The geometrical effects of stiff film patterns on surface instability modes of elastic bilayers

Surface instabilities, such as wrinkles and creases, are seen in many places, and are now understood to an excellent degree for homogeneous surfaces. However, fundamental understanding of surface instabilities for systems with in-plane heterogeneity remains lacking, despite their importance in a variety of applications (e.g. metal electrodes on an elastomer surface for flexible electronic devices). In this study, we fabricated an elastic bilayer with stiff film patterns and analyzed the effects of pattern geometry on surface instabilities. We found three distinct regimes for the behavior of the bilayer regions, depending on the pattern size compared to the natural wrinkle wavelength: wrinkling, Euler buckling, and rigid rotation. In the soft substrate regions, creases were formed in all cases. Interestingly, crease formation greatly facilitates the formation of contacts between neighboring stiff regions. The strain for contact was characterized as a function of the pattern geometry, and potential applications based on reversible formation of contact between the stiff film regions were demonstrated.