Electrorheological behavior of whisker suspensions under oscillatory shear

The rheological properties of suspensions of sphere and whisker particles were measured under oscillatory shear on the application of electric fields. Because the suspensions are converted from viscous fluids to plastic bodies with high yield stresses, the complex shear modulus is markedly increased in electric fields. When the stress amplitude is increased beyond the yield stress, the complex shear modulus shows a drastic reduction due to the structural rupture. However, the complex shear modulus of whisker suspensions electrified during oscillatory shear is increased to a great extent, under the conditions where the stress amplitude exceeds the yield value attained by electrification in a quiescent state. The development of yield stress is attributed to the chain structures of particles in direction parallel to the field vector. In actual ER suspensions, the most important factor determining the yield stress is the column structure consisting of aggregations of many chains. The microscopic observation indicates that the particle concentration in aggregated region is increased by external oscillatory motion. The enhancement of ER performance for whisker suspensions electrified under oscillatory shear can be explained by the column thickening induced by oscillatory shear.