Colloidal particles can be assembled to form crystalline or amorphous structures which exhibit their own photonic effects; crystalline shows strong iridescent colors and amorphous structure shows non-iridescent colors. Such colloidal photonic structures are promising as new class of photonic materials and color pigments. However, controlled formation of crystalline and amorphous structures and their encapsulation into 'ink' capsule still remain important challenge.Here, we report a microfluidic approach to create photonic capsules containing colloidal structures. With a glass capillary device, an aqueous suspension of polystyrene particles is encapsulated with ultra-thin oil shell in a continuous water phase, thereby resulting in water-in-oil-in-water (W/O/W) double-emulsion drops. The drops are then subjected to hypertonic condition, which pumps water out through the thin shell, thereby leading to concentration of the particles in the innermost water drops. The concentration rate strongly influences optical appearance of the drops, where the rate is precisely controlled by adjusting osmotic pressure difference. At low pressure difference, colloidal particles are slowly concentrated, thereby yielding colloidal crystals in the drops; the drops exhibit sparkling color pattern. Stepwise increase of external osmotic pressure can yield highly concentrated colloids, while maintaining crystalline phase. At high pressure difference, the particles are quickly concentrated, which suppresses crystallization of the particles; the drops still show colors but no sparkling pattern. This is attributed to short-range order; glass-like packing of uniform colloids has consistent interparticle distance, which can provide less-iridescent structural color. As an oil phase, a photocurable resin can be used, which enables the polymerization of ultra-thin shell after formation of crystalline or glass photonic structures, thereby stabilizing the photonic structures. The photonic microcapsules, containing colloidal crystalline or glass, can be used as building block to construct photonic devices or aesthetic color coating.

Microfluidic encapsulation of colloidal photonic structures toward photonic ink capsules

248th National Meeting (2014)

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