The effect of additions of sodium lauryl sulfate (10-6 to 10-4 gm/l) and the effect of the age of the interface and drop size on the rate and pattern of coalescence were investigated for toluene drops and anisole drops in water. Color movies (at 16 and 32 frames/sec) were taken of the light interference patterns produced by the changing thickness of the film of water trapped between the oil droplet and the layer of oil. The work indicates that in the trapped film there exists a complex pattern of flow behavior that is strongly influenced by the surfactant concentration. In particular, a bulk flow of fluid into the film occurs for concentrations of sodium lauryl sulfate of 10-6 gm/l. This could be caused by a momentum effect or by interfacial tension gradients that exist because the bulk flow of fluid sweeps surfactant from sections of the film. This work extends the observations of Hodgson and Lee made for the rest times of the toluene-water system by presenting the behavior of the trapped film of fluid. In addition the effect of surfactants on the rest times and the film behavior is given for the low-density difference system of anisole and water. For relatively high concentrations of surfactant (10-4 gm/l) the controlling mechanism is the expulsion of the fluid from beyond the barrier ring and not of the fluid from within the barrier ring, as has been suggested by the parallel plate model. A simple, two-dimensional model based on this observation is compared with the observed rate of drainage for separations less than 6000 A. The effect of van der Waals attraction was included in this model. A mathematical model is presented for the prediction of the diameter of the barrier ring as a function of elapsed time. © 1969.
