13. Tuning Surface Morphology of Polymer Films Through Bilayered Structures, Mechanical Forces, and External Stimuli

Surface wrinkles and many other instability patterns are ubiquitous in nature, such as fruits, vegetables, skins, and biological tissues. Understanding these instability patterns is of paramount importance in physics and mechanics fields, in particular, for their engineering and biology applications. In this chapter, we give an overview of how to tune the surface morphology of polymer thin films through bilayered structures, mechanical forces, and external stimuli through combined theoretical, computational, and experimental studies. First, we demonstrate how the compressibility of the substrate can influence the buckling and post-buckling behaviors of a perfectly bonded hard thin film. We find that Poisson’s ratio of the substrate cannot only shift the critical strain for the onset of buckling but also affect the buckling modes. Second, we explore the surface instability of bilayered hydrogel subjected to both compression and solvent absorption. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts. Third, we discuss the transition of surface–interface creasing in bilayered hydrogels. The surface or interface crease of the bilayered hydrogels under swelling is found to be governed by both the modulus ratio and height ratio between the thin film and substrate. Last, we study the surface instability of monolayer graphene supported by a soft (polymer) substrate under equal-biaxial compression. Regardless of the interfacial adhesion strength between the graphene and substrate, herringbone wrinkles have always been observed due to their lowest energy status, compared with the checkerboard, hexagonal, triangular, and one-dimensional sinusoidal modes. These fundamental understandings about the surface morphology of polymer thin films and their bilayered structures will enable their future applications in engineering and biology fields, such as flexible electronics, biofouling, and interfacial adhesion.