Curvature induced hierarchical wrinkling patterns in soft bilayers.

Sinusoidal wrinkling will occur in a planar film-substrate bilayer when the uniaxial compressive strain imposed to the system exceeds a critical value. However, when a core-shell soft cylinder is subjected to axial compression, surface wrinkling patterns may evolve from the sinusoidal mode to the diamond-like mode, depending on the modular ratio and the curvature of the system. Inspired by this phenomenon, we here propose a simple yet robust strategy to fabricate hierarchical wrinkling patterns by controlling the curvature of a film-substrate system. To quantitatively understand the experimental results, a three-dimensional finite element model has been built to track the wrinkling pattern evolution. Furthermore, a phase diagram is provided based on the theoretical analysis and finite element simulations, which may guide the experimental design. In addition, the wetting properties of the surface with hierarchical micropatterns fabricated using the proposed method are investigated. The results show that the hierarchical surface wrinkles lead to anisotropic wetting behavior, which can be tuned by controlling the imposed compressive strain. The tunable anisotropic wetting surface fabricated here may find a broad range of applications such as in the development of sensors, fluidic devices, micro-reactors and biomedical devices.