tion between the liquid state and the solid state, potentially The dc field rheological properties and frequency dependent provides the most efficient approach to controlling mechanidielectric properties of a set of electrorheological (ER) fluids comcal response by adjusting the electric field strength, which posed of oxidized polyacrylonitrile or aluminosilicate materials has a strong industrial application background. The ER propdispersed in silicone oil were examined in this paper. Our experierties and mechanisms have been addressed before, both mental investigations show that there is a complicated relationship experimentally and theoretically. The electric field-induced between the dielectric properties of dispersed particles and the ER interaction, arising from the particle polarization, is comeffect. The dielectric loss of dispersed particles, which has not monly believed to be responsible for ER behavior (1-12).
attracted much attention in previous work, was found to play a Dielectric techniques have therefore been widely used to considerable role in ER response. The large dielectric loss tangent, experimentally around 0.10 at 1000 Hz, is found to be needed for probe the polarizations generated in ER systems or to detera strong ER effect. A good ER solid material should first have mine the response times of ER fluids (13-18). The dielectric large dielectric loss, and then the higher the dielectric constant, properties of ER fluids at high electric fields or high shear the stronger the ER effect. The large dielectric loss would facilitate fields are emphasized, since ER fluids most likely operate the turning of dispersed particles, and the high dielectric constant under these conditions, and the microstructure of an ER fluid would maintain the fibrillation structure stable and strong. Two at high electric or shear field is quite different from the processes, the particle turning process and the particle polarization microstructure at low electric field or quiescent state. Misprocess, are thought to be involved in ER activity. Our findings, match of the solid and liquid phase dielectric constants (at in connection with the Wagner model, can better explain why the high frequency field) or conductivities (at dc or low frestrongest ER effect occurs at particle conductivity of 10 07 S/m; quency field) was proposed for interpreting ER phenomena why the shear stress of some ER fluids decreases with frequency (19)(21). However, the obvious differences between the exwhile with others the shear stress increases with frequency; and why trace water can enhance the ER effect considerably, which perimental results and the theoretical predictions (7) suggest would help in understanding the mechanism of the ER effect. Too that the polarization model does not adequately reflect the large a dielectric loss is thought to be unfavorable for the ER physical grounds of the ER effect. The conductance model effect, and its suitable range is worth further study. The results (22)(23)(24), in which the particle interaction is thought to be also present a method of designing high performance ER fluids, determined by the ratio of particle-to-fluid conductivities which would significantly promote development of electrorheology rather than permittivities, was also proposed, and good and its application in industrial areas. แญง 1997 Academic Press agreement between the experimental results and the predic-Key Words: dielectric properties; oxidized polyacrylonitrile and tions was obtained (22)(23)(24). Nevertheless, the conductance aluminosilicate/silicone oil suspensions; electrorheological effect. model can only be applied to the situation in which the ER suspension microstructure has been fully formed. It does not consider the microstructure transition process of the ER