An adsorption-desorption-controlled surfactant on a deforming droplet

Abstract

The effects of a sorption-controlled, monolayer-forming surfac-tant on a drop deforming in an extensional flow are studied numerically. Scaling arguments are presented for drops of 1 cm and 1 mm, indicating the applicability of these results. For all simulations, when mass transfer is slow compared to surface convection, the insoluble limit is recovered; when mass transfer is rapid, the drop behavior is the same as that for a surfactant-free drop. For a surfactant which forms a monolayer, there is an upper bound to the surface concentration, G`. The surface tension re-duction diverges as the surface concentration G approaches this limit, strongly altering the hydrodynamics. The drop deformation is studied relative to a surfactant-free drop in terms of the capillary number, Ca, the ratio of character-istic viscous stresses to surface tension. In the insoluble limit, for G! G`, droplets deform more than in the absence of surfactants at a given Ca and break-up at lower Ca. When stable drop shapes ar