We examine how the interfacial polarization of the particles affects the electric-field-induced aggregation in electrorheological (ER) fluids. We derive a microstructure-based equation for the free energy of a conducting suspension for the case of a random arrangement of the particles, and show that the thermodynamic properties of a suspension are strongly influenced by the dielectric relaxation. According to our theory, in dc electric fields the particles aggregate only provided that a certain relation between the ratio of the particle-to-suspending liquid dielectric constants and that of the conductances is satisfied. Moreover, we predict that the slow aggregation, which takes place when the conductivity effects determine the interparticle forces, will occur in relatively low dc electric fields, whereas the rapid aggregation, when conductivity effects do not contribute to the interparticle forces, will occur in relatively high dc electric fields. The theory also provides insight into why the electric-field-induced strength of the fully developed microstructure in ER fluids depends on the particle-to-suspending liquid ratio of the conductances and why there exists a correlation between the ER activity of a suspension and its dielectric spectrum. The predictions of the theory are consistent with the currently available experimental data on ER fluids.

• Received 10 January 1995

DOI:https://doi.org/10.1103/PhysRevE.52.1669