Electric effects on the rheology of insulating oils in electrodes with flocked fabric

The rheological behavior of insulating oils is studied in nonuniform electr ic fields which are generated by an electrode covered with flocked fabric. Although the oils show no electrorheological effects in uniform fields betw een metal electrodes with smooth surfaces, the flocked fabric leads to a st riking increase of viscosity in steady shear. The viscosity enhancement inc reases with decreasing zero-field viscosity and decreasing conductivity of oils. In the limit of zero shear rate, the oils with low conductivity behav e as solids with yield stress. When a very small quantity of fine particles is introduced into electrified oils without shear, a rapid and large-scale motion of particles is observed between the tips of fibers and the plate e lectrode. The local motion of fluids in high electric fields is referred to as electrohydrodynamic (EHD) convection. Periodic patterns of circulation flow are formed in static oils. The electric energy which is dissipated dur ing the circulation motion contributes to holding the periodic flow in stat ic oils. When the stress is very low, the periodic patterns are not broken down. The yield stress corresponds to the force required to rupture the dom ain structures of EHD convection. In shear fields, the additional energy ma y be required to change the periodic patterns of EHD convection. The striki ng increase of viscosity in steady shear can be attributed to the interacti ons between EHD convection and external shear.