Electrohydrodynamics and electrorotation of a drop with fluid less conductive than that of the ambient fluid

In this article, we investigated the electrohydrodynamic responses of a deformable fluid drop in another immiscible fluid under the action of a dc electric field. Both the ambient and drop fluids considered here were incompressible Newtonian and all of the drop phases were less conductive than their ambient fluids. Under these circumstances, the drops experienced the so-called electrorotation owing to the reverse dipole generated by the external electric field when the electric field strength exceeded a certain threshold value. The experimental observation showed that the threshold electric field strength was dependent on the drop size as well as the viscosity ratio. Also noted was the effect of the electrorotation on both the deformation behavior and the mode of the drop breakup. Specifically, we determined the critical electric capillary number beyond which the steady-state drop shape did not exist and the drop eventually broke up. Finally, the validity of Taylor's leaky dielectric theory was discussed in the presence of the electrorotation of the drop. (C) 2000 American Institute of Physics. [S1070-6631(00)00704-2].