Accurate evaluation of rheological constants by rheometry is an important part of the understanding of flow phenomena in complex fluids. The rheological properties evaluated by rheometry provide dimensionless numbers representative of the flow field, such as Reynolds number and Weissenberg number, and allow us to obtain an empirical equation that represents the conditions for the occurrence of flow phenomena specific to viscoelastic fluids, which are newly discovered. Using the stress tensor evaluation method based on flow birefringence, measurement methods for the second normal stress difference and planar extensional viscosity, which have been difficult to measure, have been developed. The application of these evaluation techniques to flow-induced structural changes and shear-banding phenomena has also contributed to the elucidation of these phenomena. Rheo-optic measurements were also used to elucidate flow-induced orientation and drying processes in the application of concentrated dispersion fluids and liquid crystalline dyes.

A double-network (DN) gel, first reported in 2003 by Gong and Osada, is a special kind of interpenetrating network gels showing high strength and toughness while keeping high solvent content of ~90 wt%. A tough DN gel is composed of two contrasting polymer networks, which are the brittle first network and the stretchable second network. Strengthening and toughening mechanisms of a DN gel have been extensively studied by the author and other researchers. Moreover, deformation-induced “growing” of a DN gel, which mimics metabolic reactions of living organisms, has been realized based on the unique fracture mechanism of the DN gel. In this article, how the contrasting double-network structure contributes high toughness and strength of a DN gel is briefly summarized. Also, muscle-like force-induced toughening of a DN gel through materials supply and network structure reconstruction is described.

The dependence of solution concentration on rheological properties such as relaxation time and elongational viscosity in the uniaxial extensional flow of low-viscosity dilute polymer (PEO) aqueous solutions were investigated by means of Dripping onto Substrate (DoS) and liquid dropping methods. The difference in the extensional rheology between DoS and liquid dropping methods is negligible under the present experimental and analytical conditions, but the difference in rheological properties becomes larger as increasing the solution concentration, since the blocking effect of filament stretching due to the substrate becomes larger.

For a type-A Rouse chain, a recent study derived an analytical expression of its dielectric loss εʺ under steady shear in terms of the basic Rouse parameters, the spring strength κ, the friction coefficient ζ, and the mean-square Brownian force intensity B, all being allowed to arbitrarily change with the Weissenberg number Wi (= shear rate multiplied by the longest relaxation time at equilibrium) and the latter two having a tensorial form; see Sato et al., Nihon Reoroji Gakkaishi, 50, 253 (2022). As a follow up, we have conducted a rheo-dielectric test for a type-A unentangled melt, poly(butylene oxide) with the molecular weight of 16 × 10³, to measure ε_yʺ in the velocity gradient (y) direction as well as the viscosity η and the first normal stress difference coefficient Ψ₁. Both dielectric relaxation time and intensity were found to decrease on an increase of Wi up to the onset of rheological nonlinearity (Wi ~ 1) where η and Ψ₁ exhibited moderate thinning, but the dielectric relaxation mode distribution was not affected by this increase of Wi. Analyzing those ε_yʺ, η, and Ψ₁ data on the basis of the above expressions, we found that κ increases moderately (by ~ 40 %) but the off-diagonal components of ζ remains negligibly small on that increase of Wi. We also found that the diagonal components ζ_xx (with x being the velocity direction) and B_yy hardly change but ζ_yy decreases moderately (by ~ 20 %). These results, suggesting the onset of the finite extensive nonlinear elasticity as well as a violation of a relationship B_yy ∝ ζ_yy (naively expected from the fluctuation-dissipation theorem), serves as a good starting point for deeper investigation of κ, ζ, and B.