Rheological and rheo-optical behaviors of nanocellulose suspensions containing unfibrillated fibers

We investigated the relationship between the concentration of nanosized fibers and the rheological and the rheo-optic behaviors of CNF suspensions containing unfibrillated fibers prepared by a wet refinement system (Water Jet System). The CNF suspensions behaved as a gel in which entropic elasticity was dominant, and the entanglement among fibers would be enhanced with the nanosized fiber volume fraction. The aggregates of the nanosized fibers elongated in the flow direction and deformed into elliptical shape with the applied shearing. However, the effect of these shape changes of the aggregates was barely observed on the viscosity curve. We speculate that this is due to Newtonian flow of the unfibrillated fibers (> 25% by volume) playing a significant role in the overall flow behavior of the CNF suspensions. In conclusion, for the CNF suspensions containing poorly nanosized fibers, we speculated that the nanosized and unfibrillated fibers were largely responsible for the linear response under micro-deformation and the nonlinear response such as under shear flow, respectively. Graphical abstract