The mechanism of surfactant effects on drop coalescence

We utilize numerical solutions, based on a boundary-integral scheme, to investigate the mechanisms by which surfactant influences the coalescence of a pair of equal size drops that undergo a head-on collision in a biaxial linear flow. It is known that the addition of surfactant inhibits coalescence in the sense that the time required for film drainage to the point of film rupture is significantly increased. Although there is a direct effect on the rate of film drainage due to Marangoni effects within the thin film, we find that an equally important effect is due to the fact that the hydrodynamic force pushing the drops together is increased, hence causing the film to be more strongly deformed into a dimpled configuration that slows the film drainage process. (c) 2008 American Institute of Physics.