The Advancing Understanding of Magnetorheological Fluids and Elastomers: A Comparative Review Analyzing Mechanical and Viscoelastic Properties

Abstract

Magnetorheological fluids (MRFs) and elastomers (MREs) are two types of smart materials that exhibit modifiable rheological properties in response to an applied magnetic field. Although they share a similarity in their magnetorheological response, these two materials differ in their nature, structure, and mechanical behavior when exposed to a magnetic field. They also have distinct application differences due to their specific rheological properties. These fundamental differences therefore influence their properties and applications in various industrial fields. This review provides a synthesis of the distinct characteristics of MRFs and MREs. The differences in their composition, rheological behavior, mechanical properties, and respective applications are summarized and highlighted. This analysis will enable a comprehensive understanding of these differences, thereby allowing for the appropriate selection of the material based on the specific requirements of a given application and fostering the development of new applications utilizing these MR materials.

1. Introduction

Magnetorheological (MR) materials, also known as magnetoactive or magneto-sensitive materials, belong to the category of smart materials. Their rheological viscoelastic, and mechanical properties can be rapidly and reversibly modified in response to an externally applied magnetic field. An MR material can exist in the form of a fluid, gel, or solid elastomer. Depending on the type of matrix and the size of the magnetic particles, several magnetoactive materials can be identified, including MR fluids, MR foams, and MR elastomers, which exhibit a typical MR effect. Although other systems, such as ferrofluids and ferrogels, also respond to a magnetic field, they differ in their mechanisms of action and are not strictly considered magnetorheological [1,2,3,4]. In this work, only MRFs and MREs are the subject of in-depth analysis.

Discovered in 1948 by J. Rabinow, MR fluids were first used in a magnetorheological clutch at the National Bureau of Standards in the United States [1]. MR fluids are traditionally biphasic composites consisting of magnetically polarizable solid microparticles suspended in a non-magnetic carrier medium, as illustrated in Figure 1. In the absence of a magnetic field, they behave like a low-viscosity Newtonian fluid. Once a magnetic field is applied, a rapid and reversible transition from a fluid state to a solid state can be observed, which can be described as a change in apparent viscosity, resulting in a yield stress [5,6,7,8,9,10].

Moreover, introductory research on MR elastomers was conducted a few decades later, in 1983, by [11], in which they presented pioneering work on a magnetically sensitive elastomer aimed at exploiting the MR effect. However, there seems to be a consensus that the first thorough investigation of MREs was conducted by Carlson and his colleagues in the 1990s [2,4,12,13]. Structurally, MREs can be considered solid analogs of MRFs [3]. As shown in Figure 2, they are composed of polarizable magnetic particles embedded in a non-magnetic matrix. Various additives such as plasticizers, stabilizers, or crosslinking agents may also be incorporated. The magnetic particles typically remain fixed in position after the curing process, although in very soft matrices, a significant particle displacement may still occur under an applied magnetic field [12,14,15].

To meet the growing demand in the field of these two smart materials, this review will focus on a comprehensive comparative analysis. It will examine the rheological, viscoelastic, and mechanical properties, as well as the dynamic performance of MR fluids and MR elastomers. Beyond merely listing the intrinsic characteristics of these materials, our aim is to provide clear insights into how these properties affect their mechanical behavior in specific contexts.

The fundamental factors influencing the rheological and mechanical properties of MREs and MRFs have been identified and carefully explored. Various elements, such as the state of magnetic particles, the condition of the elastomer matrix, the role of additives, stresses such as strain amplitude, excitation frequency, temperature, magnetic field intensity, and many others, have been examined and presented with supporting figures, highlighting recent discoveries in this field. A collection of experimental research has been reviewed, accompanied by an analysis of the results obtained by the authors of these studies.

In the following tables, various types of materials used to prepare MREs and MRFs are listed and summarized (

Table 1. Main MREs matrix materials, magnetic particles (nature, size, and concentration).

Matrix	Magnetic Particle			MRE Type	Ref.
	Type	Size	Content
Silicone rubber	CIP	3–4 μm	10–20–30 vol%	Anisotrop.	[2]
		2 μm	15 vol%	Isotrop./Anisotrop.	[16]
		3 μm	27 vol%	Anisotrop.	[17]
		3 μm	20 to 70 wt%	–	[18]
		2–10 μm	10 vol%	Anisotrop.	[19]
		3–5 μm	80 wt%	Isotrop.	[20]
		1.25 ± 0.55 μm	0–10–30 vol%	Isotrop./Anisotrop.	[21]
		3–5 μm	23 vol%	Anisotropic	[22]
		4–6 to 220 μm	30 vol%	Isotrop./Anisotrop.	[23]
		5–9 μm	10–40 wt%	Isotrop.	[24]
		3.9–5 μm	5 to 40 vol%	Isotrop./Anisotrop.	[25]
		3–5 μm	70 wt%	Isotrop.	[26]
		45 µm	0:20:80	Isotrop./Anisotrop.	[27]
		3.9–5 μm	12.5 to 40 vol%	Isotrop.	[28]
		2–5 μm	27 vol%	Isotrop./Anisotrop.	[29]
		3.9 to 5 μm	30 vol%	Isotrop./Anisotrop.	[30]
		5 μm	40–50–60 wt%	Isotropic	[31]
	IP	10 μm	30 vol%	Isotropic	[32]
		50–150 μm	30 vol%	Isotrop./Anisotrop.	[33]
		0 to 250 µm	30–60 wt%	Anisotrop.	[34]
		5–9 μm	30 vol%	Anisotrop.	[35]
		2.5 μm	30 vol%	Anisotropic	[36]
	IP/Fe3O4	2–3/0.2–0.3 µm	7–14/22–24–27 vol %	Isotrop.	[37]
	CIP/FeNdB	3/1–100 μm	35/68 wt%	Anisotrop.	[38]
	Bi-disp.: CIP (plates/spherical)	1–10 μm	70 wt%	Isotrop./Anisotrop.	[39]
Natural rubber	CIP	0.5–5 μm	27 vol%	Isotrop./Anisotrop.	[40]
		3–5 μm	55 wt%	Isotrop./Anisotrop.	[41]
	IP	<60 μm	0 to 37 vol%	Isotrop.	[42]
	Carbon Black	100 μm	5.6 to 38.4 vol%	Isotrop.	[43]
	Iron Sand	45–56 μm	70 wt%	Isotrop./Anisotrop.	[44]
	Bi-disp.: CIP/γ-Fe2O3	3–5 μm/leng. = 500 nm	62 wt%	Anisot.	[45]
	Ni-Zn Ferrite	6 μm	0 to 45 wt%	Isotrop./Anisotrop.	[46]
Polyurethane	CIP	6–9 μm	1.5–33 vol%	Anisotrop.	[47]
		5–8 μm	50–60–70 wt%	Isotrop./Anisotrop.	[48]
		6–9 μm	33 vol%	Anisotrop.	[49]
		5 μm	10–20–40 vol%	Isotrop.	[50]
EPDM	Core-shell: CIP/CB	3.8–5.3 μm	0:10 to 40 vol%	Anisotrop.	[51]
	Bi-disp.: CIP/BIP	3.8–5.4/5–16 μm	1.7 to 14.65 wt%	Isotrop.	[52]
	Bi-disp.: BT/Graphite BT/Mica	mica 10 μm/graphite 10 μm	30/5 to 20 vol% (both)	Isotrop.	[53]
Thermoplastic rubber	IP	60 µm	35 vol%	Isotrop./Anisotrop.	[54]
	Aramid Short Fibers	leng./diam (3 mm/10–12 μm)	1 to 10 wt%	Isotrop.	[55]
	CIP	3–5 μm	67 wt%	Isotrop.	[56]
SBR	Silica	20–50 nm	30 to 90 wt%	Isotrop.	[57]
SBR/BR	Silica	–	31 vol%	Isotrop.	[58]
PU/SR	CIP	3–5 μm	63–73 wt%	Isotrop.	[59]
Polyethylene	Bi-disp.: Mica/ Wollastonite	17 μm/49 μm	5–10–15 wt%	Isotrop.	[60]
NR/SR/Chloroprene	IP	2.5 μm	60 wt%	Anisotrop.	[61]
Epoxy resin	CIP	4.5 µm	33.3 to 71.4 wt%	Isotrop.	[62]
Acrylonitrile butadiene rubber	Fe12O19Sr	–	66.7–75–80 wt%	Isotrop.	[63]
Bromobutyl rubber	CIP	–	60–70 wt%	Isotrop./Anisotrop.	[64]
Ethylene/Acrylic	CIP	3–5 μm	0–10–40 vol%	Isotrop./Anisotrop.	[65]

and

Table 2. Main carrier fluids and dispersed phase characteristics (nature, size, and concentration) of MRFs.

Carrier Fluid	Magnetic Particle			Ref.
	Type	Size	Content
Silicone oil	MWCNT/CoFe2O4 (Nanocomposite)	fibrous shape	25 wt%	[66]
	MnFe2O4	20–30 nm	10 vol%	[67]
	Bi-dispersion: CIP(µ)/CIP(Nano)	2–4 μm/30–50 nm	40 vol%	[68]
	Cobalt(µ)-flower-like Shape	2–5 μm	12 vol%	[69]
	Fe3O4-triangular-shaped	~100 nm	7 vol%	[70]
	Bi-dispersion: IP(µ) Flake/Fe3O4 (Nano)	15–30 μm/~	5–10–20 vol%	[71]
	Core–shell: Fe3O4POA (Poly O-anisidine)	~445 nm	10 vol%	[72]
	Core–shell: Fe3O4/(PS) Polystyrene	~127 nm	10 vol%	[73]
	Core–shell: Fe3O4/(PS) Polystyrene	50–100 nm	10 vol%	[74]
	Fe-Ga Alloy: Flake Shaped Galfenol	irregular shape	30 wt%	[75]
	Core–shell: CIP/PANI (Polyaniline)	~3.66 μm	10 vol%	[76]
	Fe3O4/PANI (Polyaniline)	fibrous shape	2 vol%	[77]
	Cobalt Nanofibers	fibrous shape	12 vol%	[78]
	Bi-dispersion: CIP(µ)/Halloysite	4.25 μm/625 nm	70/1 wt%	[79]
	Bi-dispersion: CIP(µ)/γ-Fe2-O3	7 μm	70/0.5 to 2 wt%	[80]
	CIP (µ)	6 μm	20–30 vol%	[81]
Castor oil	CoNi-Nanoclusters	450 nm	15–20 vol%	[82]
	Bi-dispersion: CIP(µ)/Silica (Nano)	10–12 μm/20 nm	4.5 vol%	[83]
Paraffin oil	IP(µ)—Flake Shaped	7–8 µm	10–20 vol%	[84]
	Bi-dispersion: IP(µ)/IP(µ)—Plate Like	2 (small)/19 (large) μm	16 vol%	[85]
Ethylene glycol	Fumed Silica Hydrophile	14 nm	25 wt%	[86]
Poly(ethylene oxide)	CIP(µ)	2 μm	20 vol%	[87]
Lubricating oil (Yubase 8)	CIP(µ)/Organic Clay	4.25 μm	25/0.5 to 3 wt%	[88]
kerosene	Fe3O4	13 nm	11–17 vol%	[89]
viscoelastic matrix: Polyisobutylene (PIB)/polybutene (PB)	CIP (µ)	7 μm	25 vol%	[90]
Blend: silicone oil/Honey/organic oil	IP (µ)	1–10 µm	30 wt%	[91]
Blend: low + high viscous mineral oils	CIP (µ)	4–6 µm	80 wt%	[92]
Hong oil	CIP (µ)	6–9 µm	20–30–40 vol%	[93]
Ionic liquid	Core–shell: Silica/CIP	0.5–2.5 µm	10 vol%	[94]
Mineral oil	CIP (µ)	900 ± 300 nm	1.55 to 7.9 vol%	[95]
Grease medium	CIP (µ)	7 μm	50 wt%	[96]

). These materials include silicone oils for the base fluid of MRFs and silicone rubber for the matrix of MREs. Among a variety of magnetic particle materials, it is clearly observed that carbonyl iron powder (CIP), in micrometer size, is currently the most widely used for preparing MR materials.

2. Magneto-Mechanical Characterizations

Various parameters such as strain amplitude, oscillation frequency, magnetic field intensity, and test temperature have been studied to assess their impact on the dynamic viscoelastic behavior of MREs and MRFs. In the following section, we will examine their responses based on these parameters. The various interpretations and comments on the results were supported by examples of curves and citations from numerous and diverse references. It should be noted that the unreferenced curves of RTV 141 silicone rubber-based elastomers were obtained as part of our doctoral thesis research.

2.1. Strain Amplitude Effect

In MREs, increasing the strain amplitude leads to significantly lower storage (G′) and loss (G″) moduli, especially in the presence of a magnetic field, regardless of the excitation frequency and flux density [28,36,52,97,98,99]. For an illustrative example, Figure 3 shows a shear strain amplitude sweep test [99]. An isotropic elastomer, based on RTV 141 silicone rubber and micrometric-sized iron powder IP with an average diameter ranging from 1.8 to 2.3 μm, loaded at 40% by volume, was prepared. The tests were carried out at a fixed excitation frequency of 50 Hz under magnetic flux intensities ranging from 0 to 300 mT.

It was observed that, at a zero magnetic field, the MRE exhibits lower storage and loss moduli. The values of these two moduli increase as the magnetic field increases, a phenomenon induced by the MR effect. Furthermore, it was clearly shown that both moduli, (G′) and (G″), decrease with increasing strain amplitude. The rate of decrease of these two moduli is noticeably rapid for small amplitudes, up to 3%, whereas this decrease becomes slower for higher amplitudes. The considerable variation in dynamic moduli with increasing strain amplitude reflects the non-linearity induced by deformation.

To clearly illustrate the presence of the linear viscoelastic (LVE) region, strain sweep tests from a study conducted by [100] were presented. These curves, plotted on a logarithmic scale, show the storage modulus (G′) as a function of strain amplitude (Figure 4). The tests were performed over a strain range of 0.001% to 100%, at a constant frequency of 1 Hz and under magnetic fields of 0 and 0.8 T. Isotropic and anisotropic MRE samples containing silicone rubber and cobalt particles were fabricated.

An overall trend of decreasing storage modulus (G′) with increasing strain amplitude (γ) was observed. For both MRE elastomer samples, the (G′) modulus maintains constant plateau values in the low-strain region. In this linear viscoelastic (LVE) region, the storage modulus (G′) is independent of the applied strain, and the structures affected by magnetic fields remain unchanged [100,101].

Furthermore, the storage modulus (G′) increases with increasing magnetic field intensity. Anisotropic samples display higher storage modulus (G′) values in both the absence and presence of applied magnetic fields [29,39]. In contrast, the isotropic sample shows a wider stiffness range, with a strain threshold of about 5% compared to a threshold of 0.1% for the anisotropic sample [100]. Beyond these thresholds, a gradual decrease is first observed, followed by a more pronounced reduction. This behavior reflects the transition to a nonlinear response of the elastomer, characteristic of the nonlinear viscoelastic (NLVE) region [41,100,102,103,104].

Physically, increasing the strain amplitude induces changes in the microstructure of the MRE. The deformation of the polymer matrix increases, as does the slip between the magnetic particles and the matrix. Starting from a very low strain amplitude, the rupture and reformation of the magnetic particle networks during cyclic deformation are gradually initiated. However, as the dynamic deformation continues to increase, an increasing portion of the particle networks are destroyed and lose their ability to reform, leading to a decrease in the dynamic properties [105]. This strain dependence of the MRE elastomer is known as the Payne effect [29,103,106].

The Payne effect has been reported to be more pronounced in anisotropic elastomers, as well as when a magnetic field is activated. This phenomenon is mainly related to the destruction and reformation of a network of magnetic particles in the presence of an applied magnetic field. The bonds between fillers and the elastomer lead to an increase in the magnetic network density of MREs. This network density is expected to vary depending on the deformation of the MREs, where unstable bonds will most likely be broken under the applied mechanical stresses. This process leads to the rupture of the filler networks, thereby releasing the trapped elastomer, which results in a decrease in the shear storage modulus. Therefore, it can be concluded that in isotropic elastomers and in the absence of an applied magnetic field, the LVE region is more pronounced [39,102,103,107].

For MRE elastomers, under the application of a magnetic field, the transition from linear to nonlinear behavior typically occurs at a strain amplitude between 0.1% and 5%, depending on the intensity of the applied magnetic field, as well as the matrix conditions and the distribution of filler particles [36,97].

A similar behavior was observed for MRFs, as shown in Figure 5 [68]. In the absence of a magnetic field, the values of (G″) were reported to be larger than those of (G′) in a wide range of strain amplitude, indicating a liquid-like behavior for both MRFs.

At the applied magnetic fields, and as the field intensity increased, both (G′) and (G″) increased, but (G′) reached a value significantly higher than that of (G″). This indicates that MRFs adopt solid-like behavior due to the formation of chain-like structures.

In addition, it has been reported that both moduli (G′) and (G″) remain independent of the strain amplitude in a specific range, known as the linear viscoelastic (LVE) region. In this region, the magnetic-field-induced structures remain intact. However, beyond this LVE region, when the strain amplitude exceeds this range, the chain structure starts to break down. The storage modulus (G′) tends to decrease, while the viscous modulus (G″) shows a slight increase before decreasing. The moduli (G′) and (G″) intersect at (G′ = G″), and beyond this point, the system begins to flow [44,67,68,69,76,78,84].

The linear viscoelastic (LVE) range of MRFs is generally located between strain amplitude values ranging from 0.001 to 0.1. The stress at which this condition is satisfied also increases with increasing magnetic field [66,108,109].

2.2. Frequency Effect

The frequency dependence of the dynamic characteristics of the MRE elastomer, represented by the storage moduli (G′) and loss moduli (G″), was examined. These two moduli increase continuously with the excitation frequency independently of the strain amplitude. In the absence of a magnetic field, the progression of both moduli is slower. This phenomenon can be explained by a quasi-viscoelastic behavior similar to that of a loaded rubber [28]. However, the application of an external magnetic field reinforces the growth trend of the dynamic moduli with frequency.

In their study, ref. [28] presented an example of a frequency sweep, illustrated in Figure 6. This sweep was conducted over a range from 0.1 to 50 Hz, with different shear strain amplitudes varying from 2.5% to 20%, at room temperature and under a magnetic flux density of 150 mT. The MREs were composed of spherical carbonyl iron particles, with diameters ranging from 3.9 to 5 µm, dispersed in a silicone rubber matrix.

The increase in the storage modulus (G′) with frequency is attributed to an enhanced entanglement of the elastomer matrix. This behavior is explained by the phase lag between the deformation of the MRE molecular chains and the applied shear force due to a shorter response time as the frequency increases. This phenomenon leads to a gradual stiffening of the material at higher frequencies [28,31,39,110].

On the other hand, the increase in the loss modulus (G″) is attributed to elevated interfacial friction between the magnetic particles and the matrix, as well as internal friction between polymer chains, which promotes energy dissipation within the material [31,39,46,111].

Furthermore, a higher strain amplitude applied to the MRE results in a decrease in both dynamic moduli (G′) and (G″), regardless of the frequency or the intensity of the applied magnetic field [28,36,99].

For MR fluids, a behavior similar to that of MR elastomers was observed in the absence of a magnetic field. Both dynamic moduli (G′) and (G″) slightly increase with increasing angular frequency, especially in the high-frequency region. Moreover, the two values are remarkably close, suggesting a gel-like state. The low storage modulus (G′), which has liquid-like characteristics, results from the absence of a structure in place to support the external force [67,70,72].

Differently, this time, from MRE elastomers, in the presence of a magnetic field at an identical intensity, the vast majority of studies have observed that the two dynamic moduli, (G′) and (G″), of MR fluids remain independent and present a plateau over the entire frequency range studied [68,70,73,76,79].

To illustrate with an example, Lee and colleagues [72] performed a frequency sweep test on MR fluid based on Fe₃O₄/poly(o-anisidine) (POA) polymer-magnetic composite nanoparticles. These core–shell particles were dispersed in silicone oil, with a volume fraction of 10%. This test was performed in an angular frequency range of 1 to 100 rad/s, under different magnetic field intensities, with a constant strain of 0.005%. The results are shown in Figure 7.

The storage modulus (G′), which characterizes the elastic component of MR fluids, is mainly attributed to the magnetostatic forces between particles and chains. In the presence of a magnetic field, the hydrodynamic forces become negligible compared to the interparticle interactions, resulting in the formation of strong microstructures [112]. These microstructures are not destroyed within the measured frequency range, allowing the viscoelastic fluid to maintain a high elasticity, and the storage modulus (G′) remains constant. [72,74,85].

Furthermore, as illustrated in Figure 7a,b, the values of (G′) are significantly higher than those of (G″). This behavior suggests that these MR fluids exhibit a highly pronounced solid-like behavior rather than a liquid-like behavior, indicating a predominance of the elastic property over the viscous one. These observations are reported in the following studies [66,73,76,77,88,90].

2.3. Magnetic Flux Density Effect

The MR effect of magnetorheological materials is well known to depend on various parameters, of which the magnetic field intensity is the most relevant.

In a study conducted by [113], the dependence of the storage modulus (G′) and loss modulus G″ on the magnetic flux density B was investigated. The magneto-sweep tests were performed at a constant frequency of 10 Hz and a shear strain amplitude of 1%. A series of MRE samples based on silicone rubber and carbonyl iron powder (CIP) were prepared. The CIP particles had an average size of 5 μm, with volume concentrations ranging from 0 to 50%.

The results illustrated in Figure 8 indicate that the (G′) and (G″) moduli of the MR elastomer increase significantly with increasing applied magnetic flux density, regardless of the excitation frequency or strain amplitude. This means that the stiffness and damping properties of the MRE are enhanced in the presence of a magnetic field [36,52,114].

A significant initial increase of these moduli was observed; however, beyond a certain magnetic field strength, the curves begin to exhibit a downward trend in their slopes, resulting in a slower progression of the dynamic moduli.

As the applied magnetic field increases, the magnetic particles become magnetized until reaching saturation. At this point, the interparticle attractions no longer vary with a further increase in the magnetic flux density, indicating that the MR effect has reached its maximum [115].

The increase in magnetic flux density in MREs enhances interparticle magnetic attraction, which results in an increase in the storage modulus [28,116].

Energy dissipation in MREs mainly occurs at the interface between the matrix and the magnetic particles. A higher magnetic flux density leads to increased internal friction between the magnetic particles and the MRE matrix. This enhances the material’s ability to dissipate energy, which results in an increase in the loss modulus (G″) [103,117,118].

However, a slight decreasing trend in the loss modulus (G″) was observed at very high magnetic field strengths, particularly beyond 600 mT [119]. This reduction can be explained by several physical phenomena related to the internal structure of MREs. Under strong magnetic fields, the particle chains become more rigidly aligned, which results in an overall stiffening of the matrix [120]. Consequently, the material’s viscous damping capability decreases, as it behaves more like a “solid” and is therefore less capable of dissipating energy.

Beyond magnetic saturation, the motion of the particles becomes more restricted as magnetic forces dominate. Yet it is precisely this relative motion that contributes to internal friction, which is responsible for energy dissipation—thus explaining the observed decrease in the loss modulus G″ [121,122].

A similar behavior to that of MR elastomers was also observed in MR fluids. For curve illustration, the work of [85] was selected and is presented. Oscillatory tests, including a magneto-sweep test, were performed on MRFs. Two suspensions were prepared: one containing small iron particles with an average size of 2 µm (S-MRF) and the other with large particles of 19 µm (L-MRF). These plate-like particles were dispersed in a paraffin oil-based carrier fluid at 25 °C, with a volume fraction of 16%.

The magnetic sweep test under oscillatory shear was conducted at a constant low strain amplitude (LVE region) of 0.01% and an angular frequency of ω = 10 rad/s (f = 1.6 Hz). The Figure 9a,b respectively show the variation of the storage modulus (G′) and the loss modulus (G″) as a function of magnetic flux density.

A detailed analysis of the reports describing the magnetic sweep behavior of MRFs reveals the presence of three distinct regions in the (G′) and (G″) curves as a function of varying magnetic field intensity, as illustrated in Figure 9a,b. This overall behavior is a typical feature of MRFs and can be interpreted as a structural inhomogeneity induced within the fluid by the applied magnetic field [69,84,85,92,123,124,125,126].

At low magnetic fields, the increase in both (G′) and (G″) is very slight, as the suspended magnetic particles are unable to fully align in the direction of the field. They can be considered to be randomly dispersed within the suspensions [69,125]. Moreover, the storage and loss moduli decrease with increasing particle size, indicating behavior similar to that of a Newtonian fluid. Smaller particles have a greater influence on the rheological properties, as evidenced by the higher modulus of the S-MRF compared to the L-MRF.

At an intermediate magnetic field strength, a linear increase in the dynamic moduli (G′) and (G″) was observed, resulting from the formation of more robust aggregates. This region highlights the creation of a quasi-gel-like or solid-like structure, marking the transition from viscoelastic liquid behavior to viscoelastic solid behavior [85,126]. A predominance of the chain structure over the base liquid was obtained, with a more pronounced increase in the storage modulus (G′) compared to the loss modulus (G″) [125].

At high magnetic fields, a further increase in magnetic field intensity induces the formation of rigid chains aligned in the direction of the field, thereby impeding the flow and leading to the saturation of the MR fluid. The storage modulus (G′) becomes constant [85].

Furthermore, iron-based suspensions generally exhibit a higher storage modulus than those composed of magnetite, due to the higher saturation magnetization of iron. Larger volume fractions result in an upward shift of the curves, thus signaling a more pronounced viscoelastic behavior [123].

2.4. Temperature Effect

The dynamic characterization analysis (DMA) of MR elastomers as a function of temperature clearly indicate a decrease in both dynamic moduli (G′) and (G″) with increasing test temperature. This decreasing trend in (G′) and (G″) moduli is even more pronounced when the MR elastomer is simultaneously subjected to a magnetic field and high temperature [35,127,128,129].

In a study conducted by Tourab and colleagues [130], Figure 10 illustrates the dynamic shear moduli (G′) and (G″), measured at various temperatures ranging from 25 to 100 °C. The temperature sweep tests were carried out at excitation frequencies (from 20 to 80 Hz) under a magnetic flux intensity of 500 mT and a fixed strain amplitude of 0.01%. The MRE is based on RTV 141 silicone rubber, with a 30% volume fraction of iron particle dispersion.

The continuous decrease in the storage moduli (G′) and loss moduli (G″) can be explained by the temperature-dependent rheological properties of the elastomer matrix. The softening and progressive disordering of the molecular chains lead to the formation of defects at the interface filler matrix. As the temperature increases, the increasing thermal motion of the molecular chains in the polymer gradually overcomes the interactions between the molecules. This phenomenon causes a relative motion of the molecular chains, leading to a continuous decrease in the dynamic moduli (G′) and (G″) of the elastomer [127,130,131].

Regarding MR fluids, a study conducted by Hemmatian and colleagues [132] serves as an example of the temperature dependence of the viscoelastic properties of these fluids. Three different hydrocarbon-based fluids were prepared (MRF-122EG, MRF-132DG, and MRF-140CG) containing iron particles of 3–10 µm. The base fluid used was a synthetic oil. The test series shown in Figure 11 was performed at a fixed strain and frequency (γ = 0.01% and ω = 1 Hz).

A significant thinning induced by temperature was observed for all magnetic field strengths. In the linear viscoelastic region, and in the absence of a magnetic field, the fluid exhibits a strong temperature dependence. The dynamic moduli tend to be higher at lower temperatures, particularly below 10 °C [133].

These moduli gradually decrease with increasing temperature, reaching a critical temperature (Tcr) at which they stabilize. The value of this critical temperature varies from one MR fluid to another and decreases as the weight fraction of solid particles increases. Furthermore, it has been observed that the effect of temperature is more pronounced at low strain rates [132,133]. However, the application of an external magnetic field reduces the influence of the carrier fluid, thus decreasing the temperature dependence of the MRF fluid.

The temperature-induced changes in mechanical properties and thinning effect are mainly attributed to the decrease in carrier fluid viscosity in the absence of magnetic field, the reduction in particle saturation magnetization, and the decrease in shear stress with increasing temperature [126,134]. It should also be noted that carrier fluid viscosity is particularly sensitive to low temperatures [135].

The relatively more pronounced effect of temperature on the loss modulus (G″) compared to the storage modulus (G′) is attributed to the higher sensitivity of the carrier fluid to temperature, as opposed to the solid particles. This trend has been consistently observed, regardless of the intensity of the magnetic field [134,136].

The yield stress is defined as the shear stress threshold beyond which the fluid begins to flow. In the absence of a magnetic field, the yield stress of the MR fluid shows a slight tendency to decrease with increasing temperature, especially at low temperatures [137,138].

3. MR Elastomers’ Major Advantages

Compared to their composition or nature, MR elastomers offer exceptional properties relative to other magnetorheological materials such as MR fluids [139]. Generally, MREs exhibit a unique field-dependent material property when exposed to a magnetic field and can overcome key challenges encountered in MRFs [140,141], including the following.

3.1. A Solution Without Sediments and Deposits

Unlike MR fluids, particles within MR elastomers do not settle over time, thus avoiding sedimentation. This is a significant advantage of MREs over MRFs that is frequently emphasized by researchers [13,116,139,142].

3.2. A Solution Without Sealing Requirements

In MR elastomers, there is no need for containers to hold the MR material in place or for sealing joints, unlike systems using MRFs [113,143]. Therefore, MREs exhibit superior performance in sealing applications and are more environmentally friendly than MRFs [13]. Additionally, due to their solid state, using MREs in MR devices is more practical compared to MRFs [3,28,144].

3.3. Rapid Response Time of MR Elastomers

The response time of MREs is generally comparable to that of their fluid counterparts, representing a significant advantage of MR materials. The magnetorheological and magnetostrictive effects of MREs exhibit rapid responses sensitive to changes in magnetic field [116,145].

The intrinsic response time is typically in the order of a few milliseconds, usually less than 10 ms [146,147], depending mainly on the viscoelastic properties of the matrix [30]. The authors of [148] reported that due to the characteristic microstructure of MREs, it is possible to reduce response time and deformation values relative to magnetic field intensity. According to [149], the embedded particles in the matrix do not require time to rearrange, enabling MREs to quickly respond to an external magnetic field.

3.4. Improved Durability

MRE elastomers undergo various modes of degradation over time and under mechanical loading, including polymer chain scission, microcracking, phase separation, small-scale plasticity, and molecular-level stress relaxation [150]. The nature of the matrix (e.g., NR, SR, silicone), the particle volume fraction, the presence of additives, and the curing process directly influence wear resistance and stability against mechanical and environmental aging [151]. Moreover, improved durability can be achieved with MRE polymers subjected to repeated loading, as magnetic particles do not sediment within the polymer matrix [147]. In addition, MREs exhibit greater thermal stability than MRFs and offer better resistance to degradation [148].

3.5. Relatively Low Weight

The amount of particle loading can also be considered lower compared to MRFs. As a result, devices based on MREs for sensing and actuation have a lower weight [148].

3.6. Reduced Energy Consumption

MREs feature desirable additional characteristics such as low energy consumption and inherent safety [28].

3.7. Enhanced Response in Stiffness Control

The field-dependent properties of MR elastomers, such as stiffness, natural frequency, and damping capacity, can be dynamically adjusted with an external magnetic field. These are represented in terms of dynamic modulus rather than yield strength [4]. MREs exhibit behavior more related to stiffness rather than damping performance compared to MRFs [118,149].

3.8. Anisotropic Structure

Another remarkable characteristic favoring MR elastomers is that the embedded magnetic particles can be oriented under the application of a magnetic field during the curing process. This is a notable property, as the orientation and fixation of the magnetic component cannot be achieved in other material types such as MRFs, due to the fluid nature of the matrix. In fact, it has been concluded that anisotropic arrangement provides a better MR effect than isotropic arrangement, and the use of a flexible matrix enhances the MR effect compared to those produced with rigid elastomer matrices [56,152].

3.9. MR Effect of MR Elastomers

The effect of magnetic field intensity is also expressed in terms of the MR effect, defined as the maximum variation of the storage modulus under the application of the maximum available magnetic field intensity, under all loading conditions of tension, compression, or shear [61,153]. Depending on the applied magnetic field intensity, this dynamic modulus of MREs immediately changes due to strong interparticle magnetic forces [119].

It has been widely acknowledged that the MR effect represents one of the key performance characterization parameters for MR materials. The primary focus is on developing MREs with a high MR effect [13,61,118].

Data collected evaluating the relative MR effect of MR elastomers have reported highly varied values, ranging from a low value of 4% [154] to 24.515% [30,147,155].

The main influencing factor of the MR effect is the response of MREs to the external magnetic field [106,156]. Mitsumata and colleagues [155] reported that upon applying a magnetic field of 500 mT, MRE samples exhibited a drastic change in dynamic modulus: the storage modulus changed from 6.5 kPa to 1.6 MPa.

Recently, [28] reported MR effects up to 1672% for MREs with a silicone rubber matrix under magnetic fields up to 450 mT and a volume fraction of 40% of carbonyl iron, observed under a deformation amplitude of 2.5% at a low frequency of 0.1 Hz. For the same MRE, the storage modulus decreased to about 252% under a deformation amplitude of 20% at 50 Hz.

Nedjar and colleagues [99] prepared a silicone rubber-based MRE, achieving a maximum relative MR effect of 1908% under a magnetic field intensity of 300 mT, a deformation amplitude of 0.01%, and a frequency of 0.01 Hz.

On the other hand, the modulus at zero magnetic field reported ranges from 0.0012 MPa [157] to 5 MPa [158], contributing to the extreme variations in reported MR effects across different studies.

In addition to differences in applied magnetic field intensity and testing methods, the extreme variations in reported properties within different MRE studies can be attributed to variations in several aspects. These include differences in the fraction of magnetic particles used, their size, shape, dispersion, and the type of matrix employed [30,61,156].

As shown in the literature, absolute and relative MR effects depend on the content of magnetic particles, the frequency of oscillations, and the intensity of the magnetic field [145,159]. The MR effect is also influenced by the applied deformation amplitude because magnetic interaction forces strongly depend on the distance between magnetic dipoles [115,159]. It has been reported that the relative MR effect is higher for a softer matrix compared to a harder matrix [13,148,159], reaching its maximum for low values of deformation amplitude and excitation frequencies [160].

Additionally, anisotropic MREs have shown higher dynamic moduli and MR effects compared to isotropic ones at the same content of magnetic particles [29,160,161].

3.10. Magnetostrictive Effect

MREs can alter their viscoelastic properties and stiffness in response to external magnetic fields, but they can also undergo significant deformation states. The field-sensitive modulus of elastomers is primarily due to mechanical deformation induced by the magnetic field [17,148,162].

The capacity for shape change determines the magnetostrictive behavior. It has been reported that elastomers can reversibly stretch from 5 to 700% depending on the material used [141].

The significant stresses generated by magnetostrictive MREs can be exploited in various potential applications, including strain/stress sensors, as well as converting mechanical motion into electrical signals [163]. Moreover, vibration isolators benefit from the adaptive damping capabilities and stiffness adjustment of magnetoactive elastomers [116].

Highly flexible magnetostrictive elastomers offer numerous applications, such as variable stiffness components, high-strain actuators, and electro-magnetically active damping elements. Additionally, polymer-based gels have the potential to be used as artificial muscles and damping components [164].

It is important to note that shape-changing behavior is not as widely explored as property changes, such as modulus/stiffness modification [4].

4. MR Elastomers’ Limitations

While such advantages offer MREs great potential for designing smart devices for use in various engineering fields, they do present some drawbacks, such as the following.

4.1. Incompatibility Between Magnetic Particles and Elastomer Matrices

Magnetic particles (such as CIPs) are hydrophilic, whereas elastomer matrices are generally hydrophobic. Many studies have been conducted to increase the surface compatibility of particles with the elastomer matrix [118]. To improve their compatibility with the polymer matrix, magnetic particles typically undergo surface treatment to remove moisture and become more hydrophobic before the curing process [12,165].

4.2. Deterioration Sensitivity: Another MR Elastomer Obstacle

Another disadvantage of MREs is their sensitivity to destruction. In their stiffened state, MRFs are not damaged if the particle chains formed in the magnetic field are broken by mechanical forces, as the chains are reversible and can be reformed. However, the same process in MREs leads to irreversible destruction of the composite if the chemically cross-linked elastomer network is destroyed by excessive stretching [113].

4.3. Oxidation Issues in Elastomers

Oxidation is also a potential issue for MR elastomers. Oxidation occurs when the elastomer is exposed to air. This can be mitigated by surface treatment of the particles. To prevent these effects, silicone and polyurethane-based elastomers are good candidates due to their thermo-oxidative properties and their chemical and mechanical resistance [147]. One of the most widely used materials for the matrix is thermoset silicone rubber, both at room temperature and at high temperatures.

5. MR Fluids’ Major Positive Characteristics

MR fluids are smart materials that can exhibit non-Newtonian behavior characterized by a field-dependent yield stress and increased apparent viscosity [166]. These unique properties offer several advantages and potential applications:

5.1. Viscosity Control

The primary advantage of MRFs lies in their ability to significantly alter their rheological properties, particularly their capacity to adjust apparent viscosity reversibly in response to a magnetic field, as well as their field-dependent yield stress. This means they can transition from a fluid state to a semi-solid or solid state depending on the intensity of the applied magnetic field. The apparent viscosity of MRF changes by several orders of magnitude, typically by 3 to 4 times, with varying magnetic field intensity, exhibiting a typical MR effect [6]. This feature allows precise control over the material’s fluidity, making them useful in numerous applications [146,167,168,169].

5.2. MR Fluids Rapid Response Time

The rapid response time of MRFs when subjected to an external magnetic field is a key advantage in many magnetorheological applications [147]. The examination of these fluids reveals that they, exhibiting different properties of carrier liquids and varied particle characteristics, display distinct response performances. These performances also largely depend on the testing method used [170]. The response time of MRFs is generally recognized to be on the order of a few milliseconds [7,171,172].

Among various magnetic materials, soft magnetic particles such as carbonyl iron or Ni-Zn ferrites exhibit good performance in controlling the rheological capacity of MRFs due to their easy magnetization and demagnetization compared to hard magnetic materials [147,173].

According to the existing literature [174,175], the response time of devices using MR fluids is not limited solely to the intrinsic response time of the MRFs. The temporal response of these devices thus depends on five main factors: (a) the intrinsic response time of the MR fluid, (b) the inductance of the device’s coil, (c) eddy currents within the coil core, (d) the response time of the driving electronics, and (e) the geometry of the device. As a central component of MR devices, the intrinsic response characteristics of the MR fluid exert a significant influence on the overall response time of smart devices [170].

5.3. MR Effect of MRFs: A Key Characteristic

The most important and evident characteristic of MRFs lies in their field-dependent yield stress, which is the minimum stress required to induce the flow of the MR fluid [176]. This property is typically expressed through the relative MR effect, defined as the ratio between the increase in stress in the presence of a magnetic field and the stress in the absence of a field [13,69]. The MR effect is also defined as a tunable and reversible property of the yield stress, magnetic viscosity, and dynamic moduli of the MR fluid when exposed to a magnetic field [168,177,178,179].

Upon reviewing most reports characterizing MR fluids, it has been found that MR effect values are not often provided. Typically, only the most relevant characteristics are summarized [123,124,180,181].

It has been reported that MRFs demonstrate a considerable enhancement in their yield stresses, potentially increasing by a factor of 20 to 50 in response to external magnetic fields [168,182,183,184]. According to the available literature, MR suspensions can be classified as Bingham fluids, exhibiting flow stresses dependent on the magnetic field, which can reach values as high as 100 kPa [3,6,7,67,147,185,186,187].

The MR effect is manifested through the variation of the yield stress, as well as the dynamic moduli of MRFs, depending on the intensity of the applied magnetic field. This variation remains positive, regardless of the type, size, shape, distribution, and concentration of the magnetic particles present, as well as the medium in which these particles are suspended [184].

Another crucial parameter that significantly influences the MR effect is the viscosity of the base oil. As the base fluid constitutes a phase, its viscosity is of great importance during the preparation of the MRF. To achieve an optimal MR effect, it is necessary to keep the zero-field viscosity of the base fluid as low as possible [167,168,183].

Morillas and his coauthors also reported that the MR effect depends on both the on-state properties, characterized by the magnetization level M and the volume fraction of the particles, as well as the off-state properties, which include the viscous dissipation in the carrier fluid of the MRF [13].

5.4. Wide Operating Temperature Range

Another significant advantage of MRFs is their wide operating temperature range. It is widely reported that typical MRFs can be used in temperature ranges from −40 °C to 150 °C, depending on the application, using a low voltage of 12 to 17 V. This temperature range is considered relatively broad, as highlighted by several sources [3,147,168,188,189].

Beyond this temperature range, it has been observed that the fluid’s viscosity becomes uncontrollable, compromising semi-active functionality [190,191]. However, the viscosity of the MRF gradually decreases when the temperature exceeds 100 °C. Since viscosity is proportional to shear stress, it can also be influenced by temperature [192]. It is worth noting that shear stress remains unchanged within the optimal operating range (up to 100 °C), but it shows irregular variations beyond this threshold.

6. MR Fluids’ Limitations

Despite significant advantages and the numerous potential applications of MRFs, MRF-based devices still face inherent problems and disadvantages, primarily including the following.

6.1. Sedimentation Issue

The major and most challenging problem to overcome in MRFs is their dispersion instability and aggregation, leading to sedimentation or precipitation of magnetic particles [12,116,187]. MRFs, composed of three elements of different sizes and densities, tend to separate, creating distinct zones of composition during the sedimentation process (e.g., 7.68 g/cm³ for CIP iron) [147]. Due to this density difference and the influence of gravity, magnetic particles in an MRF tend to settle out of the suspending fluid, resulting in a loss of its magnetorheological properties [193].

Since sedimentation is a major drawback of these smart materials, additional measures must be taken to reduce or mitigate this problem in MRFs [194]. Researchers have proposed numerous stabilization strategies and methods to enhance MRF stability. Among these strategies are coating magnetic particles with a polymer shell [195,196,197] and adding nanoscale fillers [198,199,200,201].

The addition of a variety of stabilizing and surfactant additives, such as carbon nanotubes [113,202], carbon nanofibers, and organic clays [88], helps reduce the sedimentation rate by preventing direct contact between adjacent charged particles. However, the size and morphology of additives and their affinity with charge particles significantly influence the resolution of this issue. Moreover, increasing the viscosity of the base fluid or reducing the density difference also slows down sedimentation [13].

Research conducted by Böse and colleagues [113] emphasizes the necessity of using appropriate additives to stabilize particles in MRFs. However, this stabilization is limited by ensuring that MRF viscosity does not become excessively high. In a separate study, Kumar and team [191] proposed using particles in the form of wires or sheets, along with a thixotropic network and viscoelastic supports such as gel or another polymer solution.

6.2. Agglomeration Effect

One of the undesirable behaviors observed in mechanisms using MRFs is the agglomeration or clotting effect. Under the influence of high magnetic fields, magnetic particles form chains that are trapped, while the carrier fluid can flow freely. When parts of the mechanism move (e.g., a piston), the iron particles remain confined in space while the carrier fluid escapes, leaving behind the charged particles [203]. As usage continues, the amount of trapped particles in the space increases significantly, while most of the carrier fluid exits this space.

As agglomeration occurs, it leads to the separation of iron particles from the carrier fluid (FPS), which is likely to alter the force distribution within the MRF and hence reduce the compressive force. Moreover, this phenomenon is triggered by rapid changes in force transmission and displacement through the carrier fluid. Consequently, MRFs become stiffer and exhibit higher damping values [204,205].

The intensity of this effect depends on the magnetic field density, the type of carrier fluid, and its volumetric percentage [205,206]. In the design of devices using MRFs operating in compression mode, it is crucial to find a solution to mitigate the agglomeration effect [203].

6.3. Sealing Issues and Environmental Contamination

Sealing issues and environmental contamination due to the potential leakage of the liquid medium into undesired areas of MRF mechanisms limit their expansion in engineering applications [12,17,102,118,140,141].

6.4. Thickening Phenomenon

MRFs are prone to thickening after prolonged use and require replacement. This affects the durability of these fluids [141,207,208].

Erosion is a problem caused by contact friction between moving particles in fluid flow. Among the particles commonly used in MRFs, carbonyl iron particles have a structure resembling that of an onion, making them susceptible to changes due to friction and/or impacts. Erosion leads to irreversible thickening of the suspension, thereby decreasing the performance of MRFs. Significant attention has been given to surface treatments to extend the lifespan of these fluids [167,209].

6.5. High Density

The high density of iron particles, which is significantly greater than that of the carrier fluid, is a major factor contributing to sedimentation, which has severely limited their subsequent technical use [202,210]. Indeed, most MR fluids exhibit a high relative density, generally ranging from 3 to 4 g/cm³, primarily due to their high concentration of dense ferromagnetic particles, which can reach 10% to 70% by volume [3,167].

6.6. High Cost

The cost of MR fluids is potentially higher than that of elastomers [144]. Key cost-contributing factors in devices using MRFs generally include seals, surface finishing of the mechanism, precision mechanical tolerances, assembly of the electromagnet, poles, and the flux conduit, as well as the volume of MRF [189]. Additionally, high-quality fluids are expensive. These cost-related issues limit the commercial applications of MRFs [167,208].

6.7. Particle Oxidation/Corrosion Issue

The oxidation of magnetic particles is another form of failure in MRFs [116]. Oxidation is a common and negative phenomenon, particularly occurring in the case of iron particles. Iron, usually in the form of carbonyl iron, tends to undergo corrosion processes leading to the conversion of iron (Fe) into ferrous (Fe²⁺) and ferric ions (Fe³⁺), resulting in the formation of iron oxides in various forms (FeO, Fe₂O₃, Fe₃O₄) on the particle surfaces, depending on environmental conditions. For example, exposure to hydrochloric acid causes it to react, forming ferrous chloride, which in turn reacts to form solid Fe₂O₃ [197,211].

After reviewing the following studies [13,167,197,211,212], the failure of MRF-based suspensions due to thermal/chemical oxidation processes can be illustrated by several phenomena:

• Loss of magnetic properties: Iron oxides exhibit a significantly lower-saturation magnetization (MS) compared to ferromagnetic component-based particles such as CIP particles.
• Decrease in mechanical performance and MR effect due to the reduced magnetic properties of the particles.
• Reduction in yield stress induced by the magnetic field, which decreases as the degree of oxidation increases.
• Increase in friction and leakage in a zero magnetic field.
• Increase in fluid viscosity without a magnetic field.
• Changes in particle surface from very smooth to very rough have also been observed.
• Increase in the mass of magnetic particles due to the presence of oxides.
• Formation of a stabilized oxidation film on the particle surface.
• The type of dispersing liquid influences the level of particle oxidation.

Considerable efforts have been made to protect magnetic particles from oxidation, primarily involving surface modification through the application of thin organic or inorganic films. In this context, various strategies for protective coatings have been developed and applied to reduce particle corrosion rates [212].

According to the work of Plachy et colleagues [211], the addition of polymer shell or silica coatings on particles enhances their stability against thermal and chemical oxidation in MRFs. These researchers have also reported recent advancements in using inorganic films containing rare earth elements as surface coatings, providing protection against both thermal and chemical oxidation. Implementing these processes could not only strengthen resistance to oxidation and corrosion but also enhance sedimentation stability and dispersibility of MRFs.

It is noteworthy that the morphological, physical, and chemical properties of composite particles may undergo changes due to various coating processes and strategies applied to their surfaces. These changes can potentially reduce the magnetic property of the MRF due to the non-magnetic nature of the coating, thereby requiring the saturation of particles at lower magnetic flux densities [212].

In conclusion, besides kinetic stability, magnetic particles must exhibit chemical stability to prevent thermal–chemical oxidation or corrosion in extreme environments [13].

7. Other Comparative Elements

7.1. Structure and Composition

MRFs consist of suspensions of magnetic microparticles in a viscous carrier liquid, such as oil. These magnetic particles are suspended and free to move independently within the liquid [32]. Typically, one or more additives are present, often acting as stabilizers or surfactants.

On the other hand, MREs are matrices of elastomers, typically soft and elastic polymers with a defined but deformable shape [113]. Magnetic particles are incorporated into these elastomeric matrices and are immobilized once the polymer matrix is cross-linked [4,118]. These particles are embedded within the polymer matrix and, under external excitations such as deformation and magnetic fields, can only undergo local movement around their original positions [32], giving them a limited range of motion [141,170]. Various additives such as crosslinking agents, antioxidants, and mixing aids may also be used.

7.2. Pre-Elasticity and Post-Elasticity Regimes

MR elastomers and fluids operate within different ranges [147]. There is a significant difference in how MREs and MRFs are typically intended to function. It is noteworthy that the chains of magnetic particles within the elastomeric matrix are designed to operate in the pre-elasticity region, where the elasticity factor of the matrices is crucial, whereas MRFs generally activate in a post-elasticity shear or continuous flow regime. Indeed, the resistance of MRFs is explained by their field-dependent yield stress, while that of MREs is typically characterized by their field-dependent dynamic modulus [3,13,102,116,118,141,156,160,187,213,214].

7.3. MR Effect

For a comparison of the MR effects between MRFs and MREs, various theses have been presented. It has been reported that MREs exhibit a lower MR effect compared to MRFs. Raju and colleagues highlighted that the relatively low MR effect is a major drawback of MREs due to their elastomeric base [143]. Similarly, Shahrivar and colleagues noted that the trade-off for MREs is a much weaker MR effect compared to MRFs [215]. This sentiment was echoed by [139].

In contrast, Boczkowska and collaborators reported that MREs are characterized by greater deformation, where the MR effect is more significant compared to their corresponding MRFs [115].

7.4. Response Time

In terms of comparison, the response time of MREs is generally shorter than that of their MRF counterparts, typically in the range of milliseconds. This response time primarily depends on the viscoelastic properties of the matrix [30,119,173].

An analysis of the results presented by these authors suggests that MREs generally exhibit shorter response times and better reversibility compared to their fluid counterparts. This responsiveness is mainly influenced by the viscoelastic properties of the matrix, as demonstrated in references [30,119,171]. In MRFs, particles need to reorganize in the presence of a magnetic field and typically operate in a post-elastic shear flow regime, which takes time to achieve equilibrium [216,217].

7.5. Supported Stress Levels

There are significant differences in the stress levels that MRFs and MREs can withstand and, consequently, the application domains in which they are most effective.

In this context, a review study compiling the most significant works on the dynamic mechanical analysis of MREs and MRFs has been analyzed, and their results are presented. An increase in the apparent viscosity of MRFs allows for the control of their stiffness and damping, depending on the intensity of the applied magnetic field. MRFs generally have a limit in terms of the stress (yield stress) they can withstand without losing their ability to flow. This means they are typically more suitable for applications requiring relatively low to moderate stress levels. Unlike MRFs, MREs can withstand higher levels of mechanical stress under applied magnetic fields.

Dynamic mechanical properties refer to a material’s behavior under cyclic loading. For elastomers, these properties are often reported as the storage modulus, loss modulus, damping, and glass transition temperature [218]. For MRFs, they are reported as storage modulus (G′), loss modulus (G″), apparent viscosity, and dynamic yield stress.

Table 3. MR elastomers’ dynamic moduli.

G′ (Mpa)	G″ (Mpa)	Particles	Content (%)	Matrix	Loading	Mag. Field (mT)	Ref.
0.24	\	CIP-PMMA	30 (wt)	HTV SR	shear	1000	[20]
1.1	0.5	CIP	40 (vol)	SR	shear	450	[28]
2.2	0.75	CIP	27 (vol)	SR	shear	651	[29]
3	0.9	CIP	30 (vol)	RTV SR	shear	325	[36]
3	1	CIP	40 (vol)	RTV SR	shear	300	[99]
3.05/1.25	\	CIP	70 (wt)	SR	compression/shear	500	[219]
4.6	\	CIP	30 (vol)	SR	compression	750	[30]
10	3.5	CIP	30 (vol)	NR	compression	60	[21]
16	9	Fe3O4	40 (wt)	waste tires R	shear	1000	[220]
20	4	CIP	30 (vol)	SR	compression	500	[23]

and

Table 4. MR fluids’ dynamic moduli.

G′ (Kpa)	G″ (Kpa)	Particles	Content (%)	Fluid	Loading	Mag. Field (mT)	Ref.
12	5	CIP/Silica	4.5 (vol)	Castor oil	Shear	200	[83]
60	9	MWCNT/COFe2O4	25 (wt)	Silicone oil	Shear	250	[66]
300	13	Co-Ni	20 (vol)	Castor oil	Shear	500	[82]
1000	100	CIP	20 (vol)	Silicone oil	Shear	280	[71]
1000	100	Fe-Ga	30 (wt)	Silicone oil	Shear	430	[75]
1000	100	CIP	10 (vol)	Silicone oil	Shear	430	[76]
1100	300	Co	12 (vol)	Silicone oil	Shear	312	[69]
3000	100	CIP	20 (vol)	Paraffin oil	Shear	90	[84]
3000	300	CIP	40 (vol)	Silicone oil	Shear	350	[68]
4000	200	CIP	10 (vol)	Silicone oil	Shear	430	[74]
10,000	200	Ni	22 (wt)	Castor oil	Shear	1000	[133]
10,000	100	CIP	20 (vol)	Silicone oil	Shear	430	[221]

present only the dynamic moduli.

This distinction underscores the suitability of MRFs for applications where controlled stiffness and moderate stress levels are required, while MREs excel in environments demanding higher mechanical stress under magnetic fields (Table 3).

The information obtained from the review of the most significant studies dealing with dynamic mechanical analysis reveals that the elastic moduli of elastomers generally range from 0.25 MPa to 20 MPa, depending on their composition, oscillation frequency, test temperature, and applied magnetic field. The loss moduli of these elastomers typically range from 10% to 40% compared to their storage counterparts. As for MRFs, the recorded values of the dynamic storage moduli range from 10 kPa to 10 MPa, while the loss moduli range from 5 kPa to 300 kPa (Table 4).

It should be noted that the values presented in these tables were extracted from peaks in the curves presented in research reports. The dynamic properties of MRFs and MREs have been characterized under various experimental conditions. These conditions include both isotropic and anisotropic samples subjected to shear or compressive loads, with a wide range of deformation amplitudes, excitation frequencies, and magnetic field intensities, all overlapping. Different types of filler particles, with varying natures, concentrations, and sizes, have been used extensively. A wide variety of matrix materials and carrier fluids have also been employed. Wide ranges of testing temperatures have been maintained, and finally, various curing systems as well as testing and measurement methods have also been adopted.

It has also been observed that the dynamic moduli of MR elastomers in compression mode are higher than those in shear mode, allowing MRE mechanisms operating in compression mode to reduce significant deformations and vibrations [219]. Increased compression deformation along the direction of magnetic particle chains reduces the distance between particles, increases interaction energy, and consequently increases the induced magnetic modulus. The same applies to the MR effect [222].

Furthermore, the performance of MRFs under compression and tension with different magnetic fields has been compared to that under shear. Compression tests have shown that MRFs are quite stiff for small compression deformations. Compression stress and dynamic modulus increase rapidly when compression deformation is applied.

The tensile yield strength of MRFs reflects the effect of the interaction force between polarized particles and the direction of the applied magnetic field, whereas the shear yield strength reflects the effect of interaction force in the shear direction, perpendicular to the direction of the applied magnetic field. It has been reported that the tensile yield strength is approximately four times higher than the shear yield strength [133,223].

In summary, MRFs are more suitable for applications requiring relatively low levels of stress, while MR elastomers offer better capability to withstand higher stress levels while retaining their responsiveness to magnetic fields. The choice between these two materials depends on the specific application needs and the stress levels involved.

7.6. Application Areas and Opportunities

Due to their suitability based on the application and desired final function, MR devices generally require specific shapes and dimensions. One of the most advantageous features of MREs lies in their ability to adopt a variety of sizes and shapes. Manufactured from a fluid elastomer that solidifies during curing, MREs offer design flexibility, unlike MRFs, which always require reservoirs to contain the fluid [116].

Moreover, it is noteworthy that MREs can be 3D-printed, offering exceptional design flexibility. This additive manufacturing method allows for the creation of complex geometric structures by layering successive elastomer layers. The matrices of these MREs can be printed from powders, liquids, or suspensions, using various dispensing mechanisms and equipment for 3D constructions [4,26].

Furthermore, the rheological properties of MREs make them a preferred material in certain vibration cancellation applications compared to MRF devices. Unlike MRFs, MR elastomers feature two adjustable parameters: stiffness and damping. A system using MRF always requires a spring to achieve stiffness [156].

In addition to their magneto-elastic nature, MREs possess other functions such as magnetoresistance, magnetostriction, piezoresistance, and thermoresistance [13,26].

8. Other MR Behavior Mechanisms

In addition to the comprehensive comparative analysis presented earlier, a list of fundamental mechanisms governing MR behavior in magnetorheological elastomers and fluids is summarized below.

In both MREs and MRFs, stiffness and damping properties are actively adjustable. Changes in mechanical and viscoelastic properties, induced by the alignment of magnetic particles in these materials in response to an applied magnetic field, provide a unique capability to actively adjust their deformation and damping values [118,119,142,224,225,226].

8.1. Matrix MRE and Carrier Fluid

The choice of matrix material significantly influences the mechanical properties of MREs, including initial modulus, field-dependent modulus, and MR effect. A softer matrix material leads to a relatively higher MR effect [116,227,228].

The intrinsic viscosity (at zero field) of the base fluid is a critical parameter in preparing MRFs. The entire system is designed to operate with this specific viscosity. To achieve the maximum MR effect, the viscosity of the base oil should be low. However, excessively low viscosity can lead to instability and sedimentation issues, while high viscosity at a zero field can result in undesirably high viscosity of the MRF [167,183]. In addition to viscosity, the base fluid should also have reduced permeability to facilitate particle polarization with maximum efficiency, thereby enhancing the MR effect [229].

8.2. Charged Particles and Dispersed Phases

It has been reported that another parameter significantly influencing the behavior of elastomers and fluids is charged particles [32,167,173,230]. Understanding key factors related to magnetic particles, such as size, shape, concentration, magnetic properties, and spatial distribution within the matrix, allows for a comprehensive description of the behavior of MR elastomers and a deeper understanding of their properties, including their microstructure, mechanical properties, and the MR effect induced by magnetic fields [231].

Furthermore, to achieve a high MR effect in MRFs and MREs resulting from particle-particle interaction, and to ensure efficient and stable MRFs, certain fundamental magnetic properties must be present [140,159,167,173,232]:

• Low residual magnetization, which represents the remaining magnetic effect on ferromagnetic materials after the magnetic field is removed.
• High permeability, which measures the material’s magnetization relative to an applied magnetic field.
• High saturation magnetization, which corresponds to the maximum magnetization achievable by the material.

Furthermore, the choice of charge particles in MREs or dispersed phase in MRFs must consider a set of closely related intrinsic characteristics. These include magnetic behavior, size, volume fraction, shape, and density [32,109,167,173,175,230,233,234,235]. The interdependence of all these factors is highly complex but essential in establishing methodologies to optimize the performance of fluids and elastomers [236].

Traditionally, the rheology control of MRFs is primarily achieved by adjusting either the amount of charged particles or the interparticle magnetic interaction strength [237]. Therefore, the concentration of magnetic particles is a very important, if not crucial, factor in optimizing and controlling MRFs under various circumstances and applications [173,238,239].

The yield stress of MRFs primarily depends on particle size, size distribution, volume fraction, and magnetic nature (saturation) of the particles. It is directly influenced by volume concentration and quadratically by saturation magnetization. However, particle size and size distribution contribute relatively less to the maximum yield stress of an MRF [40,240,241,242].

According to the literature reviewed, the typical dispersed phase of MRFs generally ranges in size from 1 to 10 µm, with an average particle size of about 4 µm. The volume fraction of these particles varies from 10% to 70%, depending on the required stress range and allowable sedimentation [167].

Under the influence of an applied magnetic field, the magnetic force of MRFs increases with higher volume fraction and larger particle size [243]. Increasing these parameters also enhances shear stress and thus the yield stress of MRFs. Mixed-size particles have shown better results compared to small or large particles [244].

Among all the design factors, the volume fraction of particles exerts the most significant influence on the MR effect of MR elastomers. In addition to the MR effect, stiffness and damping properties are also affected. The current literature suggests that a particle volume fraction of approximately 30% within the matrix achieves an optimal MR effect [40,148,160,161,231,245].

The performance of MREs can be enhanced by optimizing particle size selection. Particle size not only affects the magnetic properties of MREs but also influences particle interface and distribution [159,232,246,247,248].

Particle size distribution also plays a crucial role in MRE performance. Magneto-mechanical properties such as shear modulus and MR effect have been significantly improved by mixing two or more particle diameters in MREs [231,249].

Mixing soft and hard magnetic particles within an elastomeric matrix allows for a combination of active and passive control over MR properties. These hybrid MREs are intended for the development of semi-active and active damping devices, actuators, and acceleration sensors [38,248,250,251,252].

The configuration of particles within the matrix significantly influences the MR effect of MREs. For the same concentration, anisotropic particle structures lead to a higher MR effect compared to isotropic structures [4,156]. The alignment of particle chains depends on the strength of the magnetic field applied during curing. A stronger magnetic field application results in wider particle chains composed of more particles [148].

The shape of the charge particles also influences the behavior of these MR materials [253,254]. Conventionally, spherical carbonyl iron particles are widely used for MR fluids and elastomers (Table 1 and Table 2). Irregularly shaped particles exhibit a higher MR effect compared to spherical particles [255]. This enhanced MR effect can be attributed to the smaller interparticle distances resulting from the irregular Critical Packing Volume Concentration (CPVC) [156,159].

At a zero magnetic field, irregular particle shapes increase the dynamic modulus by restricting the movement of MRE molecular chains [255]. Additionally, larger and irregularly shaped charge particles tend to move more slowly, producing a non-uniform distribution when a magnetic field is applied [72]. It is also preferable for particles to have an asymmetrical shape with a principal axis of anisotropy to achieve an enhanced MR effect [17,148].

Fluids composed of high-anisotropy shape particles exhibit a more pronounced MR effect, reduced sedimentation velocity, and improved stability compared to those made with spherical particles [123,180,256]. These particles facilitate the formation of stronger assembly chains by providing a larger contact surface [257,258]. Moreover, the higher interparticle friction force of aggregated particles leads to increased viscosity [259]. The larger specific surface area results in stronger mechanical and magnetic interparticle interactions [84].

8.3. Spherical Carbonyl Iron: Ideal Selection for MR Materials

Most MR elastomers and fluids described in the literature utilize soft magnetic particles, particularly carbonyl iron particles (Table 1 and Table 2). This preference is due to the excellent magnetic properties of soft particles, including relatively high initial permeability, high saturation magnetization, high short-term interparticle attraction, low magnetic coercivity, and low remanence magnetization [3,118,156,159,260]. These parameters provide a high MR effect and better reversibility. For MRFs, these factors typically characterize the maximum achievable yield stress induced by the MR effect [93,167,191,261]. Furthermore, using hard magnetic charges enables the fabrication of composites with programmable and adjustable properties, owing to their high remanent magnetization [250,262,263].

8.4. Performance Optimization Through Additives

Additives play a crucial role in enhancing the performance of MR elastomers by optimizing their magnetic, mechanical, and thermal properties. Here are some of the key contributions of additives in MREs:

• Improving both the rheological properties with and without a magnetic field, especially the MR effect [4,118,119,264].
• Facilitating compatibility between the material matrix and magnetic particles [4] by enhancing interfacial interactions between the fillers and the matrix [52,265,266,267].
• Preventing the accumulation of magnetic particles [4,142,268].
• Additives in powder or liquid form improve the properties of MREs by modifying the elastomer matrix properties (increasing polymer mobility) and the surface properties of magnetic particles (increasing polymer/filler affinity) [52,119].
• Opening up application possibilities based on MREs, offering sensing and actuation capabilities such as changes in resistance and capacitance [4].

Furthermore, compatibility between magnetizable particles and the carrier fluid, chemical stability, and dispersion are critical parameters influencing the resulting properties of MRFs. Goals include reducing sedimentation, preventing agglomeration, and improving redispersibility [191,269,270].

Additives such as stabilizers and surfactants are integrated into the magnetic suspension of MRFs to overcome particle sedimentation, a major drawback of MRFs. These additives aim to ensure dispersion stability, improve lubrication and redispersibility, and modify the initial viscosity of the fluid. Their addition also enables achieving varied shear properties, thermal coefficient, and enhanced mechanical and structural properties [270]. They are essential for preventing fluid thickening after multiple cycles of use, reducing particle oxidation, and avoiding equipment surface abrasion during operation [166,167,183].

Various solutions have been proposed to reduce the sedimentation rate, improve stability, and enhance the MR effect of MRFs, as reported in the literature [167,191,271]. These solutions include

• Coating particles with low-density materials [166,202,271].
• Using irregularly shaped particles such as wires and flakes [71,84,233].
• Using a bidispersion of nano- and micron-sized particles [125,178].
• Adding thixotropic agents (e.g., carbon fibers, silica nanoparticles) [83].
• Adding surfactants (e.g., oleic acid, stearic acid) [272].
• Adding magnetic nanoparticles (e.g., wire-like, spherical) [80,201,210].
• Using high-density carrier liquids: gels, ionic liquids, polymer liquids [83,273].

9. Conclusions

Although rheological and mechanical studies on magnetorheological elastomers (MREs) and fluids (MRFs) have been extensive over the past two decades, no rigorous analysis differentiating their mechanical behavior and dynamic properties under the influence of a magnetic field has been undertaken to our knowledge. Apart from minor aspects, such as composition and preparation methods, no in-depth comparative study has been presented. Consequently, the primary objective of this review was to provide a comprehensive comparative synthesis of the composition, preparation methods, and dynamic behavior characterization of these two materials. Current studies, as well as recent findings, which were extensively cited, on MREs and MRFs have been reported, summarized, and interpreted.

The main contributions of this review include the following.

• The various types of carrier fluids, magnetic particles, and additives and their influence on the properties of these materials have been discussed.
• An outline is given of common magneto-mechanical characterizations of MREs and MRFs, including dynamic tests, particularly shear, compression, and tension tests, conducted with and without a magnetic field.
• Among the reported studies, various experimental methods, such as rotational rheometry, double-lap shear tests, and dynamic mechanical analysis (DMA), were used. However, hysteresis characteristics, whether in stress–strain or force–displacement, were less frequently reported in the reviewed studies.
• A detailed account was given of the influence of operating conditions, such as strain amplitude and excitation frequency. Furthermore, a wide range of magnetic field intensities (magnetic sweep) and operating temperatures has been explored.
• The results discussed in these studies have generally been reported in terms of storage modulus, loss modulus, and loss factor Tan δ. Additionally, the constant shear magnetorheological properties of MRFs have been evaluated through yield stress and shear viscosity.
• A synthesis was presented in the form of a comparative analysis between MRFs and MREs.