Self-Alignment Sequence of Colloidal Cellulose Nanofibers Induced by Evaporation from Aqueous Suspensions

Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration.