Research Of The Phase Transition And The Structure Of Magnetorheological Fluids

This dissertation mainly deals with the phase transition and the structure of magnetorheological (MR) fluids. The influences of factors such as the strength and frequency of applied magnetic field, the magnetic permeability and conductivities of particles and liquid and the volume fraction of particle on the properties of MR fluids have been investigated to guide the design of high performance MR fluids. In chapter one, the history, method of study, materials and application of magnetorheological fluids are reviewed and the problems of investigation are pointed out. In chapter two, by employing a thermodynamics method and a statistical mechanics method, we have studied the solid-liquid phase transition in MR fluids based on the model of point dipoles. It is found that there exists a critical magnetic field, which shows a particle density and temperature dependence. When the applied field exceeds H c, the osmotic pressure becomes negative. The particles in the field tend to condense and the suspension experiences a phase transition to a solid phase, which is rapid and reversible. In addition, we found the phase transition can be induced by the change of temperature. In chapter three, based on Rayleigh identity, a formula of the effective magnetic permeability of MR fluids is derived. Within the Maxwell-Wagner model, the effective dielectric constant of electrorheological (ER) fluids is calculated by using the same method and the effects of conductivity in ER fluids are studied. It is found that the effective dielectric constants are controlled by the conductivities of particle and fluid when dc or low frequency field is applied, while they are determined solely by the dielectric constants of particle and fluid when the frequency of field is high. We calculate and compare the ratios of the effective magnetic permeability of BCC, FCC, SC lattices to their critical volume fractions to determine the structure of magnetic-induced MR solid. If we only consider the magnetostatic energy of MR system, we find that the face-centered cubic lattice is the most possible structure of MR solid. A simple conclusion of this thesis and some prospects on the investigation of MR fluids are presented in the last chapter.