Thin film wrinkling by strain mismatch on 3D surfaces

Wrinkling of thin films in micro- and nanoscale has found wide applications in stretchable electronics, optical gratings, material characterization, and biology. However, most wrinkling work is in planar fashion and only limited work on 3D wrinkling has been reported. In this paper, we present experimental demonstration of 3D Cr film wrinkling on PDMS micro-ridges induced by strain mismatch. We first fabricated PDMS micro-ridges using replication from a silicon mold, prepared by conventional microfabrication processes. While pre-stretching the PDMS specimen along the ridge direction, thin Cr film was deposited through physical sputtering. After releasing the prestrain, two wrinkling waves formed on the two inclined surfaces of the PDMS ridges with the same wavelength but opposite phases. These two wrinkling waves twist the top of the ridge into a wavy shape, forming wrinkling waves oscillating in the direction parallel to the substrate plane. In contrast, 2D wrinkling waves oscillate in the direction perpendicular to the substrate plane. Using finite element simulations, we demonstrate that the 180° phase shift between the two sides is favored by the dominant buckling modes. While the 3D wrinkling wavelength is consistent with 2D theoretical prediction, wrinkles on the ridge feature non-uniform wrinkling wave amplitude along the width of the inclined surface. This 3D wrinkling work opens new opportunities in manufacturing curved-shape mold for contact printing and increases surface to volume ratio for wetting and battery applications.