Rate-controlled wrinkling and folding of thin elastic films bonded to viscous substrates

We examine the buckling of a thin elastic film floating on a viscous liquid layer which is itself supported on a prestretched rubber sheet. Releasing the prestretch in the rubber at a controlled rate induces viscous stress in the liquid, which compresses the elastic film, causing buckling. This approach allows compressive strains of several ten percent to be applied. Experiments and simulations show that two different buckle modes can appear. The first is that the elastic film develops roughly sinusoidal wrinkles. Wrinkling can be captured qualitatively by a linear stability analysis starting from a stress state that is calculated from a shear lag approach (Chatterjee, Soft Matter, 2015). The second is the appearance of tall, well-spaced folds which tend to appear at small liquid layer thickness. Folds are separated by regions where the film remains more-or-less flat. To our knowledge, the appearance of such folds in elastic films bonded to viscous supports is a new discovery. While their exact origin remains unclear, we argue that folds are energetically favorable (i.e. reduce bending energy more) as compared to wrinkles. But because folds can take a long time to develop, wrinkles may appear as a temporary intermediate state.