Buckling of a stiff thin film on an elastic graded compliant substrate

An analytic model of two-level compressive buckling with applications in the assembly of free-standing 3D mesostructures

Recently developed methods for mechanically-guided assembly exploit stress release in prestretched elastomeric substrates to guide the controlled formation of complex three-dimensional (3D) mesostructures in advanced functional materials and integrated electronic devices. The techniques of interfacial photopolymerization allow for realization of such 3D mesostructures in free-standing forms, separated from their elastomeric substrate, via formation of an integrated base layer. Theoretical models for the complex modes of deformation associated with this scheme are essential in the optimal design of the process parameters. Here, we present an analytic finite-deformation model of an isolated double-ribbon structure to describe the buckling process and morphology change of the assembled mesostructures upon removal of the substrate. As validated by finite element analyses (FEA), this analytic model can accurately predict the profiles of the double-ribbon structure with a range of different design parameters. We further illustrate the extension of this model to the analyses of 3D mesostructures with different geometries. Inspired by analytic results for flexible base structures, combined experimental results and numerical simulations demonstrate that mechanical interactions between the two different layers can be leveraged to achieve hierarchical assembly of 3D mesostructures. These findings could be useful in further advances in designs of free-standing 3D mesostructures based on mechanically-guided assembly.

Surface wrinkling of an elastic graded layer

Surface instabilities have been extensively studied for homogeneous materials or film/substrate systems but with less studies on elastic graded materials. This paper studies surface wrinkling of an elastic graded layer theoretically, numerically and experimentally. A theoretical model for the onset of surface wrinkling with a sinusoidal mode is established. The predicted critical wrinkling strain and wavelength agree well with finite element analysis (FEA) for the elastic graded layer with exponentially decaying modulus. The influence of the layer thickness as well as the material properties on the critical conditions for the onset of surface wrinkling is fully investigated. The morphology evolution of surface wrinkling from FEA indicates the transitions of the sinusoidal mode to the arch mode and then to the period-doubling mode with a co-existing crease mode and folding mode, which agree well quantitatively with experimental observations. These results are helpful to provide physical insights into the influence of material inhomogeneity on surface instabilities.