Osmotic-Pressure-Mediated Control of Structural Colors of Photonic Capsules

Abstract

Crystalline or glassy materials made of colloidal nanoparticles show distinctive photonic effects; the crystals exhibit sparkling colors with strong iridescence, while the glasses show noniridescent colors. Both colors are the results of constructive interference of the reflected light by the nonadsorbing nanostructures. Such colored materials have potential applications as nonfading colorants in reflective color displays, optical sensors, coatings, and cosmetics. All of these applications require granular format of the nanostructures; however, precise control of the nanostructures from amorphous to crystalline over the submillimeter length scale remains challenging. Here, we present micrometer-level control of photonic nanostructures confined in microcapsules through osmotic-pressure-mediated concentration. We encapsulate aqueous suspensions of colloidal particles using double-emulsion drops with ultrathin layers of photocurable resin. The microcapsules are then isotropically compressed by imposing a positive osmotic pressure difference that forces the water out through the thin resin membrane. We find that the internal nanostructure of our photonic microcapsules can be kinetically controlled from crystalline to amorphous; slow concentration in small pressure gradients yields colloidal crystals with sparkling color patterns, whereas fast concentration in large pressure gradients yields glassy packing with only short-range order, which show uniform color with little iridescence. By polymerizing the thin monomeric shell, we permanently fix these nanostructures. Our findings provide new insights into the design and synthesis of optical materials with controlled structural colors. ■ INTRODUCTIONPeriodic colloidal nanostructures on the scale of half the wavelength of visual light show reflected colors, because of the constructive interference of selected wavelengths of light. 1 Such structural colors are distinguished from the usual "chemical colors" that arise from absorption spectra. When the colloidal nanostructures have long-range order, the color is iridescent; 2 when the order is short-ranged, the color is noniridescent and isotropic. 3 Although most chemical colors eventually fade, because of the oxidation or reaction of their chemical structures, structural colors can persist for as long as their periodic structures survive. Moreover, the structural colors can be dynamically tuned by controlling the periodicity of the nanostructures through chemical environment, temperature, and electric fields. 4 These unique properties of structural colors make them appealing for many applications, including colorimetric sensors, pigments, and reflection-mode displays. 4,5 Structurally colored pigments have been produced using emulsion drops to confine colloidal assembly; 6−11 the resulting pigments can be assembled to build photonic devices or can be dispersed in liquid media to make paints. Single-emulsion drops yield consolidated granules of colloidal crystals through solvent evaporation, 7 and double-emulsion drops en...

Grating Diffraction or Bragg Diffraction? Coloration Mechanisms of the Photonic Ball

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