Designing elastic photonic microbeads with high color saturation for structural coloration

Abstract

Structural colorants based on photonic microbeads containing crystalline colloidal arrays have emerged in various applications, including inks and cosmetics. However, their low color saturation and deformability generally limit their potential uses. In this study, we design elastic photonic microbeads with enhanced color saturation and high deformability. To produce the photonic microbeads, we use oil-in-oil (O/O) single emulsion droplets as a template. For structural coloration within spheres, highly concentrated silica particles in elastomer precursors are microfluidically emulsified into immiscible oil to produce monodisperse droplets. The silica particles spontaneously form crystalline arrays in the entire volume of the droplets due to interparticle repulsion which is unperturbed by the diffusion of the surrounding oil whereas weakened for oil-in-water droplets. The crystalline arrays are permanently stabilized by photopolymerization of the precursor, forming elastic photonic microbeads. The high crystallinity of colloidal arrays increases the reflectivity at stopband, enhancing color saturation. The microbeads composed of elastic polymer show high deformability under mechanical deformation. To further improve the color saturation and deformability of microbeads, elastic photonic shells are fabricated using water-in-oil-in-water (W/O/W) double-emulsion templates. The elastic shells are produced by photocuring acrylate-based resin containing non-close-packed arrays of silica particles in the middle oil layer. The presence of two concentric interfaces and high concentrations of water-soluble molecules in the inner and continuous water phases improves the crystalline order of silica particles. Moreover, the absence of relatively disordered core reduces incoherent random scattering, further enhancing color saturation. Although the shells themselves are elastically deformable, the liquid core renders them more deformable with low stiffness. Additionally, the microbeads with high color saturation and deformability can be easily formed using bulk emulsification for photonic inks. The microbeads with high deformability can be redisepersed in photocurable and printable acrylate-based resin, which is potentially beneficial for improving the fluidity of suspensions. These advancements in elastic photonic microbeads offer new opportunities for structural coloration in inks.