VISCOELASTIC RESPONSE OF ELECTRORHEOLOGICAL FLUIDS .2. FIELD-STRENGTH AND STRAIN DEPENDENCE

In this paper, we elaborated on our previously developed model for the viscoelastic response of electrorheological (ER) fluids. We compared elastic modulus data obtained for a mineral-type ER fluid over a range of dispersed phase concentrations (0.02 - 0.10), electric field strengths (0.1 - 5 kV/mm) and strains (0.2 - 15%) to model predictions for these parameters. We found that a point dipole approximation underestimated the modulus. Accounting for higher multipoles allowed for a closer prediction of the actual data. The strain dependence data were not rigorously quantitative because of experimental limitations. Here we discussed trends in the data and discussed the implications of the data on the assumptions of our model. We also pointed out similar trends in the work of others, all of which were consistent with the idea that "free strings" are an important microstructural feature of ER fluids which give rise to viscoelasticity. Microscopic observations of a variety of ER fluids were used to illustrate the structural features which we incorporated into our model.