Stability of the shape of a surfactant-laden drop translating at low Reynolds number

We examine the effect of surfactants on the evolution of the shape of an initially nonspherical drop translating in an otherwise quiescent fluid at low Reynolds number. A combination of the boundary-integral method and a finite-difference scheme is used to solve the coupled fluid dynamics and surfactant transport problems, in conjunction with the Frumkin adsorption framework to account for the effects of monolayer saturation and nonideal surfactant interactions. For sufficiently small Bond numbers, the drop achieves a nonspherical steady shape. For large initial deformations or Bond numbers, however, the drop deforms continuously, and eventually breaks up through either the formation of an elongated tail or the development of a re-entrant cavity at the trailing end, similar to the mechanisms of drop breakup reported by Koh and Leal @Phys. Fluids A 1, 8 ~1989!#, and Pozrikidis @J. Fluid Mech. 210, 1 ~1990!#, for surfactant-free drops. Surfactants are found to have a destabilizing effect on the shape of translating drops. The destabilizing effect is mitigated by the presence of strongly-cohesive surfactant interactions, and by surfactant transport between the bulk and the interface. © 2000 American Institute of Physics. @S1070-6631~00!02503-4#