EFFECTS OF SURFACTANT ON THE DEFORMATION AND STABILITY OF A DROP IN AVISCOUS-FLUID IN AN ELECTRIC-FIELD

This paper considers the effect of insoluble surfactant on the shape d eformation and stability of a droplet immersed in an immiscible viscou s fluid under the action of an applied electric field when the convect ion and surface diffusion of surfactant are balanced at the drop inter face. First, we utilize a purely analytical method of domain perturbat ions to linearize the nonlinear free boundary problem and obtain the s teady-state drop shape in the electric field. Then, the linear stabili ty analysis predicts a critical electric Weber number, which is a limi t point for the existence of the steady-state shape. When the Weber nu mber exceeds the critical value, the drop experiences time-dependent s hape deformation and eventually breaks up. In addition, the morphology evolution of an immiscible blend of polystyrene and poly(vinylidene f luoride) in a DC electric field is investigated experimentally. The ef fect of a compatibilizer, which acts as a surface-active agent, is als o examined by adding the block copolymer of polystyrene and poly(methy l methacrylate). In the limit of a weak electric field, the degree of the shape deformation is in proportion to the Weber number as predicte d from the theory. The experimental results are also consistent with t he theoretical predictions in that the drop breaks up at a lower value of Weber number in the presence of the compatibilizer than the drop w ith a clean and completely mobile interface. (C) 1995 Academic Press, Inc.