Existence of a Surface Tension Discontinuity at a Liquid Free Surface

Abstract

EXPERIMENTS have shown that, in certain conditions, a free water surface can exist in equilibrium with different values of surface tension in different regions. So far, only two such regions have been distinguished in any one water surface and their common boundary can be seen clearly in reflected light, after a little practice, as an unbroken sinuous line. One region appears to be chemically clean while the other is obviously contaminated with a surface film. This situation remains stable only if the water has a low velocity (less than 23 cm/see) which acts—see Fig. 1—in such a way that the contaminated surface region is “contained” by the shear stresses set up by the flow against the underside of the surface film. If the water velocity dies away the contaminated zone spreads out to cover the whole water surface. It is the line marking this boundary which is the subject of the present study and it will be referred to here as the “surface line”. This topic is of interest in chemical engineering where the results might be used to control the separation of minerals and other compounds by surface flotation.