Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals

Abstract

The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti-photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing.

Structural Color Materials for Optical Anticounterfeiting

“…A similar wetting phenomenon was also observed in rigid opal structures, but the intrinsic contact angles at the wetting threshold were about 90°, [ 90 ] indicating that opal structures were much easier to be infiltrated as there was no “neck” structure in opals to prevent liquid infiltration. On the other hand, the wetting phenomena were also commonly observed in soft inverse opal structures, [ 91–93 ] despite the irregular neck structures and even the nonspherical structures in these system. The wetting of these polymer‐based inverse opal structures could refer to both the swelling and wetting processes.…”

“…Zhong and co‐workers demonstrated an air flow responsive SiO 2 opal with hollow spheres. [ 91 ] The patterns were formed by chemical vapor depositions of silane coupling agents followed by selective etching by O 2 plasma. The patterns remained invisible under static environmental humidity, with very limited redshifts under a wide range of humidity.…”

“…Reproduced with permission. [ 91 ] Copyright 2018, Wiley‐VCH. c) An hydrogel interferometer in which the information can be triggered instantly by moisture, and concealed immediately after removing moisture.…”

Simple and Ultrafast Fabrication of Invisible Photonic Prints with Reconfigurable Patterns

“…Currently, various strategies have been developed to fabricate invisible photonic prints on the basis of cross‐linking, drying, or modification of the selected regions of photonic papers, and the patterns can be revealed by dint of water, deformation, magnetic field, UV light, and vapor of solvents, respectively. Most of the fabrication approaches need time‐consuming or complex fabrication processes to guarantee the success hiding and showing the pattern in the absence and presence of external stimulus, respectively.…”

“…For magnetic gel, the long‐lasting stability will be a problem as the solvents in the gel will evaporate over time, resulting in the failure of revealing the pattern. Selective etching is promising in fabricating invisible photonic prints, whose pattern can be fully hidden since there is no color contrast between the pattern and background. Nevertheless, it seems to be a big challenge in printing the patterns with high resolution.…”