. In addition, the cable vibration of the bridge can be effectively suppressed by setting the MR damper near the lower end of the cables [205] and installing an MR damper at the bottom of the bridge is adaptive to improve the bridge load-carrying capacity [206].

3.4. Brake

The brake is a conventional mechanical device used to stop or decelerate the moving part in a mechanical system. The hydraulic brake has inherent problems such as large volume and long response time. The MR brake can overcome these problems and provide higher braking torque, which has attracted the attention of many researchers. The rheological properties of MR materials under the action of a magnetic field can increase the friction between the stator and rotor of the brake, which can provide stable braking torque for most devices and has broad application prospects in vehicle braking technology. Duan et al. [207] proposed a linear control brake system with a stable braking force under long-time braking conditions based on the unique characteristics of MR fluid, whose braking response time is higher than the electro-mechanical brake driven directly by motors. The anti-lock braking system is a mandatory control system to ensure vehicle driving safety and the integration of the MR brake can avoid brake pedal vibration and enhance driving stability [208]. Ma et al. [209] and Wang et al. [210] investigated the control strategies of the brake pedal simulator for vehicle brake-by-wire control that is useful to improve the response characteristics and braking stability of MR brakes. Park et al. [211] designed a sliding mode controller to achieve fast anti-lock braking performance in the MR braking system. Dai et al. [212] proposed an improved gray wolf algorithm with a faster output response for tuning the parameters of a PID controller.

Most research focuses on the structural optimization of MR brakes and various methods have been proposed to improve the output braking torque and stability of MR brakes. Yu et al. [213] proposed an MR brake with multiple magnetic circuits, which has high torque and fast dynamic response to meet the braking requirements of vehicles. The diameter of the brake disc is 130 mm and the prototype can produce a braking torque of 27 $\text{N}\cdot\text{m}$ when the voltage is 12 V. Nguyen et al. [214] equipped multiple excitation coils on both sides of the brake housing, which can simplify the structure and reduce the total mass. Subsequently, an MR brake was designed for easier disassembly, where the excitation coils do not come into direct contact with the MR fluid [215]. Song et al. [216] proposed an MR brake with an adjustable gap (0–2 mm), and the configuration of the prototype is illustrated in Fig. 12. This work indicated that the gap size does not have a significant effect on the braking performance within the range from 0.25 to 1 mm. Chen et al. [217] also developed a gap-adjustable MR brake with a maximum braking torque of 125.08 Nm that is achieved by an electrothermal shape memory alloy spring. The gap-adjustable MR brake is more flexible and has larger controllable torque ratios. In addition, the zigzag magnetic circuit [218] and the replacement of the cylindrical rotor with a toothed rotor [219] were able to reduce the total mass of MR devices and power consumption without reducing the braking torque.

Fig. 12.

Configuration of the prototype of the MR brake developed by Song et al. [216].

Multi-disc and multi-pole MR brakes with better braking performance have come into the view of researchers [220]. Sarkar et al. [221] put forward a slotted disc MR brake with better braking performance. Hu et al. [222] developed an MR brake with double brake disc, and the braking experimental results indicated that the braking performance was stable. Shiao et al. [223] combined the double brake disc with uniformly distributed excitation coils, and the evaluation results presented that the multi-disc and multi-stage MR brakes have higher torque-power ratio. In some special applications, the brake must fulfill the requirements of small size, lightweight, high braking torque, and fast response. The definition of a small MR brake is less than 40 mm in diameter and less than 30 mm in thickness as given by Wellborn et al. [224]. In their recent work, an MR brake with a maximum diameter of about 38 mm was designed by combining disc and drum, and the MR brake introduces the excitation coil inside the rotor. The serpentine flux path allows for high braking torque and fast response while reducing structural size. Lutanto et al. [225] raised the implementation of the mass function expansion method that can be applied in the design process of MR brake and developed a miniature bladed MR brake with a diameter of 48 mm and a thickness of 28 mm. A small-size MR brake with high braking performance is suitable for small tactile or robotic devices, and provide a superior option for further development. Song et al. [226] developed a small-sized MR brake (the diameter and height are 37 mm and 33 mm) with both flow and shear modes of operation. Comparative results confirmed that the hybrid MR brake can increase the controllable torque range by 325 %. On the other hand, the hybrid magnetization of MR brake is valuable research work. Szelag et al. [227] proposed a hybrid excited MR brake with excellent reliability, in which the permanent magnets and the excitation coils were used together as the magnetic sources, the axisymmetric structures is given in Fig. 13.

Fig. 13.

Structure of the hybrid excited MR brake developed by Szelag et al. [227].

3.5. Pump

MR materials are suitable for transmission and it has been advanced to confirm that the drive of the pump is relying on the MR effect. In 2018, Stork et al. [228] explored the effect of magnetic field on fluid transport and proposed an MR peristaltic pump that can transport fluids. In fact, the magnetic field changes the physical properties of MR materials, which is an ideal method to control the flow rate [229]. Hassan et al. [230] presented an MR duckbill valve micropump, and the top wall of the pumping chamber is made of MR elastomer, which contracts downward with the application of a magnetic field, as shown in Fig. 14. The magnetic field can increase the pressure on the two duckbill valves and open the front duckbill valve. In contrast, the back pressure closes the rear duckbill valve to forming an effective one-way pumping circuit. Based on this mechanism, Cesmeci et al. [231] proposed an MR flap valve micropump whose transport capacity is almost twice that of others. According to the magnetic field driving mechanism, small-sized MR devices such as MR micropump can meet the specific requirements of practical applications.

Fig. 14.

The micropump proposed by Hassan et al. [230].

3.6. Valve

The development of MR materials provides useful references for innovating novel valves. High performance valve is essential for efficient hydraulic systems [232]. Salloom et al. [233] proposed an MR directional control valve that can achieve a direct interface between the magnetic field and the hydrodynamic force by using the characteristics of the MR fluid. Hu et al. [234] developed an MR valve with mosquito-disk type flow channel as a control unit for the valved cylinder systems that enables the valve-controlled cylinder system to output a reliable damping force. Response time is another goal of high performance valves, Wang et al. [235] proposed an MR proportional valve based on MR elastomer and the unique design ensures fast response time.

To the best of our knowledge, MR valve is used in low-flow systems with limited performance enhancement. The research has shown that multi-disc configurations can provide a high torque density in the brakes mentioned earlier. This approach also has some applications in the field of the MR valve. Yang et al. [236] designed a novel multi-ring and multi-disc MR valve based on the MR fluid with a saturation pressure drop greater than 7 MPa.

3.7. Robot

Soft robot differ from rigid robot in that it can be used in limited spaces with high curvature [237,238]. The rigid robot is widely used but is not well adapted to complex operating conditions due to its structural characteristics. Especially in the clamping work, the rigid robot is prone to cause damage to the surface of the clamped object. MR materials are promising for applications in the soft robot, as their variable stiffness properties can extend the variable stiffness range of soft robot [239]. Huang et al. [240] chose MR grease as the working medium and added an MR dielectric layer on the surface of the manipulator to control the stiffness of the gripping surface. Then, they conducted experiments to prove that the variable and high adaptability of the surface stiffness of the gripping surface can reduce the damage to the gripped objects. A flexible manipulator with MR grease based on the MR effect was developed by Ye et al. [241] and the local cross section of the gripper prototype is shown in Fig. 15. The grabbing experiments confirmed that the flexible manipulator has great adaptability to objects of different shapes and can grasp objects without damage.

Fig. 15.

Local cross section of the gripper prototype proposed by Ye et al. [241].

Figure 16 presents a cylindrical radial bellow gripper filled with the MR fluid [242]. The MR fluid is a promising force-transferring medium due to its controllable stiffness under an external magnetic field. More specifically, the MR elastomer can be directly incorporated into the soft gripper [243]. The soft grippers with high adaptability are valuable for ensuring high efficiency and quality of operations, such as picking agricultural products and gripping flexible workpieces [244]. In addition, in terms of the flexibility and adaptability of soft robot, the introduction of the MR material can simplify fluid control and improve the autonomy of soft robot [245]. McDonald et al. [246] applied the MR fluid to a multi-degree-of-freedom soft robot, demonstrating that fluid control can be achieved through the MR effect.

Fig. 16.

The gripper prototype developed by Cramer et al. [242].

Furthermore, the MR effect and MR materials also play an important role in emerging robots and prosthetic joints. Yin et al. [247] developed a novel MR bending actuator and fabricated a fish-shaped robot whose tail is composed of the actuator. They demonstrated a magnetic actuation capability of the MR bending actuator and the robot could achieve free swimming and steering. Chen et al. [248] proposed a magnetically driven MR millirobot with a large deformability, the grasping process is shown in Fig. 17. The robot can perform different behaviors in different magnetic strengths and is capable of performing tasks in complex situations such as drug delivery and thrombus removal in blood vessels. The regular-sized robot can also be used in the medical field, where robot involved in surgery need to achieve precise control and real-time feedback. The MR damper-based force feedback robot [249] and the MR fluid-based robotic haptics devices [250] have high torque and fast response. Yun et al. [251] developed a novel MR damper prototype for the manipulator of a mobile rescue robot that is used to counteract the vibration caused by the road during movement. Wang et al. [252] reported a finger rehabilitation robot with a combination of active and passive training, using the MR damper to provide the damping force during the training process. The MR damper provides resistance for the robot, power for supporting the exoskeleton system, and assists humans with lifting tasks [253]. In the field of prosthetic joints, Negi et al. [254] designed a prototype ankle-foot prosthesis based on the MR damper whose dynamic range is better than conventional prosthesis. The MR knee prosthesis is a semi-active device based on the MR damper [255–257]. MR elastomer has the potential to be an alternative material to knee menisci. Liu et al. [258] fabricated MR elastomer menisci with a superior ability to resist damage and destruction compared to conventional menisci. The combination of MR devices and prosthesis offers a semi-active control that can provide a superior experience and reduce injuries during wear.

Fig. 17.

Liquid MRF-Robot reaches out arms and grabs the plastic block under magnetic field, as tested by Chen et al. [248].

3.8. Summary

In the past few decades, research development on MR devices has been blooming. The main applications of MR materials in this review are summarized in Table 2. Numerous research works on MR devices are ongoing and scholars are still trying to improve the performance of MR devices or develop novel MR devices by using different methods. More specifically, there are various ways to achieve this goal such as multi-disc, multi-pole, magnetic circuit design, hybrid magnetization, and flow channel optimization.

It can be seen from Table 2 that hybrid magnetization combines permanent magnets and excitation coils to provide a sufficient magnetic field and reduce the total mass of MR devices. The permanent magnets provide the initial magnetic field for operation and electromagnetic coils can adjust the magnetic field strength within the working gap. Benefiting from the constant magnetic field of permanent magnets, the hybrid magnetization can reduce the vibration and shock when starting and stopping the device or switching operating modes. In addition, the magnetic field generated by the electromagnetic coils can be subtracted from that of the permanent magnets by changing the direction of the coil current. The multi-disc and multi-pole structures enable MR devices to have higher output capacity and larger output range. The introduction of liquid-cooling system or the addition of thermal insulation structures can effectively assist in heat dissipation and reduce the negative effects of high temperatures.

In the field of soft robot, the development of MR materials gained the focus of researchers and applied to construct various devices to cooperate with humans. MR materials have interesting features like softness leads to the possibilities to compensate for the lack of traditional materials and promote the huge development of flexible manipulators. In summary, the appropriate method can make MR devices operate smoothly and accelerate the practical application.

Table 2
Summary of MR devices from different researches

MR devices Characteristics Advantages References

Multi-disc MR actuator Multi-disc transmission form High torque transmission ability Wang et al. [159]

Bidirectional MR actuator Coils placed on each side of the housing Provide a desired output force in both directions Diep et al. [160]

Miniature MR actuator Lamellar excitation structure Miniaturization Liu et al. [161]

MR transmission device 2-DoF planetary mechanism Avoid shear thinning of MR fluid Zhu et al. [162]

Liquid-cooled MR clutch Two-way liquid cooling method Effectively assist in heat dissipation Wang et al. [164]

Hybrid MR clutch Hybrid magnetization Lightweight Moghani et al. [165]

Multi-plate MR clutch Two-layer multiplate transmission form Applied for high-power applications Wang et al. [170]

Hybrid multi-disc MR clutch Fluid coupling and mechanical friction Reduce the shock Park et al. [171]

Multi-plate MR clutch Friction and magnetic field control Smooth shifting operations without torque ripples Park et al. [172]

Shear mode MR clutch Different slotted clutch discs Radial groove disc facilitates torque transmission Thakur et al. [173]

MR fluid damper Thermal insulation Avoid rapid temperature rise of MR fluid Du et al. [186]

Rotary MR damper Serpentine channel More compact structure/higher damping torque Satia et al. [189]

Three-channel MR damper Parallel three annular channel A large adjustable coefficient of damping force Zhu et al. [190]

Large-capacity MR damper Annular-radial gap Higher controllable dynamic force range Abdalaziz et al. [191]

Novel MR damper Double annular damping gap Larger damping force range and dynamic range Yan et al. [192]

Axial variable damping gap Damping force can be maintained at a constant Xi et al. [193]

Novel magnetic circuit Higher control range of the damping force Lee et al. [194]

Hybrid MR damper A permanent magnet in the magnetic circuit Short response time Jenis et al. [196]

Damping rotary MR damper Variable stiffness and variable damping Reduce the vibration and improve ride comfort Deng et al. [198]

Sponge-type MR damper Utilizes a minimal amount of MR fluid Increase the stability of the boring process Saleh et al. [201]

Single-disc MR brake Multiple magnetic circuits High torque capability and fast dynamic response Yu et al. [213]

Multiple side-coil MR brake Multiple excitation coils Higher braking torque and more compact size Nguyen et al. [214]

Adjustable gap MR brake Electrothermal shape memory alloy spring Large torque adjustment range Chen et al. [217]

Disc-type MR brake Zigzag magnetic flux path Better braking performance Nguyen et al. [218]

Disc-type MR brake Slotted disc Provides a better braking performance Sarkar et al. [220]

MR devices	Characteristics	Advantages	References
Multi-disc MR actuator	Multi-disc transmission form	High torque transmission ability	Wang et al. [159]
Bidirectional MR actuator	Coils placed on each side of the housing	Provide a desired output force in both directions	Diep et al. [160]
Miniature MR actuator	Lamellar excitation structure	Miniaturization	Liu et al. [161]
MR transmission device	2-DoF planetary mechanism	Avoid shear thinning of MR fluid	Zhu et al. [162]
Liquid-cooled MR clutch	Two-way liquid cooling method	Effectively assist in heat dissipation	Wang et al. [164]
Hybrid MR clutch	Hybrid magnetization	Lightweight	Moghani et al. [165]
Multi-plate MR clutch	Two-layer multiplate transmission form	Applied for high-power applications	Wang et al. [170]
Hybrid multi-disc MR clutch	Fluid coupling and mechanical friction	Reduce the shock	Park et al. [171]
Multi-plate MR clutch	Friction and magnetic field control	Smooth shifting operations without torque ripples	Park et al. [172]
Shear mode MR clutch	Different slotted clutch discs	Radial groove disc facilitates torque transmission	Thakur et al. [173]
MR fluid damper	Thermal insulation	Avoid rapid temperature rise of MR fluid	Du et al. [186]
Rotary MR damper	Serpentine channel	More compact structure/higher damping torque	Satia et al. [189]
Three-channel MR damper	Parallel three annular channel	A large adjustable coefficient of damping force	Zhu et al. [190]
Large-capacity MR damper	Annular-radial gap	Higher controllable dynamic force range	Abdalaziz et al. [191]
Novel MR damper	Double annular damping gap	Larger damping force range and dynamic range	Yan et al. [192]
Axial variable damping gap	Damping force can be maintained at a constant	Xi et al. [193]
Novel magnetic circuit	Higher control range of the damping force	Lee et al. [194]
Hybrid MR damper	A permanent magnet in the magnetic circuit	Short response time	Jenis et al. [196]
Damping rotary MR damper	Variable stiffness and variable damping	Reduce the vibration and improve ride comfort	Deng et al. [198]
Sponge-type MR damper	Utilizes a minimal amount of MR fluid	Increase the stability of the boring process	Saleh et al. [201]
Single-disc MR brake	Multiple magnetic circuits	High torque capability and fast dynamic response	Yu et al. [213]
Multiple side-coil MR brake	Multiple excitation coils	Higher braking torque and more compact size	Nguyen et al. [214]
Adjustable gap MR brake	Electrothermal shape memory alloy spring	Large torque adjustment range	Chen et al. [217]
Disc-type MR brake	Zigzag magnetic flux path	Better braking performance	Nguyen et al. [218]
Disc-type MR brake	Slotted disc	Provides a better braking performance	Sarkar et al. [220]

Table 2 (Continued)

MR devices	Characteristics	Advantages	References
Disc-type MR brake	Double brake disc	Increase the utilization rate of the magnetic field	Hu et al. [221]
Small-scale MR brake	Combining disk- and drum-type designs	Fastest time constant and highest torque/mass ratio	Wellborn et al. [224]
Tiny vane-type MR brake	Miniature bladed	Compact size and lightweight	Lutanto et al. [225]
Hybrid excited MR brake	Permanent magnet and excitation coil	High torque capability and fast dynamic response	Szelag et al. [227]
MR duckbill valve micropump	Top wall is made from MR elastomer	Increases the pressure on the two duckbill valves	Hassan et al. [230]
MR flap valve micropump	Electromagnetic actuation mechanism	Higher transport capacity	Cesmeci et al. [231]
Radial MR valve	Mosquito-disk type flow channels	Larger pressure drop and rapid response time	Hu et al. [234]
Proportional valve	Ecoflex-based MR elastomer	Reduced weight and rigidity	Wang et al. [235]
Variable stiffness soft robot	MR grease layer	Variable stiffness of the gripping surface	Huang et al. [240]
Flexible manipulator gripper	MR grease capsule	Variable stiffness of the gripping surface	Ye et al. [241]
MR soft gripping	Soft gripper assembled with MR elastomer	High adaptability	Bernat et al. [243]
Fish-shaped robot	The tail is composed of the actuator	Free swimming and steering	Yin et al. [247]
MR millirobot	Magneto-active solid-liquid state transform	Large deformation, smooth navigation	Chen et al. [248]

4. Conclusion and outlook

MR materials have been exploited with wide applications in industrial fields due to their unique rheological characteristics. As reviewed in this paper, MR materials have the potential to replace conventional materials for efficient and high-quality applications, such as actuator, clutch, damper, brake, pump, valve, and robot, with broad prospects in the future. MR devices have been practically realized as a commercial product, while some MR devices presented in this paper only demonstrate innovative design concepts and have not been promoted. In this review, we have systematically summarized the research on MR materials and devices in recent years. From the basic composition of MR materials to the performance influencing factors and applications. With the development of experimental technologies and theoretical innovations, these research works provide a deeper understanding of the MR effect.

Applications of MR materials with adequate controllability are of great importance in many industries. The performance of MR materials mainly depends on magnetizable medium (type, shape, size, and volume fraction), base carrier liquid (type, viscosity, and density), and additives. As mentioned in Section 2, the magnetic field is the primary means of regulation and the temperature is an important influence factor that should be considered in the design process of MR devices. Increasing the number of working gaps (multi-disc) or strengthening the magnetic field (multi-pole) are efficient approaches to enhance the performance of MR devices. Nowadays applications of high-power MR devices are limited by the lack of development of cooling systems. For this reason, proper cooling methods should be proposed to assist in heat dissipation. This review presents MR devices including actuator, clutch, damper, brake, pump, valve, and robot. In addition, the adsorption and positioning of MR materials can achieve directional lubrication to avoid lubrication failure due to extreme conditions such as high temperature and rotational speed.

Based on the rheological properties of MR materials, the development of MR devices is progressing rapidly and these materials can be integrated for more emerging applications. The structural optimization of MR devices is always aimed at performance enhancement, but the sedimentation problem of the magnetizable medium must be solved to guarantee the stability of MR materials. During the research and analysis of MR materials, it can be found that these materials are the coupling of the fields of magnetism, dynamics, fluid dynamics and engineering. Understanding the evolution mechanism of MR materials is critical to their future development and applications, more practical factors are considered such as introduce the high precision control strategies of the magnetic field to construct a complex electro-mechanical system and the optimization design of MR devices could be further explored with consideration of decoupling the traditional structures. In combination with the fast response to flow state changes, this progress is also expected in anti-collision damping. In future applications, the stability of high performance MR devices will be the focus of research attention. We hope this review can provide useful information for the research and applications of MR materials in various engineering fields.

Footnotes

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 52175047) and the Anhui Provincial Natural Science Foundation (Grant No. 2008085ME140).

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