Reversible Hidingof Janus Structural Color PatternInduced by ZnS Colloidal Spheres on Inverse-Opal Structure

The Janus phenomenon observed in nature is intriguing and has garnered significant attention within the domain of structural color research. However, there have been no reports of Janus structural color with concealed properties. By spraying a layer of uniform ZnS spheres on the surface of inverse-opal photonic crystal (IOPC) films, we developed color-tunable Janus films with the ability of reversibly hiding structural color. As shown through simulation and experimental validation, the asymmetric Mie scattering effect of ZnS spheres results in different structural colors on either side of the film, thereby creating Janus characteristics. Here, the IOPC film served as a water-responsive switch to conceal or reveal the Janus structural color. The film exhibits a color corresponding to the photonic band gap and hides its Janus characteristics during the dry state, while the Janus characteristics become evident upon wetting. The incorporation of ZnS spheres with varying diameters endows the patterned Janus film with enhanced anticounterfeiting ability.