Multiscale Rheo-Optical and Scattering Signatures of Deformation-Induced Anisotropy in Aqueous Cellulose Nanocrystal Suspensions under Large-Amplitude Oscillatory Shear

Abstract

Aqueous surface-charged cellulose nanocrystal (CNC) suspensions exhibit concentration-dependent viscosity growth, gelation, and deformation-induced anisotropy, but the concentration and strain-amplitude ranges over which these responses emerge are not necessarily the same. Here, low-shear rheology and linear viscoelasticity are combined with rheo-polarized imaging (Rheo-PI) and rheo-small-angle light scattering (Rheo-SALS) to compare mechanical, optical, and scattering signatures in the same CNC system. Analysis of the operational low-rate viscosity and the Winter–Chambon criterion separates an empirical viscosity-crossover concentration, Ccross, from the sol–gel transition concentration, Cg. Under fixed-frequency large-amplitude oscillatory shear (LAOS), nonphase-resolved but directly comparable Rheo-PI and Rheo-SALS readouts show that annularly averaged Rheo-PI retardation becomes detectable at lower concentrations and lower strain amplitudes than those required for the appearance of anisotropic X-shaped SALS patterns. The first optical transition approximately coincides with an apparent contraction of initially circular SALS patterns, whereas clearly directional X-shaped SALS patterns appear only at larger strain amplitudes. The X-shaped lobe geometry then evolves continuously with the strain amplitude, providing a later SALS-based signature of directional mesoscopic anisotropy under the imposed LAOS conditions. Together, these results provide a comparative multiscale map of deformation-induced anisotropy in CNC suspensions and distinguish viscosity crossover, gelation, local optical anisotropy, mechanical yielding, and anisotropic scattering signatures as related but noncoincident responses.