Formation mechanism and morphological behavior of high aspect-ratio folds in compressed films

A thin, rigid film that is subjected to compression can exhibit a variety of buckling modes depending on the nature of the applied compression and the boundary conditions at the film surface and edges. Among the lesser-known buckling modes is the development of sharp, tall folds which are typically observed as a response to a swelling stress in a confined region of a larger film. Here, we conduct experiments to identify the essential physics underlying fold growth and the effect of constraints on the mode of buckling. We show that the folding mechanism is not limited to cases of rapid swelling, but emerges more generally in any compressed thin film which is imperfectly bound to a solid substrate. The only essential condition to achieve this mode of buckling is that the film must be able to slide along the substrate without allowing air or fluid to intrude between the film and the substrate. We demonstrate that this can occur through purely mechanical application of stress. We also show the effects of the size of the compressed region, the film thickness, and the stress application rate using swelling-based methods, and characterize our results using a model combining Flory-Rehner swelling theory with buckle initiation mechanics.