Correlations between static rheological properties (G: shear modulus, μ: viscosity, μapp: apparent viscosity) of four food fluids (thickening agent solution, yoghurt, tomato puree and mayonnaise) measured employing a newly developed non-rotational concentric cylinder (NRCC) method and dynamic viscoelastic properties (|G*|: complex shear modulus, G′: storage shear modulus, G″: loss shear modulus, |η*|: complex viscosity, tanδ: loss tangent, μapph: apparent viscosity) measured by a conventional apparatus (HAAKE MARS III) were studied. μapp and μapph for each sample fluid agreed well. μapp≒|η*| obeying the Cox–Merz rule was obtained for the thickening agent solution, μapp < |η*| for mayonnaise and μapp << | η*| for tomato puree and yoghurt. μ was lower than μapp or μapph for all samples, and reached μapp with an increasing shear rate. Plotting G and |G*| against the shear rate and angular velocity respectively in the same figure revealed G < |G*| for tomato puree, G≒|G*| for yoghurt and G > |G*| for mayonnaise and thickening agent solution. The applicability of two-element models in analyzing the viscoelastic behavior of sample fluids was investigated by comparing two characteristic times τM = 1/(ωtanδ) and τK = tanδ/ω, corresponding to Maxwell and Kelvin–Voigt models respectively and evaluated from measurements of tanδ, with τ = µ/G and τapp = µapp /Gapp calculated from rheological properties measured by the NRCC method. Here Gapp (= G(μ/μapp) is an apparent shear modulus newly defined to evaluate the shear-rate dependency of the elasticity of Non-Newtonian fluids. Results show τK agreed well with τapp, suggesting the possibility of applying the Kelvin–Voigt model to the viscoelastic behavior of sample fluids.
Extensional flow properties of dilute viscoelastic solution have attracted much attention for almost three decades. This is because extensional viscosity or relaxation time of viscoelastic solution under extensional stresses add a lot of impact on fluidity of the solution; the phenomenon is related to many industrial applications. Several techniques were proposed and many studies were conducted to measure extensional properties of polymer solutions. However, less studies were reported in the case of non-uniform solution, such as suspensions. In this study, we propose a relatively simple method to measure extensional viscosity. We adopt a syringe-shape abrupt contraction channels for the measurement; the syringe-shape apparatus is useful to contain non-uniform dilute solutions. As a first step to develop the experimental apparatus and method, we tested polymer solutions. Polymers used to prepare sample solutions were polyethylene oxide (PEO), hydroxypropyl cellulose (HPC) and polyacrylic acid (PAA). Each polymer has different flexibility, which cause different characteristics of the extensional viscosity of polymer solutions. Extensional viscosities obtained in this study detected the characteristic of each polymer solution. Extensional viscosities of PEO solution was increased by increasing extensional rates; that of HPC and PAA solution were decreased by increasing extensional rates; the tendency was explained by relaxation times of each polymer solution.
In this study, viscosity growth of entangled polymers under startup of fast shear flows was investigated by means of primitive chain network simulations. In particular, we focused on the undershoot following the well-known overshoot. The simulations reasonably reproduced the viscosity growth data reported in the literature for polystyrene melts. To investigate the origin of the observed undershoot, stress was decomposed into orientational and stretch contributions through a decoupling approximation. The decoupled results show a tiny undershoot in both orientation and stretch. Molecular tumbling was also monitored, both for single molecules and in terms of an average end-to-end orientation angle. Results appear to confirm that tumbling causes the viscosity undershoot, consistently with the suggestion of Costanzo et al. [Macromolecules, 2016, 49, 3915].
In order to clarify the mechanism of the bulge structure appearance observed in a cavity swept by a visco-elastic fluid, velocity fields were measured by a two-dimensional particle image velocimetry (PIV). The rib height, the cavity length, the flow path height and the flow path width were fixed at 20 mm, 100 mm, 40 mm and 75 mm, respectively. The Reynolds number was also fixed at 1,700 where the bulge structure appeared as reported by the previous study. The spanwise positions of the two-dimensional PIV were changed in 6 steps from the center plane to the outer region. From the results, it was found that the bulge structure has high-level fluctuation and its intermittency is related to the longest relaxation time. The bulge structure appears when the main flow separated from the upstream top corner of the cavity is intensified. The separated main flow contracted in the former cavity region expands not only toward the cavity bottom but also toward the outside walls of the flow path. In order to supply the fluid in the center plane, the backward flow occurs in the cavity. This flow motion was concluded to be a basic mechanism of the bulge structure appearance.
Poly(L-lactid acid) (PLLA) forms complex crystals (ε-crystal) with some solvents including N,N-dimethylformamide (DMF), and can be easily gelled in such solvents. The solvent of these gels can be exchanged to some other solvents which do not form ε-crystal with PLLA. The flow temperatures of PLLA gels after exchanging DMF to various solvents were investigated to clarify the relation between the exchangeability and the solubility of solvents to PLLA. After changing the ε-crystals to α-crystals (crystal form of PLLA in films without special treatments) by immersing the gels into 1-propanol, the flow temperatures of PLLA gels could be expected by Hansen solubility parameters. The results suggest that the exchangeability of the solvent of the PLLA gel is determined both by the solubility to PLLA and the ability of the new solvent to induce the change from ε-crystals to α-crystals.