Magnetic fluids prepared by suspending magnetic materials into a non-magnetic liquid matrix are examples of emerging smart materials with unique properties. Their formation is characterized by the magnetorheological (MR) effects induced by the attractions between the particles under magnetic dipolar force and their aggregate into plate-like microstructures. As such, these materials have attracted significant research attention as potential candidates for various engineering and biomedical applications. Unfortunately, the density mismatch between the particle and the matrix is an inherent challenge in the magnetic fluid and results in sedimentation problem that further limits the application of these materials.
Improving the MR effects is considered the best way to overcome the sedimentation problem. It includes the modification of the shape and size of the magnetic particles since the MR effects and the mechanical properties of the magnetic fluid largely depend on the microstructures of the magnetic particles and friction force between the particle aggregations. In fact, magnetic fluids based on both plate- and flower-like particles with a large friction coefficient (µ) demonstrated remarkable settlement stability and MR effect. However, for other magnetic fluids, a large frictional force does not always enhance the MR effect. Therefore, the development of high-performance magnetic fluid requires a thorough understanding of the effect of interparticle friction force on the MR properties of the magnetic fluids.
Different simulations have been performed to explore the mechanism of magnetic fluids and determine MR effects. Among them, particle-level dynamic simulations have been widely used to study the rheological behaviors of dynamic fluid under different conditions. However, this simulation method does not consider the interparticle friction force in steady shear and is therefore unsuitable for evaluating complex particle interactions. On this account, researchers at the University of Science and Technology of China: Dr. Lei Pei, Professor Shouhu Xuan, Dr. Haoming Pang and Professor Xinglong Gong, performed particle-level dynamic simulations to investigate the effect of interparticle friction on the MR effect for magnetic fluids. Their work is currently published in the journal, Smart Materials and Structures.
In their approach, a new numerical model considering the friction and normal elastic force between the coarse microspheres was developed. The relationships between the magnetic fluid microstructure and friction coefficient of the particles were assessed under steady shear flow, particle concentration, the external fields, shear rates and saturation magnetizations. Additionally, the optimal friction coefficient was established. Finally, comparisons were made to the existing experimental data to validate the simulation results.
The authors observed the formation of the plate-like aggregations under a moderate friction coefficient of approximately 0.2, attributed to the low particle concentration and the irregular direction of the friction forces. In contrast, thick chains with large inclinations were formed under string frictional forces greater than 1.5. The frictional forces exhibited significant influence on the rheological properties at µ > 1.5, up to 102% in steady shear flow. The optimal friction coefficient was determined as 2 ≤ µ ≤ 2.75 and was independent of the shear rate but dependent on the particle concentrations.
In a nutshell, the study was the first to apply particle-level dynamic simulations to investigate the influence of interparticle friction on the MR effects of magnetic fluid. The relative particle movements in the thicker chain as well as the formation of the plate-like microstructures, were greatly influenced by the frictional forces. Compared to the plate-like microstructures, thicker chains were characterized by larger deep angles due to stronger shear stress in the particle aggregations. Notably, the simulation results agreed well with the experimental data. In a statement to Advances in Engineering, Professor Xinglong Gong and Professor Shouhu Xuan reported that the study insights provided a comprehensive understanding of the MR mechanism and the influence of interparticle friction and would thus advance the manufacturing of high-performance magnetic fluid.
Pei, L., Xuan, S., Pang, H., & Gong, X. (2020). Influence of interparticle friction on the magneto-rheological effect for magnetic fluid: a simulation investigation. Smart Materials and Structures, 29(11), 115002.