Recent research on free surface flows in the presence of surface-active species in which a fluid interface undergoes very large deformations, e.g., as in the deformation and breakup of drops in extensional flows under conditions of Stokes flow (Stone, H. A., and Leal, L. G., J. Fluid Mech. 220, 161 (1990)) (19) and the formation of drops from capillaries (Zhang, X., and Basaran, O. A., Phys. Fluids 7, 1184 (1995)) (20) has shown that dynamic surface tension (DST) effects can radically alter the dynamics compared to situations in which the fluid interface is clean. In this paper, we present results of an experimental study that examines the impact with a solid substrate of drops of Newtonian liquids containing two commonly used surfactants. In the experiments, an ultra high-speed video and associated image analysis system is used to monitor the dynamics of the impact process. On account of the extremely large deformations that a drop exhibits and the large-amplitude oscillations that it undergoes upon impacting and spreading on the substrate, DST plays a complex and dominant role in determining the dynamics and the asymptotic state that is approached at large times. A major consequence of the presence of surfactant is that on the one hand its accumulation on the fluid interface reduces the surface tension and thereby enhances the spreading of the drop across the substrate. On the other hand, the non-uniform distribution of surfactant along the fluid interface gives rise to Marangoni stresses that inhibit drop spreading. Given the fact that many liquids used in atomization coating applications ranging from the spraying of agricultural chemicals to painting of substrates contain surfactants and/or other surface-active species, the fundamental results to be reported in this paper have important practical ramifications. (C) 1997 Academic Press.
