The compression of a heavy floating elastic film

TL;DR

This study investigates how film density influences the buckling, folding, and post-folding behaviors of elastic films at liquid interfaces, revealing new phenomena and extending existing models to account for weight effects.

Contribution

It extends the model of Diamant and Witten to include film weight, explaining symmetry breaking and new folding behaviors observed at higher compression levels.

Findings

Dense films transition from wrinkling to folding with weight-induced shape changes.

Beyond self-contact, films form closed loops encapsulating droplets, with behavior dependent on buoyancy.

Surface tension effects significantly influence fold shape, contrary to some theoretical assumptions.

Abstract

We study the effect of film density on the uniaxial compression of thin elastic films at a liquid--fluid interface. Using a combination of experiments and theory, we show that dense films first wrinkle and then fold as the compression is increased, similarly to what has been reported when the film density is neglected. However, we highlight the changes in the shape of the fold induced by the film's own weight and extend the model of Diamant and Witten [Phys.Rev.Lett. 2011, 107, 164302, arXiv:1107.5505] to understand these changes. In particular, we suggest that it is the weight of the film that breaks the up-down symmetry apparent from previous models, but elusive experimentally. We then compress the film beyond the point of self-contact and observe a new behaviour dependent on the film density: the single fold that forms after wrinkling transitions into a closed loop after self-contact, encapsulating a cylindrical droplet of the upper fluid. The encapsulated drop either causes the loop to bend upward or to sink deeper as compression is increased, depending on the relative buoyancy of the drop-film combination. We propose a model to qualitatively explain this behaviour. Finally, we discuss the relevance of the different buckling modes predicted in previous theoretical studies and highlight the important role of surface tension in the shape of the fold that is observed from the side -- an aspect that is usually neglected in theoretical analyses.