Macroporous Hydrogels for Fast and Reversible Switching between Transparent and Structurally Colored States

Abstract

Colloidal crystals have been used for creating stimuli-responsive photonic materials. Here, macroporous hydrogels are designed, through a simple and reproducible protocol, that rapidly and reversibly switch between highly transparent and structurally colored states. The macroporous hydrogels are prepared by film-casting photocurable dispersions of silica particles in hydrogel-forming resins and selectively removing silica particles. The silica particles spontaneously form a nonclose-packed array due to repulsive interparticle interaction, which form the regular array of cavities after removal. However, the cavities are randomly collapsed by drying, losing a long-range order and rendering the materials highly transparent. When the hydrogels are swollen by either water, ethanol, or the mixture, the regular array is restored, which develops brilliant structural colors. This switching is completed in tens of seconds and repeatable without any hysteresis. The resonant wavelength depends on the composition of the water-ethanol mixture, where the dramatic shift occurs in one-component-rich mixtures due to the composition of the hydrogel. Micropatterns can be designed to have distinct domains of the macroporous hydrogels, which are transparent at the dried state and disclose encrypted graphics and unique reflectance spectra at the wet state. This class of solvent-responsive photonic hydrogels is potentially useful for alcohol sensors and user-interactive anti-counterfeiting materials.

Microfluidic Production of Mechanochromic Photonic Fibers Containing Nonclose‐Packed Colloidal Arrays

“…PEGPEA forms an elastomer when it is polymerized by UV irradiation in the presence of a photoinitiator, whereas it makes a solvation layer on the surface of silica particles through hydrogen bonds, which enables the silica particles to form a nonclose‐packed colloidal array. [ 42,43 ] Therefore, photopolymerization produces a photonic elastomer containing a nonclose‐packed colloidal array. As a carrier fluid, we use hexadecane containing 4 w/w% surfactant of ABIL EM 90, presaturated by PEGPEA.…”

3D Chiral Photonic Nanostructures Based on Blue‐Phase Liquid Crystals

“…[ 30–32 ] The bottom‐up approach, however, involves the controlled self‐assembly of judiciously designed building blocks into periodic photonic nanostructures. [ 33–37 ] Although significant advancements have been made in the development of various artificial photonic nanostructures, the fabrication of 3D nanostructures with intrinsic chirality remains a challenging task. Based on the theoretical prediction of large complete 3D photonic bandgaps in high‐index contrast silicon square‐spiral structures, [ 38,39 ] Thiel et al fabricated 3D chiral photonic crystals by a two‐photon direct laser writing technique; the high‐quality 3D polymeric helices arranged in a periodic array exhibited polarization stop bands with large circular dichroism in transmission in the infrared regions.…”

Photonic Microbeads Templated by Oil‐in‐Oil Emulsion Droplets for High Saturation of Structural Colors

“…[4][5][6] Moreover, the colors are adjustable with a single set of materials, non-toxic, and never fade. [7,8] These novel properties render the colloidal crystals appealing for various coloration applications of printings, paintings, and PEGPEA has a solvation layer on the surface, which renders the particles to have repulsive interparticle potential and causes spontaneous crystallization. The emulsion droplets of silica-in-PEGPEA suspension turn photonic microbeads by irradiation with UV.…”