Cholesteric Spherical Reflectors with Tunable Color from Single‐Domain Cellulose Nanocrystal Microshells

Abstract

The wavelength‐ and polarization‐selective Bragg reflection of visible light exhibited by films produced by drying cholesteric liquid crystal (CLC) suspensions of cellulose nanocrystals (CNCs) render these biosourced nanoparticles highly potent for many optical applications. While the conventionally produced films are flat, the CLC‐derived helical CNC arrangement would acquire new powerful features if given spherical curvature. Drying CNC suspension droplets does not work, because the onset of kinetic arrest in droplets of anisotropic colloids leads to severe buckling and loss of spherical shape. Here these problems are avoided by confining the CNC suspension in a spherical microshell surrounding an incompressible oil droplet. This prevents buckling, ensures strong helix pitch compression, and produces single‐domain cholesteric spherical reflector particles with distinct visible color. Interestingly, the constrained shrinkage leads to spontaneous puncturing, leaving every particle with a single hole through which the inner oil phase can be extracted for recycling. By mixing two different CNC types at varying fractions, the retroreflection color is tuned throughout the visible spectrum. The new approach adds a versatile tool in the quest to utilize bioderived CLCs, enabling spherically curved particles with the same excellent optical quality and smooth surface as previously obtained only in flat films.This article is protected by copyright. All rights reserved

Modulation of Tannic Acid on the Cholesteric Structure of Cellulose Nanocrystals

“…To obtain a well-organized CNC-based liquid crystal material with fewer defects and higher quality via flow-based processes, it is recommended to use a CNC suspension with a concentration in the biphasic region. − Otherwise, the significant electroviscous effect in the highly concentrated CNCs suspensions will induce gelation in the system and make the process hard to conduct. This process window is in the range of concentrations higher than the critical concentration for cholesteric phase formation but lower than the concentration for gelation.…”