(187z) Effect of Surfactants on Drop Breakup In Flow through Capillaries

We present the results of an experimental investigation of the deformation and breakup of viscous drops and air bubbles translating through cylindrical and square capillaries. The critical conditions for the onset of drop breakup, as well as the modes of breakup, are examined. The experiments are performed for different surfactant concentrations in order to identify the effect of surfactants on the four breakup mechanisms identified earlier for surfactant-free systems. These mechanisms include transient stretching, propagation of capillary waves along the interface, formation of a re-entrant cavity at the trailing end of the drop that evolves into a jet of suspending fluid penetrating the drop, and tail-streaming wherein the drop develops a pointed tail which emits a stream of very small droplets. In most cases, the presence of surfactant is found to accelerate the onset of drop breakup. Namely, surfactant-bearing drops are found to break up at smaller critical capillary numbers compared to their surfactant-free counterparts.