Building blocks that govern spontaneous and programmed pattern formation in pre-compressed bilayers

Surface wrinkling in stiff-film/soft-substrate bilayers is a common phenomenon in biological systems and is increasingly being exploited in thin-film technology. While the onset of surface wrinkling in end-compressed bilayers is well understood, questions remain with regards to the evolution of the wrinkling pattern in the intermediate and deep post-wrinkling regimes, especially when the substrate is strongly pre-compressed. Here, we explore the bifurcation landscape of end-compressed bilayers with strongly pre-compressed substrates, using hyperelastic, plane strain finite elements and generalised path-following algorithms. After bifurcating from a flat into a sinusoidally wrinkled state, bilayers undergo further n-tupling bifurcations into stable wrinkling patterns of longer wavelength whose periodicity n = {4, ..., 8} is a function of overall bilayer length. These five n-tupling wrinkling patterns are shown to be independent localisations of the deformation mode and are accordingly identified as stable `building blocks' that govern the intermediate post-wrinkling regime. Additional end-shortening into the deep post-wrinkling regime then leads to further period doubling and coalescence of the building blocks. Beyond a certain length threshold, a bilayer can form a combinatorial side-by-side arrangement of the five building blocks. In the limit of an infinitely long bilayer, this leads to the phenomenon known as spatial chaos with the emergence of an infinite set of possible wrinkling patterns. In reality, though, the precise side-by-side arrangement of the building blocks is governed by the initial conditions. We show that the morphological evolution of the wrinkling pattern can be programmed by a judicial placement of manufactured dents in the thin film, creating new manufacturing capabilities for textured bilayers.