Photonic Microbeads Templated by Oil‐in‐Oil Emulsion Droplets for High Saturation of Structural Colors

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2022-02, Vol.18 (8), p.e2105225-n/a
• Main Authors: Kim, Jong Hyun, Kim, Jong Bin, Choi, Ye Hun, Park, Sanghyuk, Kim, Shin‐Hyun
• Format: Article
• Language: English
• Subjects: Arrays; Beads; Biocompatibility; colloidal crystals; Color; Cosmetics; Crystal structure; Crystallinity; Droplets; Elastomers; Emulsions; Incoherent scattering; Inks; microbeads; microfluidics; Microspheres; Nanoparticles; Nanotechnology; Optics and Photonics; Photonic crystals; photonic inks; Photons; Photopolymerization; Precursors; Saturation (color); Silicon dioxide; Stiffness; structural colors; Water drops
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202105225

Photonic microbeads containing crystalline colloidal arrays are promising as a key component of structural‐color inks for various applications including printings, paintings, and cosmetics. However, structural colors from microbeads usually have low color saturation and the production of the beads requires delicate and time‐consuming protocols. Herein, elastic photonic microbeads are designed with enhanced color saturation through facile photocuring of oil‐in‐oil emulsion droplets. Dispersions of highly‐concentrated silica particles in elastomer precursors are microfluidically emulsified into immiscible oil to produce monodisperse droplets. The silica particles spontaneously form crystalline arrays in the entire volume of the droplets due to interparticle repulsion which is unperturbed by the diffusion of the surrounding oil whereas weakened for oil‐in‐water droplets. The crystalline arrays are permanently stabilized by photopolymerization of the precursor, forming elastic photonic microbeads. The microbeads are transferred into the refractive‐index‐matched biocompatible oil. The high crystallinity of colloidal arrays increases the reflectivity at stopband and the index matching reduces incoherent scattering at the surface of the microbeads, enhancing color saturation. The colors can be adjusted by mixing two distinctly colored microbeads. Also, low stiffness and high elasticity reduce foreign‐body sensation and enhance fluidity, potentially serving as pragmatic structural colorants for photonic inks. Elastic photonic microbeads with high color saturation are produced by employing oil‐in‐oil emulsion droplets. As the continuous oil phase does not lessen the interparticle repulsion, particles form crystalline arrays in the entire volume. In addition, refractive‐index matching further reduces incoherent scattering. High color saturation and elasticity render the microbeads promising as structural colorants in cosmetics and inks.