Analysis of the Flow Behavior of Mechanically Fibrillated Cellulose Nanofibril Suspension by the Rheo-Polarized Imaging Technique (Rheo-Iris)

Understanding the coupling between the dynamics of microstructural deformation and the bulk flow behavior of colloidal suspensions is crucial for both fundamental studies and practical applications of this important class of soft matter systems. In this study, we investigated the flow behavior of cellulose nanofibril (CNF) suspensions─renewable, sustainable materials with low environmental impact─using the "Rheo-Iris," a rheo-polarized imaging system we developed to visualize two-dimensional microstructural changes in fluid under applied stress. Creep tests under constant shear stress revealed an initial elastic material response, a yield transition, followed by viscoplastic flow at long times. Simultaneous polarized imaging identified three distinct retardation patterns, depending on the applied shear stress. At low stresses (≤10 Pa), or small strain immediately after stress application, the phase retardation remained uniformly low, and the orientation axis of the microstructure was randomly distributed, indicating that the homogeneously dispersed CNFs form an isotropic, entangled network structure. At a stress near the yield point (40 Pa), a spiral-shaped region of high retardation appeared, and the orientation axis shifted to 60-85° away from the flow direction. This corresponds to the formation of rosary-like structures aligned in the vorticity direction, exhibiting spatially nonuniform birefringence and an oriented microstructure. At higher stresses far above the yield point (200 Pa), this "log-rolling" mesostructure collapsed, and smaller CNF aggregates became aligned in the flow direction, leading to spatially uniform oriented birefringence across the entire field. Both cases represent distinct fiber orientation phenomena, and our noninvasive rheo-polarization method clearly distinguishes how the spatial orientation distribution in the field changes with applied stress. The Rheo-Iris system enables real-time, quantitative analysis of internal microstructural evolution under imposed shear strain or stress and offers a powerful tool for exploring the orientation dynamics in soft matter systems, opening a new eye on complex fluid rheology in colloidal dispersions.