Surface buckling delamination patterns of film on soft spherical substrates.

This study aims to provide a fundamental understanding of the morphological transition of film buckling-delamination in an elastomeric bilayer spherical shell system. We developed an experimental system in which surface delamination buckles emerge because of biaxial compression of the elastomeric bilayer spherical shell driven by an air-pressured (pneumatic) device. A flat PDMS plate was first isotropically expanded and shaped into a hemisphere by air pressure. Subsequently, the hemisphere substrate was covered with a thin PDMS film. By releasing the air pressure, the substrate contracts and the outer film surface were subjected to biaxial compression; this resulted in various surface patterns of film buckling-delamination. It was found that the surface morphology transitions from initial delamination sites and that the buckles propagate on the entire surface of the sphere. This pattern formation is dependent on the surface strain distribution, i.e., radial strain and circumferential strain. In order to control the surface pattern, we systematically changed the material and system parameters such as film thickness, Young's modulus, and interfacial adhesion condition. In addition, finite element (FEM) computation was carried out to simulate the surface pattern and to elucidate the mechanism of buckling-delamination morphological transition.