Reconfiguring Colors of Single Relief Structures by Directional Stretching.

Color changes can be achieved by straining photonic crystals or gratings embedded in stretchable materials. However, the multiple repeat units and the need for a volumetric assembly of nanostructures limit the density of information content. Inspired by surface reliefs on oracle bones and music records as a means of information archival, here, surface-relief elastomers are endowed with multiple sets of information that are accessible by mechanical straining along in-plane axes. Distinct from Bragg diffraction effects from periodic structures, trenches that generate color due to variations in trench depth, enabling individual trench segments to support a single color, are reported. Using 3D printed cuboids, trenches of varying geometric parameters are replicated in elastomers. These parameters determine the initial color (or lack thereof), the response to capillary forces, and the appearance when strained along or across the trenches. Strain induces modulation in trench depth or the opening and closure of a trench, resulting in surface reliefs with up to six distinct states, and an initially featureless surface that reveals two distinct images when stretched along different axes. The highly reversible structural colors are promising in optical data archival, anti-counterfeiting, and strain-sensing applications.