Studies On Mechanical Property Of Electrorheological Fluids

This dissertation has investigated the static and dynamic mechanical properties of the electrorheological(ER) fluids and discussed the relationship between the mechanical property and the structure evolution.Abrupt shear stress change of the ER fluids has been observed at very small shear rates, which is ascribed to the structure evolution between a loose and a tight strip/lamella structures. It is governed by the electrostatic force, viscous force, and Brownian force of particles. A modified Mason number is proposed to act as a characteristic parameter to represent the happen of this abrupt shear stress and structure change. It shows that the static yield stress of ER fluid is not an intrinsic property, while the sheared induced tight chain structure is the real origin of high shear resistances of ER fluids.The neck shrink like chain aggregation of ER fluids during tension caused that the relationship between tensile yield stress and electric field has an exponent of high than the traditional polarization model prediction of two. The tensile velocity affects the hydrodynamic force on particles to change the chain structure evolution during tensions, which can change the tensile stress significantly. In the compression of ER fluids, chains aggregate more seriously at a lower compressive speed and result in an obvious increase of compressive stress and nominal shear yield stress. A pre-compression can significantly enhance the shear yield stress of ER fluids through increase the interconnections between particle chains.The transient response of ER fluids under stepwise electric fields in tensile flow and compressive flow has been studies. The electric field strength, tensile or compressive speed effects on the transient stresses have been experimentally determined. The response of ER fluid in transient tension and compression flows has been found to be much quicker than in shear flow.Slip at the interface of solidified ER fluids and electrode in compressive flow has been observed. Various manufacturing methods have been used to modify the profile of electrodes to suppress the slip in compressive. It works well by increase the local electric field near the electrode surface. It can also significantly suppress the slip in shear flow and tensile flow. Results show that the surface modification of electrodes is an effective way to obtain high mechanical performances of ER fluids in applications.