Marangoni Flow-Driven Angular Self-Assembly of Cellulose Nanocrystals: The Tale of Tilted Tactoids and Folded Domains.

Cellulose nanocrystals (CNCs) can spontaneously self-assemble into cholesteric photonic films with vibrant colors with multidomain structures and variations in cholesteric pitch. Herein, an angular deposition technique is employed to harness capillary and Marangoni flows to fabricate CNC photonic films with spatially tunable structural colors spanning from red to blue. A crucial relation between the substrate angle, the development of color zones, film coverage and film thickness is discovered. The color range of the photonic films can be shifted by tuning the size distribution of CNC particles is also demonstrated. As the CNC particles and tactoids are deposited on the substrate, a central deformation line emerged with tilted and folded domains, which is a consequence of Marangoni flow-induced deformation of the tactoids at the early stages of deposition. Further in the process, well-aligned domains emerged at the bottom of the substrates, indicating the simultaneous kinetic onset of multiple gelation processes which depend on size segregation across different color zones. Such insights allow us to tune the color domains using angular deposition and manipulate the kinetic arrest phase transition to produce more uniform and homogeneous films.