(607j) Flow-Induced Birefringence in Cellulose Nanocrystals (CNC) Suspensions

Cellulose nanocrystals (CNC) with a high aspect ratio can align under shear deformation. The degree of alignment depends on the concentration of suspension, rheological properties, ionic strength, sonication, and applied shear deformation. In this work, we explore the effect of sonication and shear deformation on the arrangements of the CNC particles in suspensions at different concentrations (3, 5, 7, and 9 wt.%). Our results show that at medium CNC concentration (5 and 7 wt.%), the sonication of the suspension can change the self-assembly of the CNC particles from nematic to chiral nematic. At low concentration (3 wt.%), however, the sonication treatment results in the isotropic structure and random orientation of the CNC particles. At high CNC concentration (9 wt.%), the sonication reduces the system viscosity without inducing a chiral nematic structure. We have also studied the effect of shear deformation on the alignment of the CNC particles. Under a pressure-driven flow in a capillary tube, the CNC suspensions exhibit iridescence patterns, which can be observed by transmitted polarized light microscopy. These iridescent color variations can be attributed to the flow-induced alignment of the CNC particles. While the emergence of the iridescent patterns in the chiral nematic suspensions are strongly shear rate dependent, the nematic and isotropic suspensions are independent of the applied average shear rate. The rheological characterizations are exploited to understand the microstructure of the gel-like and liquid crystalline suspensions and establish a relationship between the flow-induced birefringence patterns and the applied shear flow.