A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio єI/єf. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/єf, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of єI/єf. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists between the measured shear yield stress, current density, and the applied breakdown field for oxidized aluminum particles suspended in silicone oil and those predicted by the present model. Recommendations are given for the design of ER fluids with high yield strength and reasonable current density.

Dielectric and conduction effects in electrorheological (ER) fluids with ohmic conductivity of the host liquids and with an applied AC electric field are considered. It is found that the conductivity ratio Γσ, the dielectric constant ratio Γε and the applied field frequency fare three important parameters which determine the ER effect. If Γσ>Γε, a stronger ER response is obtained with a low applied field frequency than with a high frequency. If Γσ<Γε, the opposite occurs. Empirical equations are given for the attractive force between the particles in an ER fluid and for the shear yield stress. The calculated current density is independent of the applied field frequency when the frequency is smaller than a critical value, but becomes a linear function of the frequency beyond the critical value. The predicted attractive force of two nearly touching polymer spheres inmineral oil is in good agreement with that measured; also the predicted shear yield stress of some common ER suspensions is in accord with measurements.

Dielectric and conduction effects in electrorheological (ER) suspensions with non-Ohmic conductivity of the host liquid and with ac applied electric field are investigated. It is found that the conductivity ratio Гб, the non-Ohmic conductivity parameters A and Ec of the host oil, the dielectric constant ratio Гб, and the applied field frequency are important parameters that determine ER response. The effects of these parameters are more complex than in the case of host liquids with simple Ohmic conductivity. If the ac field frequency is high, the conductivity effect disappears; if the frequency is low the dielectric effect disappears. The current density is independent of the frequency below a critical value, but it increases with frequency beyond the critical value. The critical frequency is larger for ER suspensions having non-Ohmic conductivity of the host liquid than with Ohmic conductivity. Good agreement occurs between experimental measurements of the current density and attractive force between particles (including shear yield stress) and predictions by our model.

Hydrodynamic lubrication of a bilinear rheological fluid plays an important role in the so-called electrorheological (ER) smart' bearings. There may not. however, be a reliable and efficient algorithm for the non-Newtonian lubrication problem of bilinear rheological fluids. In this paper, a finite element trethod (FEM), together with a mathematical programming solution technique, are proposed for solving such a non-Newtonian lubrication problem. This method was used to study the hydrodynamic lubrication of a journal bearing lubricated by a bilinear rheological lubricant. The computed results were compared with Tichy's work for the Bingham rhcological model. It was shown that the Bingham model should be regarded as a limiting case of the bilinear rhcological model.

A modified conduction model is proposed for predicting the maximum local electric field, current density and attractive force between two spherical particles in a host oil. The dependence of the conductivity of the host fluid on electric field is expressed by a simplified equation based on Onsager's theory. The model gives the ratio of the maximum local field to the applied field as Em/E0=30(Г/A)0.1 (Ec/E0)0.9, where Г=бp/бf (0) and бf (0), A and Ec are the conductivity parameters relating to the host fluid. бp is the conductivity of the particles and E0 the applied field. Good agreement is found between the predicted and measured values of the current density and the yield stress of electrorheological (ER) suspensions. To obtain a strong ER effect, a high value of the combined conductivity parameter of the host fluid λ=Kf [бf(0)A]-0.1 (Ec)0.9 is desired.