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Electrorheological Material under Oscillatory Shear

Two typical electrorheological materials, which are anhydrous dispersions of silica particles in silicone oil with surface active additives, are sheared in oscillatory motion in a gap, formed by an outer fixed cylinder and an inner, axially movable cylinder. The rheometer is operated at tuneable resonance frequencies, such that only the reaction of the material is measured. Long time oscillatory shear measurements show that the electroviscous stress deteriorates under d.c. elec tric field, while electroviscous stress is stable when a.c. fields are used. Electroviscous stress is found to depend on field frequency through the dependence of the dielectric constant of the disperse phase on field frequency. A dimensionless quantity, which relates the electroviscous shear stress to the electric field strength, serves to correlate all the experimental data for different shear amplitudes and shear frequencies for each fluid. The phenomenological theory of Huang and Spurk predicts this efficiency with satisfactory accuracy.