Simulation of the optimal diameter and wall thickness of hollow Fe3O4 microspheres in magnetorheological fluids

Nanosphere Chain Model of Magnetic Fluid and Its Dynamic Performance

The magnetorheological effect is a critically important mechanical property of magnetic fluids. Accurately capturing the macroscopic properties of magnetorheological fluids with elongated particle forms, such as nanosphere chains, remains a challenging task, particularly due to the complexities arising from particle asymmetry. Traditional particle dynamics primarily utilize spherical particles as computational units, but this approach can lead to significant inaccuracies, especially when analyzing nonspherical magnetorheological fluids, due to the neglect of particle asymmetry. In this work, an advanced particle dynamics model has been developed by integrating the rotation and collision of these asymmetric particles, specifically tailored for the configuration of nanosphere chains. This model exhibits a significant reduction in error by a factor of 3.883, compared to conventional particle models. The results demonstrate that alterations in the geometric characteristics of magnetic nanosphere chains can cause changes in mesoscopic structures and magnetic potential energy, thereby influencing the mechanical properties at the macroscopic level. This work has developed an accurate mesoscopic simulation method for calculating chain-type magnetorheological fluids, establishing a connection between mesoscopic structures and macroscopic properties, and unveiling the tremendous potential for accelerating the design of next-generation magnetic fluids using this approach.

Magnetorheological properties of Fe-Co nanoparticles with high saturation magnetization and low coercivity

Fe-Co alloys exhibit an excellent saturation magnetization, which makes them become a potential candidate for the high property magnetic particles in magnetorheological fluids (MRFs). How to decrease their coercivity and residual magnetization without sacrificing the saturation magnetization is a crucial problem to be solved. In this study, Fe-Co nanoparticles were prepared by DC arc discharge and further disposed through low temperature annealing in Ar atmosphere. The successful synthesis of Fe-Co nanoparticles was proved by x-ray diffraction and EDS. The vibrating sample magnetometer results revealed that the prepared Fe-Co nanoparticles had a saturation magnetization of 208 emu g^-1, while the coercivity and remanent magnetization were 58 Oe and 5.8 emu g^-1, respectively. The MR properties of Fe-Co nanoparticles based MRFs (FeCoNP-MRFs) with 10% particles by volume fraction were systematically investigated. The FeCoNP-MRFs showed up to 4.61 kPa dynamic shear stress at 436 kA m^-1magnetic field and an excellent reversibility. The MR properties of FeCoNP-MRFs were fitted well by Bingham and power law model, and described by Seo-Seo and Casson fluid model. Meanwhile, the sedimentation ratio of FeCoNP-MRFs was still 87.3% after 72 h, indicating an excellent sedimentation stability.

Mechanism analysis of the carrier viscosity effect on shear stress of magnetorheological fluids

Shear stress is an important index to evaluate the rheological behavior of magnetorheological fluids (MRFs), which is not only related to the properties of ferromagnetic particles, but also the viscosity of the carrier. However, the research related to the carrier viscosity is quite lacking, and the mechanism of its effect on shear stress is still unclear. In this work, the carrier viscosity effect on the microstructure of MRFs under shearing was investigated via numerical simulations, and the relationship between chain inclination and carrier viscosity was presented for the first time. It was found that the deflection angle of the chain increases with the increase of carrier viscosity. Based on the simulation results, the relationship between the shear resistance induced by the magnetic field and the deflection angle of the chain was studied. Finally, a constitutive model incorporating the mechanism of the viscosity effect on shear stress was proposed, and the calculated results agreed well with the experimental data. This work provides new insights into the effect of carrier viscosity and can help us to better understand the corresponding microscopic mechanism.

Molecular Dynamics Simulations and Experimental Studies of the Microstructure and Mechanical Properties of a Silicone Oil/Functionalized Ionic Liquid-Based Magnetorheological Fluid

Magnetorheological (MR) fluids are smart materials that show enormous potential in vibration control, mechanical engineering, etc. However, the effects of the solid-liquid interface strength and the interaction strength between carrier liquid molecules on the mechanical properties and sedimentation stability of MR fluids have always been unresolved issues. This work presents a new type of MR fluid that has a novel carrier liquid, i.e., silicone oil (SO) mixed with a hydroxyl-functionalized ionic liquid (IL-OH). An all-atomic Fe/SO/IL-OH interface model for studying the relationship between mechanical properties and interface strength and intermolecular interactions is established. On the basis of simulation results and theoretical analyses, the mechanical properties and sedimentation stability of the SO/IL-OH-based MR fluids are thoroughly investigated by experiments. The results show that functional ionic liquids significantly improve the mechanical properties and sedimentation stability of MR fluids. These results are essentially attributed to the stronger solid-liquid interface strength, van der Waals forces, and hydrogen bonds between the silicone oil and the functional ionic liquid. The explicit results not only help elucidate the numerous phenomena involved in the research process for MR fluids at the atomic scale but also provide insightful information on the fabrication of high-performance MR fluids.

Magnetorheology: a review

Magnetic Soft Matter is a rapidly evolving discipline with fundamental and practical interest. This is due to the fact that its physical properties can be easily controlled through external magnetic fields. In this review paper, we revisit the most recent progress in the field (since 2010) emphasizing the rheological properties of these fascinating materials. New formulations and flow kinematics are discussed. Also, new members are integrated into the long-lived magnetorheology family and suggestions are provided for future development.

Experiments and Simulations on the Magnetorheology of Magnetic Fluid Based on Fe3O4 Hollow Chains

This work reports an experiment/simulation combination study on the magnetorheological (MR) mechanism of magnetic fluid based on Fe₃O₄ hollow chains. The decrease of shear stress versus the increasing magnetic field was observed in a dilute magnetic fluid. Hollow chains exhibited a higher MR effect than pure Fe₃O₄ hollow nanospheres under a small magnetic field. A modified particle level simulation method including the translational and rotational motion of chains was developed to comprehend the correlation between rheological properties and microstructures. Sloping cluster-like microstructures were formed under a weak external field (24 mT), while vertical column-like microstructures were observed under a strong field (240 mT). The decrease of shear stress was due to the strong reconstruction process of microstructures and the agglomeration of chains near the boundaries. The chain morphology increased the dip angle of microstructures and thus improved the MR effect under a weak field. This advantage made Fe₃O₄ hollow chains to be widely applied for small and low-power devices in the biomedical field. Dimensionless viscosity as a function of the Mason number was collapsed onto linear master curves. Magnetic fluid in Poiseuille flow in a microfluidic channel was also observed and simulated. A qualitative and quantitative correspondence between simulations and experiments was obtained.

Shape-Controlled Syntheses of Magnetite Microparticles and Their Magnetorheology

Magnetic microspheres in a concentrated suspension can be self-assembled to form chain structures under a magnetic field, resulting in an enhanced viscosity and elasticity of the suspension (i.e., the magnetorheological (MR) effect). Recently, interest has been raised about the relationship between nonspherical particles, such as octahedral particles and the MR effect. However, experimental studies have not made much progress toward clarifying this issue due to the difficulty associated with synthesizing microparticles with well-defined shapes and sizes. Here, we presented a method for the shape-controlled synthesis of magnetite (Fe₃O₄) microparticles and investigated the MR effects of two suspensions prepared from the two shape-controlled samples of Fe₃O₄ microparticles. Our method, which was based on the polyol method, enabled the preparation of spherical and octahedral Fe₃O₄ microparticles with similar sizes and magnetic properties, through a reduction of α-FeOOH in a mixed solvent of ethylene glycol (a polyol) and water. The water played an important role in both the phase transition (α-FeOOH to Fe₃O₄) and the shape control. No substantial difference in the MR effect was observed between an octahedral-particle-based suspension and a spherical-particle-based one. Therefore, in this study, the shape of the microparticles did not strongly influence the MR effect, i.e., the properties of the chain structures.

Yielding behavior of model magnetorheological fluids

The yielding behavior of magnetorheological fluids is revisited through the use of finite element method calculations on model structures and carefully conducted experiments in a magnetorheometer. Model structures investigated in this work are monoclinic lattices with simple and body centered bases. From the simulation point of view we emphasize the influence of the interparticle gap separation. From the experimental point of view we elucidate the importance of the magnetic field application and the occurrence of slip at the confining surfaces. Simulations demonstrate that the yield stress τ0 scales with the interparticle center-to-center distance h as τ0/M2 ∝ h-6 where M is the particle magnetization. A good agreement is found when the simulated yield stresses are compared with the experimental ones, independent of the particular packing and interparticle gap.