Capillarity-induced fold localization in film-substrate systems.

Surface instability and elastocapillarity represent critical phenomena in biological and engineered systems. In this study, we investigate capillarity-induced fold localization in film-substrate systems through experiments and finite element simulations. Upon water droplet deposition, globally ordered wrinkles transform into localized folds. The fold morphology and dimensions depend on the aspect ratio of initial wrinkles. Our results demonstrate that high-aspect-ratio wrinkles facilitate spontaneous formation of closed channels beneath the surface upon fold emergence. Additionally, the morphological transition between wrinkles and folds exhibits reversible control through applied strain adjustment. These findings enable technological applications such as the creation of fold nanochannels and graphene oxide folding. This work establishes a fundamental framework for understanding the interplay between surface instability and elastocapillarity, which represents a crucial mechanism in biological and engineered systems while providing design principles for functional surfaces and devices.