Carlson ^[4] wrote a review article introducing commercial magnetorheological fluids (MRFs), made by Lord Company, stating a series of figures of merit. It introduced commercial MRFs and the first MRF application device, which was a small controllable brake applied to aerobic exercise equipment. In 1998, a small MR seat damper was commercialized for heavy-duty trucks, followed by several types of MR dampers for the suspension systems of race and sedan vehicles. The ingredients of the MRFs developed by Lord Corporation in 2004 and their field-dependent rheological properties are also presented. For example, an MRF consisting of a hydrocarbon carrier liquid and an iron particle volume fraction of 22% can produce a maximum yield stress of 23 kPa at 200 kA/m and a response time of less than 1 ms. Several advantages of semi-active controllable actuators, significantly triggered numerous research works, in diverse areas with a creative design philosophy, which replaced traditional systems or devices with high performance systems showing design simplicity, a low power consumption, and an excellent adaptability to various control strategies. Muhammad et al. ^[5] summarized several requirements for making high-performing MRFs, by stating principal properties including the lowest coercivity, the highest saturation magnetization, the fastest response time, non-abrasive particles, spherical-shaped particles, and high purity particles (carbonyl iron powder: CIP). In addition, the particle volume fraction and particle size dependence of the viscosity was described, explaining that a higher volume fraction, led to a higher viscosity, but also to a faster sedimentation due to the increased density. Some constitutive models of MRFs, relating to the shear stress versus the shear rate, were also given by classifying its geometry into concentric cylinder, parallel plate, cone and plate, and double concentric models. Ierardi and Bombard ^[6] introduced the optimized mixture method for CIPs and a carrier liquid (hydrocarbon) to achieve minimal off-state viscosity, as well as maximal field-dependent yield stress. In their investigation, three different BSAF CIPs were been used to obtain the proposed target, because the CIPs had several merits over other magnetic particles, such as excellent absorption of radar and microwaves, high purity, outstanding quality and consistency, reliable delivery, compatibility with most polymer matrices, and being easily compoundable. Therefore, CIPs are currently used to develop innovative solutions for a whole spectrum of different applications, including automotives, since BASF discovered the CIP processing recipe 85 years ago. Three samples were classified into Sample A (coarse), Sample B (medium), and Sample C (fine), and dispersing thixotropic additives were used in all mixture formulations. It was found in this investigation that Sample A produces the highest field-dependent magnetization effect, indicating that a larger particle size contributes to a high yield stress. de Vicente ^[7] comprehensively revisited the most salient properties of the field-dependent rheological fluids of MRFs, especially focusing on researchers’ understanding of flow motion, yield stress, and viscoelastic behavior under shear mode operation. Firstly, they reviewed the manufacturing methods of MRFs via traditional approaches, followed by new methods aimed at reducing particle sedimentation by reducing the particle size to the nanometer range. Then, they discussed the particle magnetization model (a constitutive model of MR fluids) from both a microscopic and macroscopic perspective, by using the Brownian motion, Mason number, Reynolds number, Peclet number, van der Waals body forces, Bingham model, Herschel–Bulkley model, and the Casson method. Subsequently, the relationship between the shear stress and shear rate, and between the viscosity and the Mason number were reviewed, at various magnetic field intensities, to understand the constitutive models of MRFs. Ashtiani et al. ^[8] reviewed the overall content of MRFs, especially emphasizing the different methods of preparation and stabilization, the field-dependent rheological models, and the applications. The MR effect was found to be characterized by a reversible increase in the viscosity and the yield stress, due to the introduction of a magnetic field, which can be explained by the particle chain formation. The MR effect could be controlled by the magnetic field’s intensity and the rheological characteristics of the MR fluid constituents. The review article pointed out the difficulty of practical usages, due to the particle sedimentation caused by the density mismatch between the magnetic particles and the carrier liquids. As a solution to mitigate this problem, several approaches were suggested: particle coating, the use of various additives, the use of various carrier liquids, the use of porose particles, the use of nano-sized particles, the use of different particle shapes and so forth. A detailed schematic image of the MR effect were shown, both with and without adding nanoparticles to MRF dispersed in the voids between microparticles, resulting in yield stress by strengthening the particle chains. The significance of the carrier liquid was also discussed, and one potential carrier liquid was suggested: polyethylene oxide (PEO), which is a widely used polymer since it is a linear polymer soluble in organic media. Other potential carrier liquids included hydro-carbonic oil, ferrofluid, silicone oil, mineral oil, ionic liquid, cedar wood oil, and so on.

Choi ^[9] comprehensively reviewed and analyzed the methods to improve sedimentation stability by altering the aspects of three ingredients: particle modification, carrier liquid modification, and the adjustment of additives. In addition, a few conceptual methodologies to prevent the sedimentation that occurs during the bottle’s storage and in the application systems were also discussed as possible obstacles to developing successful MR applications. For the particle modifications, several types of magnetic particles, including CIP, iron oxide, iron carbide, low carbon steel, silicone steel, nickel, and cobalt, were used and coated particles were employed; as for the carrier liquid, many types of carrier liquids aimed at reducing the density mismatch, such as mineral oil, modified silicone oil, hydrocarbon oil, polyalphaolefin (PAO), 1-ethyl-3-methylimidazolium diethyl phosphate, and 1-hexyl-3-methylimidazolium chloride, were investigated. The additives also play a crucial role in the sedimentation stability of MRF and, hence, the effect of using additives, such as thixotropic agent, carboxylate soap, antioxidant, lubricant, viscosity modifier, metal oxide powders, sulfur-containing additives, thioesters, xanthan gum, and stearate carboxylic acid, on MRF sedimentation were discussed. The article ^[10] also analyzed the sedimentation stability of MRFs, focusing on the use of different additives: iron oxide additives, ferrofluids, organic additives, carbon allotropes, and inorganic additives. In addition, the relationship between the carrier liquid and sedimentation was investigated by using several different carrier liquids: linear polydimethylsiloxane, hyperbranched polycarbosilane, CI dispersed in silicone oil (Si), synthetic oil (Sy), sunflower oil, and polytetrafluoroethylene (PTFE). Pei and Peng ^[11] summarized the diverse constitutive models of MRFs, which are useful tools for the prediction and analysis of MRFs’ field-dependent characteristics, such as yield shear stress and complex modulus. After describing the constituents and rheological properties of MRFs, two types of constitutive modeling methods were discussed: macroscopic models and microscopic models. The macroscopic models, which are generally data-based models, have been widely used due to their simplicity and accuracy. Among the many macroscopic parametric models, the Bingham model is the simplest, but provides useful properties, including the shear stress versus shear rate at different magnetic field intensities. The macroscopic parameter models include the biplastic model, Casson model, biviscous model, Herschel–Bulkley (H–B) model, Eyring model, Robertson model, Pa–Casson model, Mizrahi–Berk (M–B) model and so forth. The prediction of the accuracy of these models is dependent on many factors such as particle type and size, temperature, types of carrier fluids, and the properties of the additives. In general, the parametric models lose their prediction accuracy at both very high and low shear rates. Thus, there are several types of macroscopic nonparametric models, which can be applied for the prediction of the field-dependent shear stress of MRFs which are subjected to uncertainties of varying magnetic fields and varying currents. Unlike the parametric models, nonparametric models have difficulty expressing specific equations containing experimental coefficients. One of the best ways to deal with this nonparametric problem is to use the artificial neural network (ANN) and support vector regression (SVR) techniques. Using these methods, the time-varying data are trained and tested by defining the temperature and shear rate as the inputs and the shear stress as the output. Another approach to resolve the uncertainties is to use an extreme learning machine, which can demonstrate the properties of both the ANN and SVR with a smaller training time. Additionally, the nonlinearity and saturation of particle magnetization and the calculation of the resistance force of the chains in the field-on microscopic models have been studied by several scholars. Some representative microscopic models include the finite element model, micro–macro model, dipole model-based micro–macro model, initial tilt chain model, and the micro model based on a hexagonal closed packed structure.

Kumar et al. ^[12] summarized a few challenging methods for the development of advanced MRFs, considering the types of magnetic particles obtained by the chemical vapor deposition of iron pentacarbonyl, and the particle shapes that affect the wear on the walls of the container or device inside in which MRFs operate. To increase the sedimentation stability of MRFs, several methods were discussed: the use of coated particles and the use of additives such as carboxymethyl cellulose, polyethylene oxide, polyvinyl butyral, fibrous carbon, oleic acid, cholesteryl chloroformate, and magnetic nanoparticles. When the additives were used to reduce the sedimentation, the percentage of the volume or weight needed to be carefully adjusted. In addition, the fast time responses of MRFs, dependent on the input current and the friction between the particles in fluid flow, were carefully investigated. Matharu and Sehgal ^[13] summarized the field-dependent rheological MRFs focusing on the particle type, particle shape, and particle size, because these factors significantly influence the yield shear stress, sedimentation stability, wear and durability, and the magnetic intensity. Firstly, MRFs were classified into monodispersed, bi-dispersed, poly-dispersed, ferrofluid-based and dimorphic fluid-based, stating inherent characteristics such as sedimentation and off-state viscosity. Particulate materials included carbonyl iron, Fe₃O₄, cobalt, nickel, carbon nano tube, graphene oxide, glass, hard magnetic metal, soft magnetic metal, magnetic stainless steel, and alloy iron. The size of the particulates ranged from nanometers to 30–50 μm. The durability of MRFs caused several disadvantages, such as degradation due to the oxidation of the particles, degradation due to the wear of the particles, degradation due to breakage or spalling of the particles, and the decrease in loss modulus over time due to shear thinning. On the other hand, Khairi et al. ^[14] reviewed numerous studies that focused on the ability of these materials to alter their rheological properties in response to applied magnetic fields. In particular, the influence of additives on the rheological properties of MR solids, including MR elastomers and MR greases, were presented. It was shown that plasticizers soften the rubber matrix, increasing the MR effect by lowering the zero-field moduli, and carbon-based additives provide superior bonding with the rubber matrix and improve the dispersion of CIPs at the same time. Chromium-based additives enhance the stability of CIPs in the dispersion media by acting as coating agents, resulting in the prevention of agglomeration. It was also demonstrated that a combination of plasticizer, multi-walled carbon nanotube (MWCNT), and carbon black used during the fabrication of anisotropic MR elastomers enhance the MR effect. Notably, MWCNTs provide an enhancement of MR elastomers, MR gels, and MR plastomers. It was noted that, to obtain better additives to improve the MR effect in various aspects, mathematical relationships must be established to predict the best composition of additives, with systematic investigations on the interparticle forces between particles and additives, as well as additives and matrices.