Elastic Photonic Microcapsules Containing Colloidal Crystallites as Building Blocks for Macroscopic Photonic Surfaces

Abstract

Colloidal crystals develop structural colors through wavelength-selective diffraction. Recently, a granular format of colloidal crystals has emerged as building blocks to construct macroscopic photonic surfaces or architectures with high reconfigurability through the secondary assembly. Here, we design elastic photonic microcapsules containing colloidal crystallites along the inner wall as a building block. Water-in-oil-in-water double-emulsion templates are microfluidically prepared to have an aqueous dispersion of polystyrene particles in the inner droplet and polydimethylsiloxane prepolymers in the shell. Colloidal particles are enriched in the presence of depletant and salt by osmotic compression, with the crystallization at the inner interface by depletion attraction. The number of nucleation sites depends on the rate of the enrichment, which enables control over the size and surface coverage of the crystallites with osmotic conditions. The enrichment is ceased by transferring the droplets into an isotonic solution, and the oil shell is cured to form an elastic membrane. As the elastic microcapsules have a large void in the core, they are deformable without structural damage in the crystallites. Therefore, the microcapsules can be closely packed to form macroscopic surfaces while achieving a high quality of structural colors with a collection of crystallites aligned along the flattened membrane.

Recent advances in the microfluidic production of functional microcapsules by multiple-emulsion templating

“…To design photonic building blocks, colloidal particles are assembled by depletion attraction in an aqueous core of microcapsules with an elastic shell using W/O/W double-emulsion drops. 132 Depletion attraction is caused by depletant molecules dissolved in the dispersion of particles due to the exclusion of the depletant on the surface of colloids. To avoid clogging and secure flow assurance, the concentration of the depletant is sufficiently reduced in the aqueous dispersion during the microfluidic operation, which prevents the formation of colloidal crystallites by weakening the attraction.…”

Designing photonic microparticles with droplet microfluidics

“…6d. 78,79 The crystalline grains are stabilized by making a solid membrane from the oil shell. The average size of the grains is controllable via the rate of enrichment, as shown in Fig.…”

“…To address this limitation, photonic microcapsules have been designed to have crystalline grains along the elastic membrane and a large void in the middle via depletion attraction. 79 When the array of elastic microcapsules is compressed using a pair of plates, the interstitial voids among the microcapsules are filled by the deformation. Moreover, the crystalline grains are aligned to be parallel to the flattened membrane in each microcapsule, which maximizes the region for specular reflection, resulting in high reflectivity.…”

Direct Determination of the Phase Diagram of a Depletion-Mediated Colloidal System

“…As the depletion attraction provides a facile means to tune interparticle interaction, it has been used to study colloidal crystallization and gelation and produce colloidal photonic materials. 15,17,20,21,[23][24][25]28,33 To systematically study the influence of depletion interaction on the phase behavior of colloidal particles, we suspend polystyrene particles with negative surface charges in water containing sodium chloride and poly(acrylamide-coacrylic acid) (p(Am-co-AAc)) as a charge-screening agent and depletant, respectively, and explore the phases by controlling both concentrations. The depletant, p(Am-co-AAc), has an average molecular weight of 5,000,000 g/mol and a hydrodynamic radius of r h = 32.5 nm.…”