PHASE-SEPARATION AND FLOW-INDUCED ANISOTROPY IN ELECTRORHEOLOGICAL FLUIDS

We present experimental results on the formation of different sheet st ructures obtained in a colloidal suspension of silica particles in the presence of an electric field and of an oscillating shear flow. The c haracteristics of the structure depend on the amplitude of the imposed strain; at low strain the sheets are perpendicular to the velocity li nes, whereas they become perpendicular to the vorticity axis at high s train. The electrostatic forces between the particles, which are induc ed by the electric field, give an elastic modulus to the suspension. W ith an appropriated experimental device, we have measured for a given structure the elastic component of the shear modulus with a strain eit her parallel or perpendicular to the direction of the sheets. The stor age modulus for a strain in the plane of the sheets is found to be sev eral times larger than for a strain in the perpendicular direction. A possible explanation of this strong anisotropy in elastic shear modulu s is given in terms of the microscopic structure inside the sheets.