Mapping of the Marangoni effect in soap films using Young's double-slit experiment

We report on the thickness variation measurement of a soap film due to a local perturbation, using Young’s double slit experiment configuration. We map a laser-heated deformation of a vertical free-standing draining thin soap film using the differential change of optical path in the interferometer. The experiment has a resolution of about 0.1 nm and enables to follow the liquid flow dynamics. We evidence a bottleneck formation in the heated region of the film that perturbs the usual flow. Such an experimental set-up could then be adapted to measure other tiny variations in fluctuating hydrodynamics such as capillary waves for example. Introduction. – On the one hand, tears of wine are a superb manifestation of the Marangoni effect [1]. The variation of the surface tension due to the change of alcohol concentration leads to these tears [2–5]. Such an effect can also originate from a temperature gradient at the air/liquid interface and is sometimes called Rayleigh-Bénard convection [6,7] when it is linked to thermal convection. Whereas this effect is often invoked, quantitative measurements are scarce [8–17]. These measurements have been mainly performed, directly using either the naked eye, or with the help of a microscope, or using temperature sensitive markers, or probing the film thickness with interference techniques. On the other hand, tiny length variations in general can be evidenced using optical interferometry measurements [18, 19]. Relative variations in the 10 to the 10 range have already been reported [20,21]. Optical interferometry has also been used in the measurement of soap film thicknesses in the 10 μm-200 nm range [22–24], with a precision of 20 nm. Among all these interferometry techniques, Young’s double slit experiment is one of the most popular, fascinating and easy to set up technique. In particular, in quantum mechanics, it has been used to demonstrate the wave particle duality [25] and to evidence phase variations, like in Aharonov-Bohm type experiments [26]. R. Feynman himself said from this experiment: ”It is a phenomenon which is impossible to explain in any classical way and which has in it the heart of quantum mechanics” [27]. Back in the optical domain, due to its high simplicity and versatility, one may thus wonder whether Young’s double slit experiment could be implemented to measure the thickness variation of soap films via the change of the phase due to the modification of the effective optical length. The aim of this letter is then to investigate potentiality of the double slit experiment by probing the dynamics of the Marangoni effect on a soap film locally heated by a laser. In particular, we will try to map its thickness variations with a