Encoding and Decoding of Invisible Complex Information in a Dual‐Response Bilayer Photonic Crystal with Tunable Wettability

Abstract Combining responsive colloidal photonic crystals into invisible wettability patterns is important in steganography and watermarking for information encoding and decoding. Using a bilayer photonic crystal as a regionally functionalized porous carrier, a dual‐responsive surface with tunable wettability, reversible switching between wetting and no wetting, and responsivity to pressure and solvents are reported. The mechanism of the design involves reversible switching of micropore shapes and optical diffraction in the fabricated periodic nanostructures, and uses the infiltration and capillary evaporation of fluids to realize the spectral diversity of reflectance. It generates different invisible wettability patterns that are regulated by induced pressure and the Donnan equilibrium osmotic pressure. Immersion of the sample in water induces the varying degrees of infiltration and immediately displays multiple colors. It enables instantaneously transparentizing the full infiltration parts of the top layer and simultaneously releasing the reflected light of the corresponding bottom layer. Moreover, those invisible patterns can be reset by wiping with ethanol. Multiple ways to encode and reveal invisible complex information can easily be realized due to these characteristics. This work opens a new avenue for encoding complex information in a single material platform and extends the design approach of invisible patterns for information coding.