INFLUENCE OF A SURFACE FILM ON CONDUCTING PARTICLES ON THE ELECTRORHEOLOGICAL RESPONSE WITH ALTERNATING CURRENT FIELDS

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 eI/ef. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/ef, 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 eI/ef. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists ...