EFFECT OF FREQUENCY ON A WATER-ACTIVATED ELECTRORHEOLOGICAL FLUID IN AN AC ELECTRIC-FIELD

The electrorheological (ER) behaviour of particulates in an external a.c. electric field can be explained by the structuring of dispersed particles caused by the oscillations of induced dipoles in the particles. In this work, the effect of an a.c. electric field on a water-activated ER fluid was investigated under both continuous and oscillatory shear. An a.c. frequency-dependent behaviour of a model ER fluid was found at large deformations (continuous shear). Specifically, it was observed that the Bingham yield value decreased as the a.c. frequency increased and above 500 Hz was independent of frequency, implying that the fluid has a response time of 2 ms. However, there was no significant dependence on the a.c. frequency at small deformations (oscillatory shear); the elastic modulus was found to be constant over the range 10-1500 Hz. This dichotomy has been resolved by recalling that the Bingham yield value is a measure of the energy needed to break the structure, whilst the elastic modulus is more a measure of the extent of structure. The data suggest that in the oscillatory experiments,which were obtained in the linear viscoelastic region, the structure is not destroyed, whilst in the continuous shear experiments the structure is broken and at high frequencies, above 500 Hz, the structure does not have time to reform.