Depletion-mediated colloidal crystallization in double-emulsion drops for structural coloration

Abstract

Structural colors are developed through wavelength-selective diffraction with regular micro- or nano-structure of diverse living things in nature. It can be imitated with colloidal crystals generated by self-assembly of colloidal particles. Recently, macroscopic photonic surfaces or architectures with high reconfigurability is designed through the second assembly of microcompartment containing photonic nanostructures. Here, we design elastic photonic microcapsules containing colloidal crystallites along the inner wall as a building block for macroscopic photonic surfaces. Water-in-oil-in-water double-emulsion drops are microfluidically generated that have an aqueous dispersion of polystyrene nanoparticles in the inner droplet and polydimethylsiloxane prepolymers in the membrane. Colloidal particles, coexisting with depletant and salt, are enriched by osmotic compression with the depletion-mediated crystallization. The size of crystallites and surface coverage of microcapsules can be adjusted by compression rates. Generated droplets are transferred into an isotonic solution to cease the enrichment, and the shell is thermally cured to form an elastic membrane and protect inner structure. As the elastic microcapsules have a large void in the core, they are deformable without structural damage of inner structure or unwanted discoloration. Finally, microcapsules are closely packed to form macroscopic photonic surfaces that have a high quality of structural colors and high reflection intensity.