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.

Retroreflection and Wettability Controlled Smart Indicator Based on Responsive Bilayer Photonic Crystals for Traffic Warning

“…Preparation of Monodispersed SiO 2 Nanospheres, SiO 2 Arrays, Monolayer Inverse Opals, and Bilayer Films: SiO 2 nanospheres, SiO 2 arrays, and monolayer inverse opals were prepared under a modified Stöber method (detailed information is listed in Figures S1-3, and Table S1, Supporting Information). [36,54] For the preparation of bilayer photonic crystals, "template A" and "template B" were combined into the sandwich structure after dislocation of 90°. [36] By utilizing the capillary force, the interstitials in-between SiO 2 microspheres of the "sandwich" template were filled up with oligomer mixtures consisting of ETPTA, PEGDA (200), and BMA with varying volumetric ratios from 1:4:0.25 to 1:6:0.25.…”

“…, where γ wa is the water-air surface tension, ϕ is the central angle of the position of solid-water-gas junction point at any moment, θ c is the contact angle. According to the previous research, [32,54,55] infiltration of a pore occurs if the intrinsic contact angle is less than ϕ 0 , while no infiltration occurs if θ c > ϕ 0 . The contact angle of water on the surface is θ c ≈ 48° (>30.7°, ϕ 0 ), which means that water can partially infiltrate the hydrophilic surface.…”

“…The surface wettability plays a decisive role in the color changing of the responsive photonic crystals. [54] The wettability response mode is a method to realize the controllable infiltration of the liquid medium by using the unique pore structure of opal photonic crystal. [55,56] Because of the existence of the free-energy landscape, [57] for normal conditions, the global nonwetting state should occur when there is an energy barrier associated with the propagation of the liquid front through a micropore.…”

Facile fabrication of encryption composite materials with trilayer quasi-amorphous heterostructure

“…As an important component of information security, information encryption technology refers to the use of mathematical or physical means in protecting informa-tion from illegal access. In recent years, unique functional materials have been developed for information encryption, including organic afterglow materials with distinguishable lifetime codes or reversible solid-state emission features [3][4][5], responsive photonic watermarks [6][7][8][9][10][11][12][13][14][15][16][17][18][19], rare-earth-doped materials with luminescence modulation [20], phase-change thermochromic materials [21], and plasmonic metamaterials [22][23][24]. Although these encryption strategies can be promising tools for strict security protection, their integration into real-life applications still faces great challenges due to their inability to meet some aspects of information security simultaneously.…”

“…Recently, a bilayer polymer photonic crystal with shape memory effect has been developed for the water-responsive encoding and decoding of information systems [7]. However, the pressure-assisted fabrication of coding regions increases the effective refractive index due to the change of lattice spacing.…”