EFFECT OF SURFACTANTS ON FLOW CHARACTERISTICS OF FALLING LIQUID FILMS

Experimental values of the wave length and wave velocity have been obtained for dilute solutions of valeric and hexanoic acid for a vertical falling liquid film. The wave length was unaffected by the surfactants for Reynolds numbers in the range 5 to 100; however, the wave velocity was decreased for increased surface concentrations of the two acids. This is in direct contradiction to previous theoretical work, and the explanation for the anomaly is that the free surface velocity is greatly retarded by the adsorption of the surface active agents. In an effort to determine the extent of this retardation, an approximate form of the diffusion equation and the equations of motion were solved subject to boundary conditions describing the effect of the absorbed surfactant on the surface stress. The results indicate that the entrance length can be increased several orders of magnitude by the addition of small amounts of surfactant. Copyright © 1969 American Institute of Chemical Engineers