Effect of Nonionic Surfactant on the Deformation and Breakup of a Drop in an Electric Field.

We have examined deformation and breakup of fluid drops suspended in another immiscible fluid under the action of an electric field. The contiguous fluids are incompressible Newtonian and the fluid-fluid interface is populated by nonionic surfactant molecules. The presence of the nonionic surfactant affects both the degree of deformation and the modes of breakup through the so-called Marangoni flow resulting from its nonuniform distribution on the interface. The drop is deformed into either a prolate or an oblate spheroid depending upon the electrical properties of the fluids and sustains a steady-state shape until the electrical Weber number is above a certain critical value. Two distinctively different modes of the drop breakup are observed depending on the surfactant concentration. When the interface is clean or contaminated by a very small amount of surfactant molecules, the drop bursts into several small droplets after forming bulbous ends. There exists a certain range of the surfactant concentration in which tip-steaming is a prevalent drop breakup mode. If the surfactant concentration exceeds this range, the breakup mode goes back to the fragmentation with bulbous end formation. This shows that, although not pronounced in the small deformation limit, nonuniformity in the surfactant distribution is a decisive factor for the breakup mechanism of a prolate spheroid. The results also show that when the drop deforms into an oblate spheroid, the effect of nonuniform distribution of surfactant can be significant. Copyright 1998 Academic Press.