Surface tension coefficient and thickness measurements in planar soap-film flows

Steady planar soap-film flows have been used widely in experimental investigations of fluid mechanics, e.g., as models for two-dimensional flows or supersonic flows. Despite the significant advances in the measurement techniques for soap films over the years, there is still room for improvement, especially for flowing soap films. Here we present two developments in measuring the surface tension coefficient and the film thickness of flowing soap films. For a vertically flowing soap film between two elastic wires, based on the force balance of the boundary wires, we derived an exact solution for the curve shape of the boundary wires. We verified our solution by direct measurement of the wire shape in our flowing soap film. Our exact solution thus provides support to a recently proposed semi-empirical method. Moreover, based on our solution, we proposed a new, easy-to-implement method for measuring the surface tension coefficient of flowing soap films. For film thickness measurement, we proposed a new method based on the interference of the incident light with a single-wavelength. When coupled with a conventional vertical velocity profile measurement, such as particle image velocimetry (PIV), the interference method can provide the film thickness profile using a single monochrome camera.