The surface discontinuity at an air/water interface near the boundary of an adsorbed film

Abstract

When a free surface water flow contains small amounts of surface active agents (surfactants) these surfactants are adsorbed onto the surface of the water. If the free surface flow is towards a physical barrier or a stagnation point the surface contaminants form a film upstream from the barrier or stagnation point. The manner in which the film forms was discussed by the experimenters Merson and Quinn, Merson, R.L., and Quinn, J.A. (1968). Stagnation in a fluid interface: Properties of the stagnation film. A. I. Ch. E. Journal. 11, 391 - 395, who gave detailed results on the way such a film forms with time They showed that the length of the film grows linearly with time for an incompressible film with no desorption; is proportional to the square root of time if the film is compressible and no desorption occurs; and that the growth rate decays exponentially with time if Langmuir desorption kinetics are assumed: see Davies and Rideal Davies, J.T., and Rideal, E.K. (1963). "Interfacial Phenomena", Academic Press, New York, and London. In measuring the lengths of the surface films, Merson and Quinn sprinkled talc on the upstream surface. The talc was swept downstream and at the edge of the film formed a thin line, the talc then moving to the sides of the channel and on entering the film moving in a circulating pattern. Mockros and Krone Mockros, L.F., and Krone, R.B. (1968). Hydrodynamic effects on an interfacial film. Science (U.S.A.), 361 - 3 gave experimental evidence that the films equilibrium length, at steady state, is inversely proportional to the square of the surface velocity of the free stream (i.e. the upstream surface velocity), that the film exhibits a linear decrease with distance downstream in the surface tension and that the interfacial shear stress in the film is proportional to the velocity. In these experiments, Mockros and Krone observed that the upstream edge of the film is marked by a small ripple, the ripple being such that the film surface appears to be elevated at the leading edge.