The molecular mechanism by which emulsifiers inhibit starch retrogradation has not been fully understood. The retardation effect of emulsifiers on potato starch retrogradation was investigated using dynamic viscoelastic measurements and differential scanning calorimetry. Molecular-level investigations of the adsorption between starch and amylose with sucrose fatty acid esters were conducted using isothermal titration calorimetry to elucidate the mechanism by which emulsifiers attenuate starch retrogradation. Starch gels containing highly hydrophilic and poorly hydrophilic emulsifiers exhibited a small increase in storage elastic modulus at 4 degrees C. Amylose chains exist in water with their helices loosened, and the hydrophilic groups of sucrose fatty acid ester molecules adsorb onto the amylose chains via hydrogen bonding and van der Waals interactions. The adsorbed emulsifiers acted as steric hindrance and inhibited amylose chain reaggregation, thus suppressing retrogradation. Through a multi-scale investigation-using rheology, thermal analysis, and calorimetry-this study effectively elucidates the molecular-level inhibitory effects of emulsifiers on starch retrogradation.
In-plane isotropic laminated materials, such as sheet-molding compounds (SMC) and randomly oriented strand (ROS) composites, exhibit plug-flow-like behavior during compression molding. Our previous study proposed a linear anisotropic viscosity model that decomposes the strain-rate tensor D into three orthogonal flow-mode tensors: out-of-plane compression (Eout), out-of-plane shear (Gout), and in-plane flow (Din). Each mode is treated independently and symmetrically, and the linear effective viscosity is obtained by decomposing the second invariant (D:D) into contributions from each flow-mode. We extend this framework into the nonlinear regime by incorporating coupling among the flow-modes. The nonlinear stress tensor is formulated through a quadratic expansion of D, introducing self-interaction and cross-coupling terms among the modes. The corresponding nonlinear effective viscosity is derived from the flow-mode decomposition of the third invariant, detD, which reveals an asymmetry in mode coupling while maintaining thermodynamic consistency through the equivalence of viscous dissipation. Disc compression simulations using an interaction parameter demonstrate that flow-mode coupling enhances plug-flow formation and increases the compression load relative to the linear baseline. This hierarchical formulation unifies the linear (independent-mode) and nonlinear (interactive-mode) viscosity models by linking them to the second-and third-invariant decompositions, respectively, thereby providing a consistent framework for anisotropic viscosity in laminated materials.
The yielding behavior and microstructural characteristics of cellulose nanofiber (CNF) aqueous dispersions were investigated through stress-increase tests, post-yield structural recovery measurements, and evaluation of Cox-Merz rule applicability. In CNF dispersions, a second yielding-like behavior was observed after the initial yield upon continued stress application. Both first and second yield stresses exhibited strong concentration dependence, with the first yield stress showing a more pronounced trend. Notably, short-fiber CNF dispersions did not exhibit second yielding even at the same concentrations as long-fiber samples. Static resting after the first yielding event resulted in a recovery of the apparent yield strength, and the elastic behavior approached its original level within approximately 1800 s. However, elasticity lost due to second yielding showed incomplete recovery even after prolonged resting, suggesting irreversible structural changes. CoxMerz rule validity strongly depended on CNF concentration and fiber length. The rule held only for specific high-concentration, long-fiber samples, indicating robust inter-fiber entanglements resistant to flow-induced disruption. These findings elucidate concentration- and morphology-dependent network dynamics in CNF dispersions.
