Photonic Microcapsules Containing Single-Crystal Colloidal Arrays with Optical Anisotropy

Colloidal particles with a repulsive interparticle potential spontaneously form crystalline lattices, which are used as a motif for photonic materials. It is difficult to predict the crystal arrangement in spherical volume as lattices are incompatible with a spherical surface. Here, the optimum arrangement of charged colloids is experimentally investigated by encapsulating them in double-emulsion drops. Under conditions of strong interparticle repulsion, the colloidal crystal rapidly grows from the surface toward the center of the microcapsule, forming an onion-like arrangement. By contrast, for weak repulsion, crystallites slowly grow and fuse through rearrangement to form a single-crystal phase. Single-crystal structure is energetically favorable even for strong repulsion. Nevertheless, a high energy barrier to colloidal rearrangement kinetically arrests the onion-like structure formed by heterogeneous nucleation. Unlike the isotropic onion-shaped product, the anisotropic single-crystal-containing microcapsules selectively display—at certain orientations but not others—one of the distinct colors from the various crystal planes.