A new method for direct measurement of the film tension of newton black films on a diminshing bubble

Abstract

The problem of the measurement of the film tension of a Newton black film at the top of a diminishing bubble at the solution surface is discussed. It has been concluded that the shearing interference method cannot be used for this measurement. A new experimental method has been developed. It is based on the fact, that the film at the bubble top (a convex mirror) and the air/solution surface at the bubble bottom (a concave mirror) reflect, simultaneously, the incident light. They produce two images of the collimating aperture slit, which can be distinguished and photographed separately. The ratio of the lengths of both images is equal to the ratio of the curvature radii of the film and the bottom bubble surface, hence it is equal to the ratio of the film tension and the solution surface tension. Measurements with a sodium dodecyl sulfate aqueous solution show that the film tension of the Newton black film does not depend on the bubble size or the capillary pressure over a wide range (400 to 80 μm) of bubble radii. The measured film tension values coincide well with previous data.