Spatially Programmable Chirality in Cellulose Nanocrystal Films via Rotational Magnetic Flow.

Programmed assembly of natural materials on a large scale is often limited by inherent factors, including dimensional dispersity, complex hierarchical organization, and slow processing kinetics. In this study, we demonstrate a scalable strategy to preprogram the chiral assembly of cellulose nanocrystals (CNCs) by applying a rotational magnetic field during evaporation-induced self-assembly. To facilitate magnetic responsiveness, CNCs are decorated with magnetic nanoparticles and subjected to a rotational magnetic field. This magnetically induced azimuthal shear flow aligns the nanocrystals with a remarkably high local orientational order parameter of 0.96. On the macroscopic scale, the rotational flow generates a gradual, azimuthal alignment, resulting in large-area orientational ordering with identical helicity extending across centimeter-scale regions. Notably, the handedness of the chiral structure and the emergence of distinct optical textures, such as centimeter-wide Maltese crosses, can be controlled by adjusting the direction and strength of the induced large rotational magnetic vortex. This approach provides a versatile route for the larger-scale fabrication of programmable chiral photonic materials from bioderived building blocks.