Phase separation and flow induced anisotropy in electrorheological fluids

We present experimental results on the formation of different sheet structures obtained in a colloidal suspension of silica particles in the presence of an electric field and of an oscillating shear flow. The characteristics of the structure depend on the amplitude of the imposed strain; at low strain the sheets are perpendicular to the velocity lines, whereas they become perpendicular to the vorticity axis at high strain. The electrostatic forces between the particles, which are induced by the electric field, give an elastic modulus to the suspension. With an appropriated experimental device, we have measured for a given structure the elastic component of the shear modulus with a strain either parallel or perpendicular to the direction of the sheets. The storage modulus for a strain in the plane of the sheets is found to be several times larger than for a strain in the perpendicular direction. A possible explanation of this strong anisotropy in elastic shear modulus is given in terms of the microscopic structure inside the sheets.