Magneto-Responsive Chiral Optical Materials: Flow-Induced Twisting of Cellulose Nanocrystals in Patterned Magnetic Fields.

Magnetic fields have been used to uniformly align the lyotropic chiral nematic (cholesteric) liquid crystalline (LC) phase of biopolymers to a global orientation and optical appearance. Here, we demonstrate that, in contrast, weak and patterned magnetic field gradients can create a complex optical appearance with the variable spatial local organization of needle-like magnetically decorated cellulose nanocrystals. The formation of optically patterned thin films with left- and right-handed chiral and achiral regions is observed and related to local magnetic gradient-driven vortices during LC suspension flow. We trace the localized flow directions of the magnetically decorated nanocrystals during evaporation-induced assembly, demonstrating how competing evaporation and field-induced localized flow affect the twisted organization within magnetically induced vortices. The simulations suggested that localized twisting inversion originates from the interplay between the direction and strength of the local-depth-related magnetic gradients and the receding front through peripheral magnetic gaps. We propose that this finding will lead to magnetically patterned photonic films.