Elastocapillary folding in film–substrate bilayers

Wrinkles in thin elastic films on soft substrates can transition into folds when exposed to water. This instability, referred to as elastocapillary folding, has recently been observed experimentally and holds important implications for understanding biological morphogenesis and designing functional interfaces. However, the mechanisms underlying this transition remain insufficiently understood. In this study, we investigate the governing factors of elastocapillary folding using finite element simulations. Our results demonstrate that the onset and shape of folds are controlled by film thickness, contact angle characterizing the surface wettability, liquid surface tension, substrate elasticity, and the compressive strain required for wrinkling. Phase diagrams are constructed to predict the onset of folding instabilities and the spacing between folds. Our findings indicate that thinner, more hydrophilic films on softer substrates under larger global compressive strain preferentially exhibit elastocapillary folding, where fold spacing is determined by the combined effects of these parameters. These results provide guidelines for creating functional interfaces and suggest new approaches for water-assisted fabrication of micro-/nanostructures. • Folding in film–substrate bilayers can be triggered and controlled by water. • Factors controlling fold onset and dimensions are uncovered. • Phase diagrams are developed to predict folding behavior. • Findings enable water-assisted fabrication of micro-/nanostructures.