Controlling viscosity to engineer focal conic domains in photonic cellulose nanocrystal films

TL;DR

This study demonstrates how controlling viscosity through salt concentration and sonication influences the self-assembly of cellulose nanocrystal films, enabling the engineering of specific photonic structures like focal conic domains with tunable optical properties.

Contribution

It introduces a viscosity-based approach to steer CNC self-assembly, allowing reproducible fabrication of films with desired photonic and defect structures, advancing sustainable photonic material design.

Findings

Low viscosity yields large, uniform cholesteric domains.

Intermediate viscosity conditions produce focal conic domains.

Viscosity controls the extent of tactoid coalescence and optical response.

Abstract

Cellulose nanocrystals (CNCs) form cholesteric architectures that can have color specific reflectivity and enable sustainable photonic films. However, achieving uniform color, suppressing iridescence, and accessing ordered defect structures such as focal conic domains remain challenging. Here, we control the photonic properties of CNC films by steering the self assembly process. Across 24 dish-cast films with varying salt concentrations and sonication doses, we combine viscosity measurements, timelapse polarized optical microscopy, and angle-resolved reflectance spectroscopy to correlate evaporation dynamics with photonic structure. We show that viscosity, jointly controlled by NaCl-mediated electrostatic screening and sonication-induced bundle fragmentation, dictates the extent of tactoid coalescence. Low-viscosity suspensions generate large, homogeneous cholesteric domains and narrow spectral responses, while high viscosity leads to arrested, heterogenous domains and increased diffuse light reflection. Critically, within a narrow parameter window of intermediate ionic strength and moderate sonication, we reproducibly engineer photonically active focal conic domains. These results identify viscosity-driven flow as a key, previously underappreciated factor in CNC self-assembly and establish design rules for producing structurally colored films with tunable photonic response, reduced iridescence, and controllable defect architectures.