THE SMALL SHEAR RATE RESPONSE OF ELECTRORHEOLOGICAL SUSPENSIONS .2. EXTENSION BEYOND THE POINT-DIPOLE LIMIT

Electrorheological (ER) suspensions display dramatic changes in their rheological properties when subjected to large electric fields. In an accompanying paper, a molecular dynamics-like simulation technique was developed to investigate sheared ER suspensions. The results of that study predicted a dynamic yield stress that saturates at large concentrations of the disperse phase. Experimental results are presented here which confirm the predicted saturation of the yield stress. The experimentally determined yield stresses are larger than the values predicted by the simulations in the point-dipole limit. This discrepancy is accounted for by reconciling approximations made in the simulation model. Incorporation of multipole and multibody contributions to the electrostatic interaction between spheres through a perturbation analysis suggests that accurate predictions of yield stresses can be made. However, we find that the ER response is sensitive not only to the material dielectric properties, but also the details of the pair interactions between particles at vanishing separations.