Flower Inspiration: Broad-Angle Structural Color Through Tunable Hierarchical Wrinkles In Thin Film Multilayers

The petals of some flowers form hierarchical structures when nano-scale cuticular ridges overlay bulged epidermal cells. These hierarchical structures can broaden the observable angles of iridescence. The resulting optical effect enhances the foraging efficiency of pollinators. Although efforts have been devoted to mimicking this unique broad-angle structural color, the intrinsic tunability offered by natural systems to control such a broadened spectrum is still absent in synthetic models. A hierarchical system is developed that provides hierarchical wrinkle-based structures that tune the observable angles for structural color. Laser diffraction measurements demonstrate that the observable angle of reflectance is broadened in proportion to the square root of the applied compressive strain. The morphology controls the diffraction pattern: the small wrinkles control the diffraction angles and the large wrinkles broaden the observable range. The development of a multi-mode wrinkling system to produce this broad-angle structural color only occurs within a limited range of conditions, which are experimentally discovered and theoretically modeled. Without diffractive small wrinkles, single wrinkling modes do not display structural colors. The control of wrinkling modes mimics the tunability of petals, which gives new insight into the natural system and provides a robust foundation for tunable structural color control.