Line tension in a thick soap film - Théo Lenavetier (Institut de Physique de Rennes)

When a soap film is set into motion by gravity, capillary forces or air movements, it is common to observe patches of very different thicknesses being put into contact. In this case, they tend to minimise the perimeter separating them, making them adopt circular shapes (cf. Fig. Left). This is due to the presence of a line tension of purely capillary origin, of which we provide an analytical expression depending only on the thickness gradient between the patches.

This line tension has never been quantified to the best of our knowledge, and we have built an experiment which allows us to do so by creating a situation where a thin piece of film is embedded into a much thicker one. We get access to the line tension by measuring the thickness profile between the two pieces using a hyperspectral camera, yielding a force of the order of a tenth of nanonewton.

To validate this novel measurement, we also look at the relaxation of the in-plane motions in the film which are generated by the presence of the line tension (cf. Fig. Right). This latter acts as the driving force of the motion whereas the damping force is due to the friction of the air on the interfaces. We find a quantitative agreement between the measured value of the force and the dynamics of relaxation, thus validating our measurement.

Contact: Élise Lorenceau