Optimization, Fabrication Of Magnetic Fluids And Molecular Dynamics Study Of Their Rheology

Magnetic fluids (Ferrofluids) exhibit a combination of normal liquid behaviors and magnetic controllable properties. Because of their unique physical properties and mechanical performances, magnetic fluids have attracted much attention in wide fields. This research aims to optimize, fabricate, and characterize a new type of magnetic fluids by dispersing Fe@SiO₂ magnetic particles in a carrier liquid. Both experimental approach and molecular dynamics simulation analysis were conducted to investigate rheological and mechanical properties of magnetic fluids under various working conditions. This research has led to the improvement of antioxidant capacities of magnetic fluids while keeping good magnetoviscous effects of the magnetic fluids.This dissertation firstly reports the optimization and fabrication of magnetic fluids. The olivary core-shell magnetic particles Fe@SiO₂, instead of traditionally used Fe3O4 particles, were synthesized and used for the fabrication of a novel kind of magnetic fluids. The olivary composite particles Fe@SiO₂ exhibit improved physical and chemical properties because they have both the prominent properties of"core"and"shell"components. The amorphous silica''shell''can effectively inhibit particles aggregation and immunize the encapsulated species against environmental degradation effects while retaining their intrinsic properties. Meanwhile, the nonspherical iron''core''would endow the particles with magnetic anisotropy character. Rheological properties and viscoelastic behaviors of the fabricated magnetic fluids were characterized with a rheometer. Experimental results showed that this kind of magnetic fluids has a high magnetoviscous effect and a good stability. Their viscoealstic properties are different from the conventional ones because of the particular characters of the functional Fe@SiO₂ magnetic particles. Additionally, the interaction between the magnetic particles and carriers also strongly affects the magnetorheological properties of magnetic fluids.Molecular dynamics simulation analysis was developed to predict the microstructures and mechanical behaviors of materials on atomic scale. The hydrodynamic equations of magnetic nanoparticles under a multi-field coupled in a complex fluid were derived. By taking into account a variety of influencing factors, including the effect of particle size, shape and distribution of the microstructures in magnetic fluids, rheological properties of magnetic fluids under various working conditions, such as quasi-static, steady shear, and compression, were analyzed. The simulation results indicated that the magnetic interaction dipole forces drive the nanoparticles to form chain-like structures. The strength of the chain-structure and its distribution varies with the magnetic field strength and the loading conditions. The particles packing in the chain-structure consequently influence the yield stresss of magnetic fluids. The higher the packing density, the higher the yield stresses. This was verified by the simulation results that the magnetic fluids exhibit enhanced yield stresses when the materials were compressed along the magnetic field.