Théo Lenavetier
Université de Rennes, Institut de Physique de Rennes, Département Matière-Molle

“Line tension in a thick soap film”

Soap films and bubbles have extreme aspect ratios: around 1µm in thickness and up to several centimetres in the other directions. An important piece of knowledge about these objects is that the main flows in them are uniform across the thickness (plug flows), meaning that each patch is being advected in the film without exchanging any fluid with its neighbours. This “sliding puzzle-like dynamics” is important to better understand the global mechanics of soap films and bubbles, as well as liquid foams, as it is an important transport mechanism for the surfactants decorating their liquid-gas interfaces.

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 1, 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 (cf. Fig 1, right). 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. 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.

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