Rheological responses of oil-in-oil emulsions in an electric field

In the present article, the rheological responses of oil-in-oil emulsions in a de electric field were investigated experimentally. Specifically, the dispersed phase of the emulsions considered in this work was less conducting than the continuous phase. Depending on the relative strength between the shear flow and electric fields, three distinctive responses of the emulsions were observed in steady and dynamic oscillatory shear tests. First, the apparent viscosity enhancement (positive electrorheological effect) was produced when the electric field was predominant. Second, when the shear flow was strong and dominant, the electric field played a trivial role on the rheological behavior of emulsions. Finally, the viscosity reduction (negative electrorheological effect) was generated when the shear flow and electric fields were competitive. The viscosity enhancement was induced by the formation of chain-like microstructures of the dispersed droplets as in a typical electric-field responsive particle suspension. Meanwhile, the viscosity reduction was closely associated with the electric-field-induced rotation of the dispersed droplets, which was confirmed by the electrohydrodynamics of a single conducting drop in a more conducting ambient fluid. Finally, the rheological responses of the model emulsions to a dynamic small-amplitude oscillatory shearing were considered in conjunction with the morphology evolution under the action of the electric and flow fields. The results showed that the interfacial contribution to the rheological response appeared quite differently depending on the conductivity ratio of the two contiguous fluids. (C) 2000 The Society of Rheology. [S0148 -6055(00)00502-2].