Magneto-Responsive\nChiral Optical Materials: Flow-Induced\nTwisting of Cellulose Nanocrystals in Patterned Magnetic Fields

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