Structural Coloration of 3D Irregular Surfaces via Spraying of Liquid Photonic Crystals for Iridescent Effect and High Colorfastness

Biomimetic structural coloration with photonic crystals (PCs) possesses the advantages of eco-friendly coloration, brilliant structural colors, and high colorfastness to light, etc. It is appealing for advanced coloration of various substrate surfaces. However, the fabrication of PCs with iridescent effect and high colorfastness on three-dimensional (3D) irregular substrate surfaces is still challenging. In this work, liquid photonic crystals (LPCs) with high mass fraction (40   wt.%) of colloidal nanospheres were used as the spraying solution to assemble long-range ordered PCs with iridescent effect. Thanks to the fluidity and dynamic reversibility, the pre-crystallized LPCs could disassemble under the applied spraying pressure, which promoted smooth spraying and enabled the formation of atomized micro-droplets. The colloidal nanospheres present in these micro-droplets were able to quickly reassemble and restore their pre-crystallized state. This allowed micro-droplets to connect with each other on the curved or 3D irregulated substrate surface into orderly assembled solid PCs upon the water evaporation, exhibiting brilliant structural colors with iridescent effect. Meanwhile, the use of the spraying solution with high mass fraction of colloidal nanospheres was shown to be effective against the slipping problem of the micro-droplets on 3D substrate surfaces, enabling the fabrication of PCs with pronounced iridescent structural colors on various 3D substrate surfaces (metal, ceramic, rubber, glass and so on). In addition, pre-spraying a specific polymer slurry to form a bearing layer on the 3D substrate prior to spraying the LPC allows improving the stability of the assembled PC structure under external forces (rubbing, washing, etc.), while retaining the refractive index contrast of PCs and thus achieving the consistency between their structural stability and color saturation.