Interpretation of color fringes in flowing soap films

Soap films were introduced to carry out classical hydrodynamics experiments on two-dimensional (2-D) flows by Couder [J. Phys. Lett. 42 (1981) 429-431] nearly two decades ago. The thickness of the film, ranging from 1 to 10 μm, varies slightly with the velocity field. This small thickness variation, however, results in fascinating color fringes, thus providing an excellent means for flow visualization. Here, a theoretical investigation is conducted to interpret the physical meaning of the color fringes that appear in soap film flows. It is shown that the color fringes resemble streamlines in the case of steady flow, under some domains of the parameter space. Also, photographs of flows over a circular cylinder and a backward-facing step taken in a horizontal soap film tunnel and numerical simulations under the same conditions of the experiments are presented. The color fringes shown in the photographs are in good agreement with numerically computed streamlines.