Dynamics of gravity-induced gradients in soap film thicknesses

We demonstrate a direct measurement of thickness gradients in vertical soap films with a resonant differential interferometer, i.e., the Jamin-Fabry-Perot interferometer. Two regimes are investigated: thick colored films with gravity- and capillarity-induced gradients, and silvery-gray to common black films which are quasi-independent of gravity. In the colored zone, our differential method is an ideal tool with which to isolate the large thickness instabilities of the film reaching $17nm∕mm$ that characterize the end of its drainage. Using the so-called $F2$ law of such an interferometer, thermal-induced thickness variations as small as $1nm$ are isolated in the gradient-free common black film.