| To develop an understanding of the mechanisms associated with electrorheology, a theoretical model for the electrostatic interaction force between particles in a chain is developed and compared with the measured force between particles in a single chain of a model ER fluid. The calculations show that electric field concentrations are about an order of magnitude higher near the particle contacts. The force between particles predicted by this model is in reasonable agreement with that measured experimentally. Studies on the behavior of zeolite-based ER fluids indicate that the strength of ER fluids is determined by: (1) the force of interaction between particles in a single-row chain and (2) an enhancement of the force due to the observed clustering of particles into multi-row chains. The force enhancement factor (structure parameter) varied with the shear rate and the concentration of particles, but was relatively independent of the electric field, temperature and host fluid. When the appropriate structure parameter was taken into account, reasonable agreement existed between the predicted and the measured shear stress-shear strain curves and the concentration dependence of the yield stress. The present theoretical and experimental considerations thus indicate that the electrostatic polarization force between the particles is a major factor in determining the shear strength of ER fluids under static shear conditions. The mechanical and electrical properties of ER fluids can thus be determined from the physical properties of the phases that are present. Finally, an estimate of the maximum attainable yield strength of ER fluids is given. |
