Bidisperse magnetorheological fluids with strong magnetorheological response, long-term stability and excellent in-use performance

There is a critical demand for magnetorheological fluids (MRFs) with high particle loading, long-term stability, and high magneto-viscous properties to be used in industrial MRF devices. Bidisperse MRFs composed of highly magnetizable micron-sized carbonyl iron particles and poly(acrylic acid) coated superparamagnetic iron oxide nanoparticles (SPIONs-PAA) that can chemically interact are proposed to achieve such MRFs, here. Coating bare, commercial CI with lauric acid (LA) enhanced its dispersibility in a hydrophobic carrier fluid, allowed high magnetic loading and significantly prevented the sedimentation of the particles when mixed with 9–12 wt% SPION. Different carrier fluids (mineral oil, paraffin, and hydraulic oil) were tested, and hydraulic oil was determined as the best for this particle combination. The most stable bidisperse MRF was achieved at 83%–84% magnetic content with 12 wt-%SPION-PAA, LA-coated-CI and 3 wt% polyvinyl alcohol. Such MRFs outperformed the commercial benchmark, 140-CG® from Lord Corp., in long-term stability (4 months) and stability under dynamic loading. Bidisperse MRFs were stable between 20 °C and 60 °C. Most importantly, the excellent performance of the bidisperse MRFs in dampers designed for washing machines suggests that these MRFs may provide comparable damping forces with much better stability, ensuring longer shelf-life and longer lifetime in use.


Introduction
Magnetorheological fluids (MRFs) are smart materials that undergo a quick and reversible liquid-to-solid-like transition when placed in an external magnetic field, an outstanding property for many applications, including shock absorbers and clutches, engine mounts, flexible fixtures, etc [1][2][3][4][5][6].MRFs are non-Newtonian fluids consisting of magnetizable particles, usually micron-sized, in a carrier fluid, usually a hydrophobic oil such as hydraulic oil.These micron-sized particles are typically ferromagnetic, and in an applied magnetic field, they magnetize, and due to the strong interaction of magnetic dipoles, they align anisotropically in the direction of the magnetic field lines forming chain-like structures.This not only causes the liquid to semi-solid transition but also causes a magnetic field strength-dependent increase in the yield strength and resistance to deformation, such as shear flow, etc [7].
Major problems of MRFs are the sedimentation of highdensity micron-sized particles and the oxidation of magnetic particles, which weakens the magnetorheological (MR) performance.Therefore, various approaches were tested to improve MRFs' colloidal and oxidative stability.The use of alternative carrier fluids is one such approach.For example, using ionic liquids [8] or thixotropic oils [9].An alternative approach is to add a stabilizer like fatty acids [10] and fumed silica [11].Direct coating of the carbonyl iron (CI) has been tested, as well [12][13][14][15].Alternatively, adding nano-sized particles to MRFs consisting of micron-sized magnetizable particles has been suggested to improve MRFs stability, which are called bidisperse MRFs [16][17][18][19][20][21][22][23][24][25][26].Although some of these procedures have effectively reduced aggregation and precipitation, it has also been proved that the magnetic chaining responsible for the magneto-viscous effect can be weakened as the concentration of stabilizing agents increases [5].A summary of methods for enhancing MR properties of MRF discussed here is given in table 1.In addition, perfluorinated polyether-based bidisperse MRFs with superior off-state viscosity for prosthetic knee applications and clutches have been summarized in table SI [27][28][29][30].
In some of the studies mentioned above, researchers reported that stability improvement in MRFs was observed to be accompanied by a weaker MR response, suggesting a complex interplay between stability and MR performance.However, it is essential to note that this observation is not a universal statement, and various factors, such as particle size, surface coating, dispersion characteristics, and particle-particle interactions, can significantly influence the relationship between stability and MR response [16].The main focus of this study is to improve the long-term stability and redispersibility of MRFs with high magnetic particle loading without sacrificing the magneto-viscous properties of the designed MRFs to be used in industrial MR machines.To achieve this, we suggest bidisperse MRFs composed of magnetizable micron and nano-sized particles that can interact with each other and a small amount of polyvinyl alcohol (PVA) as an additive.Small superparamagnetic iron oxide nanoparticles (SPION) were produced in small sizes with a PAA coating capable of interacting with CI.Indeed, previously we have improved the stability of commercial MRF, Lord Corp. 140-CG ® by adding PAA coated SPIONSPION-PAA), however, there was still a significant amount of sedimentation in long-term [31].
In this study, we suggest preparing bidisperse MRFs with high magnetic content (above 80 wt%) from CI and SPION-PAA using different carrier fluids instead of trying to improve the commercial one.Firstly, we mixed SPION-PAA with bare CI to exploit the electrostatic interaction between micron-sized and nano-sized magnetic particles, aiming to improve stability without compromising the MR properties, even at high particle loadings.This approach demonstrated improved short-term stability compared to MRFs prepared solely with CI.Subsequently, we coated CI with lauric acid (LA) to enhance its dispersibility in the carrier fluid and prevent particle aggregation, thereby promoting better interaction between nano-and micron-sized particles.The MR response of the resulting bidisperse MRFs, along with their stability, specifically resistance to sedimentation, was comprehensively investigated at various compositions.Remarkably, the stability of bidisperse MRFs produced from LA-coated CI and SPION-PAA, with the additional use of PVA as a stabilizer, exhibited a substantial improvement over the benchmark, 140-CG, highlighting the efficacy of our proposed modifications.The MR properties of bidisperse MRFs were also measured at different temperatures since the stability of MRFs for hightemperature applications is also a concern [32][33][34].Lastly, the performance of the bidisperse MRFs in an MR damper was also compared with the commercial MRF 140-CG suggesting better stability, good high-temperature performance, and comparable performance in an MR damper.This new bidisperse MRFs may be accepted as new, stable MRFs for practical MR applications.
The procedure detailed before was adopted for the synthesis of PAA coated SPIONs [36].Simply 0.496 mol (9.16 g) FeCl 2 .4H 2 O and 0.993 mol (25 g) FeCl 3 .6H 2 O were mixed in Ar purged 500 ml de-ionized (DI) water and heated up to Higher damping force when used in a MR damper [35] 80 • C.Then, 0.0038 mol (20 g) PAA was added to the solution, followed by 77 ml ammonia solution.After 30 min of reaction, the black solution was cooled down and placed on a magnet overnight.Any precipitate was removed by decantation, and the colloidal solution was washed using centrifuge filters (10.000 kDa Sartorius) with DI water.The total volume was replaced twice by DI water.Nanoparticles were dried by a rotary evaporator (Laborota 4000) at 60 • C under 60 rpm rotation speed.

Lauric acid coating of carbonyl iron (CI-LA)
Commercial CI (200 g) was treated with 500 ml of 0.5 M HCl for 10 min and then washed with deionized water (5 × 600 ml), ethanol (3 × 250 ml), and finally, acetone (3 × 200 ml each time) using a decantation method.Then, the residual solvent was removed using a rotary evaporator at 60 • C (Heidolph) to remove [37].Next, surface-activated dry CI powder (190 g) and 600 ml ethanol were put into a 1 l three-neck roundbottomed flask equipped with a mechanical stirrer and a reflux condenser and heated in an oil bath at 110 • C. Lauric acid (LA) (25 g) was added to the flask when the mixture's temperature reached 80 • C and kept stirring at 600 rpm for 6 h.Then, the solution was cooled down to room temperature, and the particles were removed and washed with DI water three times using the decantation method.Finally, LA coated CI particles were dried at 60 • C using a rotary evaporator.

Preparation of MRFs
Bidisperse MRFs composed of CI and SPION-PAA or CI-LA and SPION-PAA with 9-12 wt% nanoparticle and a total of 80-84 wt% particle loading were prepared in hydraulic oil (HO).Briefly, micron-sized particles were added to the carrier fluid at different wt% and dispersed with a light-duty homogenizer (ISOLAB) at 12 000 rpm for 1 min.Then, SPION-PAA was added and homogenized for 1 min.When PVA was used in the MRF formulation, first PVA (3 wt% of the carrier fluid) was added to the carrier fluid, homogenized for 1 min, then CI-LA and SPION-PAA were added sequentially and homogenized for another minute after each particle addition.
To test the impact of different carrier fluids at fixed particle loading and CI-LA/SPION ratio hydraulic oil (HO) was replaced with mineral oil (MO) or paraffin.a Hydraulic oil (HO) and mineral oil (MO) as carrier fluids.

Characterization methods
The hydrodynamic size of SPIONs was measured using Malvern Zetasizer dynamic light scattering instrument.Crystal sizes of particles and morphology were analyzed using Tecnai G2 F30 bright field high resolution (acceleration voltage = 200 kV) transmittance electron microscopy (TEM) and ZEISS Ultra plus scanning electron microscopy (SEM).
For the SEM analysis, MRFs were washed with methanol and dried at 60 • C. SPIONs were dropped cast on C-coated Cu-grids for the TEM analysis.Magnetic properties of particles were measured at 305 Kelvin with Quantum Design PPMS9T vibrating sample magnetometry.TA Q500 thermogravimetric analysis was used to determine the organic content of SPIONs by heating samples between room temperature and 900 • C at a heating speed of 10 • C min −1 under Argon gas.Thermo Scientific Nikolet iS 10 FTIR instrument was used for the functional group analysis of particles.
MR properties of MRFs were measured by 302 MCR Anton Paar rheometer.MRF`s mechanical behavior was studied in steady shear and frequency sweep modes using a tween gap parallel plate.The diameter of the plate was 10 mm and the gap between the two plates was 0.339 mm for all the measurements.Before each MR measurement and data collection, approximately 0.1 ml of the MRF was carefully dispensed onto the lower plate using a clean syringe.To ensure proper dispersion and consistency, a pre-shearing process of 0.05 s −1 was applied for 10 s without the application of a magnetic field.This preliminary pre-shearing step was implemented to achieve optimal particle dispersion within the MRF, ensuring reliable and consistent results during subsequent measurements.
The Bingham model was used to calculate the yield stress of MRFs as a function of magnetic field strength.In this method, MRFs' measured shear stress-shear strain curves were used to calculate their yield stress [38][39][40].According to the Bingham model given in equations ( 1) and ( 2), the shear stress is given by: where τ is shear stress, τ y is the yield stress under magnetic field, η p is the Bingham plastic viscosity defined as the slope of the flow curve and γ is shear rate.The sedimentation ratio was calculated using equation ( 3) [41] Sedimentation ratio (%) = height of the particlerich phase height of the entire fluid ×100. (3)

Synthesis and characterization of magnetic particles
SPION-PAA with average crystal sizes of 3 nm and average hydrodynamic size of 120 nm (SPION-PAA) were obtained (supp.info.figure S2).SPION-PAA were synthesized with a co-precipitation method in aqueous solutions.Surface functionalization with PAA provided a hydrophilic coating with carboxylate functional groups on the nanoparticle surface.The average crystal size of the SPION-PAA was measured as 3 nm by TEM (supp.info.figures S1(a)-(d)), and an average hydrodynamic size of 120 nm (SPION-PAA) was determined by DLS (supp.info.figure S2).The presence of a polymeric coating, polymer conformation, bound solvent and formation of small clusters may contribute to such larger hydrodynamic size than the crystal size [42,43].The organic content of SPION-PAA was determined as 27 wt% by thermogravimetric analysis (supp.info.figure S3).We have confirmed the PAA coating of SPIONs with vibrational spectroscopy: FTIR spectrum of SPION-PAA has a band at ca 535-580 cm −1 corresponding to Fe-O bond vibration and peaks at ca 1570 cm −1 for the carboxylate groups adsorbed on SPION crystals.C-H stretching bands were around 2800-3000 cm −1 and O-H stretching bands were at 3200-3400 cm −1 (supp.info.figure S4).CI was coated with LA to improve its dispersion in hydrophobic carrier fluids.FTIR spectrum of CI-LA indicates the presence of LA with C=O stretching at 1400-1700 cm −1 and C-H bands at 2800-3000 cm −1 (supp.info.figure S5).SEM images of 140-CG LORD ® and CI reveal the presence of 1-6 µm size particles (figures 1(a) and (b)).SPION-PAA nanoparticles adsorbed on the micron-sized CI-LA particles (MRF-5) (figure 1(d)) can be easily seen in the SEM images.This suggests that the PAA coating on SPIONs provided an attractive interaction between the nanoparticles and micronsized ones [36,44].Part of the carboxylate groups on the surface of PAA coated SPIONs may adsorb on the CI-particles due to strong attraction between Fe and carboxylate groups.In the case of CI-LA, this attraction may displace the LA from the surface, providing an electrostatic binding between SPION and CI [45].
The value is within different M sat values reported for such nanoparticles [42,43].All micron-sized magnetic particles, 140-CG LORD ® , CI, and CI-LA, possess 205 emu g −1 saturation magnetization, so it can be assumed that LA coating of CI as a non-magnetic organic material has not reduced the magnetization of CI significantly, which is highly desirable.Magnetization of bidisperse MRF-5 is slightly weaker than CI and CI-LA since SPION has lower magnetization than CI.

Stability of MRFs
Initially, CI was dispersed in hydraulic oil at 80-85 wt% particle loading as a homemade reference material.Bidisperse MRFs were then prepared with CI-LA, which is more dispersible in the carrier fluid than the bare CI.MRF-5-7-8 have the same particle composition, 71 wt% CI-LA, 12 wt% SPION-PAA and 3 wt% PVA in hydraulic oil, mineral oil, and paraffin, respectively.After four months at rest, MRF-5 did not show any sedimentation, while MRF-7 and −8 showed clear sedimentation starting in 1st week and after 10 d, respectively (figure 3(b)).Hence, hydraulic oil with the highest viscosity emerged as the best carrier fluid to ensure the stability of the bidisperse MRFs composed of SPION-PAA and CI-LA.A comparison of MRF-3 and MRF-5 shows improved stability of the latter, which not only has PVA but also uses CI-LA instead of CI at even higher particle loading.This suggests a positive impact of LA coating on CI on the stability of highly loaded bidisperse MRFs.Lauric acid coating of CI particles in bidisperse MRFs further enhances stability and has an impact on the MR effect.Lauric acid, as a surfactant,  enhances the dispersibility of CI particles in the carrier fluid.It reduces the propensity of CI particles to form aggregates, which can result in uneven MR responses and settling problems.The enhanced dispersibility and reduced aggregation of CI-LA improve the interaction between the magnetic particles, which results in more effective chain formation under the applied magnetic field.The stronger and more predictable MR effect produced by this improved particle arrangement is essential for obtaining accurate and dependable performance in MR devices and systems [46][47][48].MRF-4 had a 1 wt% higher magnetic loading than MRF-5 but with lower SPION-PAA content and did not show the same stability as MRF-5, indicating the role of SPION-PAA in the long-term stability of the bidisperse MRFs.However, MRF-4 stayed stable for 10 d, and then about 5% sedimentation was observed (figure 3(b)).PVA based aqueous ferrogels are known in the literature utilizing physical or chemical crosslinking of PVA providing soft gels stabilizing particles and preventing agglomeration but at the same time allow particle movement under magnetic field.PVA also provides particle stabilization via surface adsorption and steric stabilization and aids reducing density, hence, reduce sedimentation [49,50].Here, its possible interaction with CI-LA and SPION-PAA via adsorption and H-bonding may be improving the stability, preventing long-term agglomeration and settling.Our bidisperse MRFs as a result have improved long-term stability, making them ideal for a variety of real-world applications where stability is important [46,48,[51][52][53].
Based on these results, further studies were performed with MRF-4, 5 and 6.MRF-6 was produced as a replica of MRF-5, but it had 72 wt% CI-LA instead of 71 wt%.

Rheological properties
MR performances of MRFs were investigated under various magnetic fields.Shear stress was measured as a function of shear rate at various magnetic field strengths.All samples exhibit consistent behavior across the complete shear rate range.However, it is important to clarify that the flow curves of concentrated suspensions, as evidenced by the double logarithmic representation employed, typically deviate from linearity due to their complex nature (figures 4 and 5).All MRFs studied here show typical MR behavior.Shear stress as a function of shear rate rises with the applied magnetic field for all samples due to the formation of particle chains due to strong dipole-dipole interaction along the direction of magnetic field to resist the external shear forces [25,31,54,55].In figures 4(a)-(d), bidisperse MRFs stress vs. shear rate with different carrier fluids under different magnetic fields are compared.It is evident that as the applied magnetic field increases stress increases under the applied shear rate.This can be attributed to the formation of robust columns within the fluid, a consequence of the stronger dipole-dipole interactions occurring between adjacent magnetic particles under higher applied magnetic fields [19].As the applied magnetic field increases from 125 kA m −1 to 150 kA m −1 , the shear stress increase is not significant.Most probably the saturation of the sample was already reached, so the use of higher intensities (150 kA m −1 ) does not have a significant effect on the rheological properties [8,10].MRF-5, 7 and 8 represent a set of bidisperse MRFs that differ only in the carrier fluid.In that case, the shear stress increased in the order of hydraulic oil, mineral oil, and paraffin demonstrating that a higher oil viscosity corresponds to a more significant MR effect [56].Overall, hydraulic oil with the highest shear stress under different magnetic field strengths gave the best MR response and enhanced stability.
It is also valuable to compare the shear stress of 140-CG and MRF-1, -2, which are not bidisperse, and were Overall, decreasing shear stress with decreasing CI content of the fluid could be explained by weaker chain formation between the particles in a diluted solution, while the same behavior can be seen in related articles with MRFs containing different content of CI loadings [8,12,13,18,23,[57][58][59].On the other hand, adding SPIONs to CI alters the shear stress-shear rate behavior and stability.When comparing the MRF-1 and MRF-2 with 85 wt% CI loading with bidisperse MRF-3, (mixture of 68% CI and 12% SPION-PAA), stability is enhanced dramatically, but shear stress decreased significantly.Increasing the CI content by using CI-LA to 71-72 wt% along with SPION-PAA and 3 wt% PVA (MRF-4-5-6) increased the shear stress again to the level of MRF-2 with 80 wt% CI only, but more importantly, provided dramatically improved stability compared to poor stability of MRF-2.Among these three bidisperse MRFs, the best stability was observed in MRF-5 and MRF-6, which may be related to higher SPION content (12 wt%); MRF-4 had 9 wt% SPION.An increase in the shear stress with additives was reported in several publications, and the increase in shear stress was assigned to friction or flocculation-induced enhanced interaction between particles [60].In our samples, PAA coating on SPION may strengthen the interaction between particles, which results in higher shear stress.increased slightly with the addition of SPION-PAA at low shear.On the other hand, bidisperse MRFs prepared with CI-LA, SPION-PAA and PVA addition (MRF 4,5,6) displayed significantly higher viscosity, which is higher with higher SPION-PAA content (12 wt % in MRF 5 & 6).Increasing interaction of the CI-LA with the carrier fluid, interaction of As expected, at a given shear rate, the apparent viscosity of MRFs rises with an increase in the applied magnetic field.This phenomenon is attributed to the creation of chain-like structures, a result of the attractive interaction between the magnetic particles [22].At a magnetic field strength of 33 kA m −1 , bidisperse MRF5 and MRF6 exhibit the highest viscosities.Among all samples, the highest viscosity is observed for commercial 140-CG MRF at an applied magnetic field higher than 33 kA m −1 due to higher magnetic content and higher saturation magnetization of the MRF.The bidisperse MRF5 and MRF-1 with no CI surface coating have higher viscosities after 140-CG MRF.The elevated viscosity in MRF-1 can be attributed to its higher content of CI particles (85 wt%), as well as a robust saturation of magnetic particles and stronger bonds within the carrier fluid.Conversely, in the case of bidisperse MRF-5, the increased viscosity results from the effective filling of the spaces between CI-LA particles with SPION-PAA.This filling enhances the interaction between particles, leading to an increase in friction and a consequent rise in flow resistance [22].The viscosity of MRFs was observed to decrease as the shear rate increased, indicating a clear shearthinning behavior [61].A noticeable difference was observed between bidisperse MRF-5, 7 and 8 (figure 7), which have the same compositions but were in different carrier fluids with different viscosities that as the viscosity of carrier fluids increase MR effects increase consequently [56].At all magnetic field strengths and in the absence of magnetic field, the viscosity decreased in the order of MRF-5, MRF-7, and MRF-8, indicating once more that bidisperse MRF in hydraulic oils better for stability and for MR properties, as well.
Apparent viscosities at different magnetic field strengths under constant 250 1/s shear rate are given in figures 8(a)-(c).As expected, the apparent viscosity of all MRFs increases as the applied magnetic field strength increases showing MR effect.This is because the magnetization of the particles increases with the increasing strength of the applied magnetic field, which in turn causes a stronger alignment of the particles in the direction of the applied field [62].
Yield stress is the minimum stress required for transforming from solid-like to fluid-like behavior.At the yield stress, the internal structure is broken.The yield stress was calculated using the Bingham model, and its dependence on the magnetic field is given in figures 9(a)-(c).The results presented in figure 9 reveal noteworthy insights into the dynamic yield stress characteristics of the MRFs.When compared to MRF-1 and MRF-2, both characterized by high CI loading (85 and 80 wt%) and MRF-3, all utilizing uncoated CI particles, 140-CG demonstrates slightly higher dynamic yield stress under different magnetic field strengths and a consistent shear rate of 250 1/s (figure 9(a)).Several factors may contribute to this observation, such as the specific CI particle type employed, its saturation magnetization, or distinctions in carrier fluid viscosity.
Bidisperse MRF-5 and MRF-7, operating in hydraulic oil and mineral oil environments, exhibit significantly higher yield stress values, particularly at magnetic field strengths below approximately 60 kA m −1 (figure 9(b)).This phenomenon suggests the development of stronger and thicker alignments of micron-sized CI-LA and nano-sized SPION-PAA particles along the direction of the applied magnetic field, indicating an enhanced MR effect.In contrast, at higher magnetic fields, 140-CG surpasses the others in terms of yield stress, owing to the greater saturation magnetization of its bare CI particles.
Notably, bidisperse MRF-4, despite having a 1 wt% higher magnetic particle loading than MRF-5, yet lower SPION-PAA content (9 wt%), exhibits lower yield stress (figure 9(b)).This discrepancy underscores the influential role played by SPION-PAA in determining the MR behavior of these fluids.Furthermore, when comparing the dynamic yield stress of bidisperse MRF-5, MRF-7, and MRF-8 in different carrier fluids (figure 9(c)), it becomes evident that MRF-5, utilizing hydraulic oil, exhibits superior MR effects at higher magnetic field strengths, aligning with the trends observed in shear stress and viscosity results.These findings collectively contribute to a comprehensive understanding of the dynamic yield stress behavior in diverse MRF formulations and environments.
The effect of temperature on the yield stress of CI-LAbased bidisperse MRFs was also investigated at 20 • C-45 • C-  Oscillatory experiments were conducted to explore the frequency-dependent viscoelastic characteristics of both the commercial 140-CG MRF LORD ® and the CI-LA-based bidisperse MRFs (MRF-4, -5, -6).The influence of strain, magnetic field, and frequency on these MRFs was examined.Figures 11(a)-(f) presents the results of storage modulus and loss modulus as functions of angular frequency within the range of 0.1-100 rad s −1 under applied three different magnetic field strengths (92, 167, and 206 kA m −1 ).
Under a 10% strain at lower frequencies (figures 11(a), (c) and (e)), the shear storage modulus remains relatively steady until it encounters a specific frequency denoted as the critical frequency ω cr .Beyond this point, the storage modulus gradually declines as some of the chains or clusters within the MR fluids break, signifying a shift towards a more viscous response at higher frequencies, as indicated by an increase in the loss modulus [63,64].
When subjected to a 100% strain, both storage modulus and loss modulus were dropped down, regardless of the applied magnetic field strength (figures 11(b), (d) and (f)).This behavior is primarily attributed to the disintegration of aggregated networks as unperturbed chains of magnetic particles begin to fragment.
Regarding the effect of applied magnetic field strength, as the magnetic field strength intensifies under both 10% and 100% strain, storage modulus increases.This increase is attributed to enhanced interactions between magnetic particles, indirectly resulting in elevated mechanical stiffness owing to the MR effect and a more substantial elastic component [64][65][66].These changes in modulus reflect the intricate interplay of strain, frequency, and magnetic field strength, unveiling the complex behavior of these 140-CG and bidisperse MRFs.Indeed, the change in the storage and loss modulus of 140-CG is larger than the bidisperse MRFs.Table 3 summarizes the dynamic yield points of MRFs based on applied frequency under 100% strain and different applied magnetic field strengths.Results show that synthesized bidisperse MRFs have a higher yield point than 140-CG commercial LORD ® .
It is essential to acknowledge that such experiments, especially at large deformations, introduce complexities due to the nonlinear viscoelastic nature of materials.In this regard, our research is primarily focused on the practical application of MRFs in real-world scenarios, where the materials often experience non-ideal conditions that are challenging.Results obtained in this study are more indicative of the performance of MRFs in practical applications rather than comprehensive physical characterizations.

Use of bidisperse MRFs in a magnetorheological (MR) damper for washing machine
After studying and analyzing the MR behavior of each synthesized bidisperse MRF sample, the sample with the best possible characteristics, such as; MR response, good stability, and high yield strength, was selected and tested in the MR damper application.For this purpose, a shear mode MR damper having a sponge filled with MRF sample was designed for the vibration suppression of the washing machine.A 3D CAD model of MR damper is shown in figure 12(a), whereas a 2D crosssectional view of the double coil MR damper is provided in figure 12(b).The MR gap height is 2 mm which is occupied by a sponge layer filled with MRF sample.The axial length of side and middle poles are 6 mm and 12 mm, respectively.For the sake of conciseness, the magnetic flux lines are shown which emerge from the side poles of MR damper, pass through MR sponge and cylinder, and then converge at the middle pole of MR damper.Two MR dampers were manufactured and further tested in the washing machine at the Manufacturing and Automation Research Center, Koç University.The vibration patterns and noise levels of the washing machine were demonstrated in our previous work [67,68], however, the details of damping force at different values of electric current for various synthesized MRF samples are provided here.
An experimental setup was established, as shown in figure 13.The MR damper is actuated by a linear actuator with different command signals.Since the strength of the magnetic field is directly dependent on the electric current, the electric current is supplied by the DC power supply to energize the double coil of MR damper in real time.The magnetic field strength value is varying along the surface of electromagnetic poles at a particular value of electric current, the mean values of magnetic field strength at the middle and side poles of the piston are provided in table 4. The generated damping forces are measured by the table-type dynamometer.Three MRF samples were tested in the MR damper at five different values of applied electric current.For the comparative analysis of bidisperse MRF samples with commercial MRF, damping force values were recorded at different values of electric current by using the commercial MRF 140-CG from LORD ® as a benchmark.The peak values of the damping force at each value of electric current are presented in table 4.
It is observed that MRF-3 sample showed the lowest offstate force value at 0 A, which is desirable for the washing machine application operating at high RPMs as mentioned by Ulasyar and Lazoglu [68].However, at higher electric current values such as 0.4 A, 0.5 A and 0.6 A, it showed lower amplitude of force values than the other samples.On the other hand, MRF-4 and MRF-5 showed better performance in terms of damping forces when the damper is operated at high values of electric current.According to the study reported in [68], higher values of damping forces were effective, when the washing machine encountered resonance.Although the offstate force values of 21.01 N and 20 N for MRF-4 and MRF-5 samples are slightly higher than the off-state force value of 14.03 N for MRF-3 sample, it did not affect the overall performance of the washing machine operating at higher RPMs.Therefore, MRF-5 would be the best selection for the MR damper, because the force values of MRF-5 (83 wt% magnetic content and only 71% LA-CI) are very close to the commercial MRF 140-CG (85.4 wt % Fe) with less than 10% difference at 0.3 and 0.4 A, and with less than 3% difference at 0.5 and 0.6 A.
In addition to that, the hysteresis curves of force versus velocity and force versus displacement of one of MRF samples (MRF-5) are shown in the figures 14(a) and (b) which are the part of a previously published article by our research group [67].It is important to mention that the characteristics of MR damper were analyzed by using one of our best MRF sample (MRF-5) and the experimental setup given in figure 13.The linear actuator was given a sinusoidal signal of amplitude of 5 mm and frequency of 1 Hz.The hysteresis behavior of     of MR damper was recorded at 0.6 A due to the high area value of hysteresis loop [67].Keeping in view the aforementioned characteristics of MRF 5 sample, it has comparable MR properties to the commercial one with significantly lower content of the micronsized particles, and with better shelf-life and, equally important, a better lifetime in use due to the improved sedimentation resistance thanks to SPION-PAA.

Conclusion
In this work, different bidisperse MRFs using CI and SPIONs were prepared.The performance of the new MRFs was compared with the commercial 140-CG ® from Lord Corp. CIbased MRFs in hydraulic oil at 80-85 wt% loading, similar to the commercial one, did not provide good stability, which was improved with the addition of SPION-PAA (MRF-3).It is suggested that anionic SPION-PAA adsorbed on the CI surface with carboxylate groups preventing interaction of micronsized particles with each other, however since the content of the CI decreased, MR properties such as viscosity, shear stress, and yield strength fell below the commercial one.Then, a new set of bidisperse MRF was prepared using LA coated CI for better dispersion and with 3 wt% PVA, which allowed an increase in the CI content to 71-72 wt% along with 9-12 wt% SPION-PAA.Among these, MRF-4,-5,-6 with 12 wt% SPION-PAA showed excellent stability coupled with MR performance similar and even better than the commercial one.The best formulation, which consists of 71-72 wt% CI-LA, 12 wt% SPION-PAA, and 3 wt% PVA in hydraulic oil, showed complete stability for 4 months at rest.
The influence of carrier fluids was studied using 71 wt % CI-LA and 12 wt% SPION-PAA particle loading.Among different carrier fluids studied, mineral oil, paraffin and hydraulic oil, hydraulic oil was best in stabilizing particles and achieving good MR properties.Bidisperse MRFs were also stable at high temperatures.The performance of the new bidisperse MRF was successfully demonstrated in MR dampers [68,69].
In other real-world applications, it should be kept in mind that gap differences may influence the results.
Here, it was demonstrated that bidisperse MRFs produced by all magnetic particles with interacting surfaces provide significantly enhanced resistance to sedimentation, temperature stability and desirable MR behavior to be utilized in practical applications.Overall, the new bidisperse MRF-5 outperformed the commercial benchmark.
Sedimentation of MRF-1,-2 and -3 in the first 48 d are shown in figure 3(a).MRF-1 and MRF-2, which have 85 and 80 wt% CI in hydraulic oil, showed significant sedimentation, and sedimented completely after 60 d.Stability improved dramatically when 12 wt% SPION-PAA (MRF-3) was added to CI (68 wt%) in hydraulic oil.MRF-3 showed dramatic enhancement in stability with about 5% sedimentation after 48 d and no further sedimentation in the next 4 months.Overall, MRF-3 was slightly better than 140-CG.

Figure 3 .
Figure 3. Time-dependent sedimentation ratio of the commercial MRF and bidisperse MRFs (a) containing bare CIs or (b) CI-LA.Insets: (a) picture of the MRFs after 48 d, (b) after 4 months.

Figure 9 .
Figure 9. Dynamic yield stress versus magnetic field strength of (a) CI-based MRFs, (b) bidisperse.MRFs in hydraulic oil as a carrier fluid, (c) bidisperse MRFs in different carrier fluids.

Figure 13 .
Figure 13.Experimental setup for testing MRF samples in MR damper.

Figure 14 .
Figure 14.(a) Hysteresis graph of force vs velocity (a) and force vs displacement (b) for changing value of input current of MRF-5 sample in the MR damper.[68] John Wiley & Sons.© 2021 John Wiley & Sons, Ltd.

Table 1 .
Summary of methods developed to produce improved MRFs.

Table 2 .
Composition of the synthesized MRFs and commercial LORD 140-CG ® as a benchmark.

Table 4 .
Damping forces of the MR damper by using different bidisperse MRF samples at various values of supplied current.damperwasrecordedwithrespect to different values of electric current (0 A, 0.3 A, 0.4 A, 0.5 A, and 0.6 A) as shown in figures 14(a) and (b).It is evident from figure14(a) that a significant hysteresis was observed during the MR damper's operation in the pre-yield region.Moreover, due to the MR damper's operation in the post-yield region, the rate of change of total damping force was comparatively low with respect to velocity.Besides this, it was noticed in figure14(b) that the area of force versus displacement hysteresis loop was changed by varying the electric current.The highest dissipated energy MR