Study On Improvement Of Sedimentation Stability Of Magnetorheological Fluids

With the development of modern science and technology, a lot of new materials with unique properties were developed continuously. Because of its magnetorheological (MR) effect, Magnetorheological Fluids (MRFs) show superior value in new materials, and have been more and more attractive.MRF exhibits good fluidity without applying any external magnetic field, but the liquidity can be greatly changed to that of solid-like materials in a very short time interval once an external magnetic field is applied. When the magnetic field is removed, the fluidity can be recovered immediately. Because of this characteristic, MRFs have been used in active and semi-active control purposes in many fields. However, the sedimentation of the ferromagnetic particles due to the big difference between the apparent densities of particles and carrier fluid is a serious problem for MRFs to be applied in practice, since Brownian motion of magnetic particles is insignificant for micron-sized particles, although a lot of work has been done for the enhancement of the sedimentation stability of MRFs.Two kinds of attemps were made in this thesis for the enhancement of the sedimentation stability of MRFs. The first one is the preparation of MRFs by using different types of additives. Considering that the stability of MRFs includes sedimentation stability and aggregation stability. It is known that lauric acid and oleic acid play respectively available roles in improving these two kinds of stabilities. Several groups of samples were prepared, with both lauric acid and oleic acid as additives. The variations of the zero-field viscosity, the critical shear stress and sedimentation rate of these samples were tested. The effects of volume fractions of lauric acid, oleic acid and carbonyl iron particles on the stabilization and mechanical property of the MRF samples were investigated. The results indicated that a proper addition of lauric acid and oleic acid could significantly improve the mechanical property of magnetorheological fluids.The second one was based on physical consideration. Making use of magnetic attraction provided by permanent magnets, which is in the opposite direction against gravity, the sedimentation can be reduced. Both theoretical and experimental investigations were made. The theoretical analysis also includes two parts: Firstly, a magnetized particle is simplified as a magnetic dipole, the magnetic force of the dipole in a magnet field generated by a permanent magnet was determined, and the stability region and velocity of the magnetic dipolar particle were discussed; and Secondly, the magnetic force on a ferromagnetic particle induced by a sufficiently big permanent magnet was computed with commercially available FE code ANSYS, based on which the force on a chain of particles was computed. The results show that, due to the presence of non-uniform magnetic field and nonlinear magnetization effects, as well as the interaction between magnetized particles, the magnetic force of the entire chain is greater than the sum of magnetic forces of all individual particles without considering the interaction between particles. Finally, a testing device had been designed and used to verify the effect of this new method. The sedimentation of MRF samples at different distances to permanent magnet was investigated and the results reveal that choosing a suitable permanent magnet to generate the magnetic field and setting an appropriate distance between the sample and permanent magnet can greatly improve the sedimentation stability of MRFs.