Birefringence-Based Evaluation of Stress-Optical Behavior in CNC Suspensions under Rheometric Shear Flow

Cellulose nanocrystals (CNCs) are rod-like, optically anisotropic colloids that exhibit birefringence when aligned under shear flow. At rest, Brownian motion randomizes their orientation, making the suspension optically isotropic. Under shear, however, CNCs align and induce flow birefringence—an optical response that enables non-invasive visualization of local stress and shear-rate fields.

In this study, we investigated the birefringence behavior of CNC suspensions under two-dimensional laminar shear using a stress-controlled rheometer with parallel-plate and concentric-cylinder geometries. Polarization imaging from multiple directions revealed variations in the projected refractive index ellipsoid that are inaccessible in conventional single-direction setups.

Analysis of birefringence intensity Δn and orientation angle as functions of shear rate showed that Δn follows a power-law scaling with shear rate, where the exponent decreases as alignment progresses. This change in slope indicates a structural transition in CNC orientation.

Furthermore, we extracted the stress-optical coefficient—a physical quantity that links mechanical stress to birefringence. The coefficient varied with the degree of CNC alignment, highlighting the coupled evolution of microstructure and macroscopic stress.

These results offer new insight into the structure–property relationship of CNC suspensions and support the use of flow birefringence as a diagnostic tool in soft matter rheology.