Structural Color Mixing in Microcapsules through Exclusive Crystallization of Binary and Ternary Colloids

Abstract

Colloidal crystals have been designed as photonic microparticles for various applications. However, conventional microparticles generally have only one stopband from a single lattice constant, which restricts the range of colors and optical codes available. Here, we create photonic microcapsules that contain two or three distinct crystalline grains, resulting in dual or triple stopbands that offer a wider range of colors through structural color mixing. To produce distinct colloidal crystallites from binary or ternary colloidal mixtures, we manipulate the interparticle interaction using depletion forces in double-emulsion droplets. Aqueous dispersions of binary or ternary colloidal mixtures in the innermost droplet are gently concentrated in the presence of a depletant and salt by imposing hypertonic conditions. Different-sized particles crystallize into their own crystals rather than forming random glassy alloys to minimize free energy. The average size of the crystalline grains can be adjusted with osmotic pressure, and the relative ratio of distinct grains can be controlled with the mixing ratio of particles. The resulting microcapsules with small grains and high surface coverage are almost optically isotropic, and exhibit highly-saturated mixed structural colors and multiple reflectance peaks. The mixed color and reflectance spectrum are controllable with the selection of particle sizes and mixing ratios.