Angular dynamics of cellulose nanofibrils in channel flow*

New materials from nanocellulose have the potential to replace many high performance materials today. The mechanical properties of nanocellulose are highly dependent on the internal structure of nanofibril network, which in turn can be controlled through hydrodynamic alignment of cellulose nanofibers (CNFs). This process is typically followed by a transition from the dispersion to the arrested (gel) state with the structure locked in a non-isotropic configuration and the subsequent drying of the aligned gel. The orientation distribution of the fiber depends on many parameters, including concentration, flow geometry and flow rates. The present study takes the particular focus on the orientation of CNF in the shear layers of a channel flow. The orientation of CNF is studied using small-angle X-ray scattering, where the results are compared with simulations of dilute anisotropic Brownian particles. Furthermore, we demonstrate how polarized microscopy can be used to both characterize the rotary diffusion of the birefringent CNF dispersion as well as study the average orientation direction in the channel. The results from this study provide an insight into the dynamics of dispersed CNF and lead to new strategies for controlling the assembly of nanofibrous materials.