Rheologica Acta

Using a piezo rotary and axial vibrator (PRAV) the viscoelastic properties of a fresh Al2O3 coating that is deposited on a well-chosen plate can be measured simultaneously during drying. There are three types of vibrations available to be excited and detected in their resonance modes, by measuring resonance frequencies f k and half-widths h k, before and after coating as a function of time: axial vibration, bending vibration to get the evolving Young’s modulus and rotary vibration to follow the viscosity increase during drying of the thin coating. This information is contained in the complex frequency shifts Ωk = 2Δf k /f k + iΔh k /f k of the three vibration modes caused by the coating on the plate. A derivation of the relationships, their validation on Newtonian liquids and experimental applications carried out using the PRAV are given in the paper.

This paper describes the work carried out to investigate the pore pressures occurring in secondary consolidation. A theoretical approach and an experimental technique was developed in order to conduct the study. By considering compression to occur only due to water leaving the soil it was possible to derive an expression for the dissipation of pore pressure in the secondary phase. By further simplified assumptions which are based on experimental observations, the above general solution was reduced to a simple formula which predicted the observed behaviour of pore water pressures during secondary consolidation.

The molecular-weight dependence of the intrinsic viscosity and the diffusion coefficient of polymers in dilute solutions can be obtained by assuming the polymer chains to be rigid spheres of radius equal to their radius of gyration. This observation has been interpreted as implying that the solvent within a polymer coil is dragged along with the coil, and that the coil with the occluded solvent acts like a hydrodynamic entity. In order to question this very common interpretation, we calculate the velocity field in nondraining polymer chains. Our explicit results are at odds with the simple picture of nondraining polymer coils acting like rigid spheres.

A stress-controlled commercial rheometer has been equipped with small-angle light scattering (SALS) instrument working in a range of wave-vectors between 0.5 and 4.2 μm−1 By a specially designed optical system the scattered light is focused directly on the square chip of a CCD-detector with high dynamical range allowing quantitative two-dimensional (2 D) intensity measurements of polarized and depolarized scattering. The rheo SALS system is suited for simultaneous fast measurements of 2D SALS and rheological material functions of polymer melts and liquid crystals. As a first application results on the influence of shear rate on the late stage of spinodal decomposition of a polybutadiene/polyisoprene blend are reported. A second application concerns a creep experiment in the liquid crystalline phase of a lyotropic non-ionic surfactant with simultaneous measurement of the time-dependent compliance and associated changes of the 2D intensity patterns of the polarized and depolarized scattering.

A constitutive model for elasto-viscoplastic thixotropic materials is proposed. It consists of two differential equations, one for the stress and the other for the structure parameter, a scalar quantity that indicates the structuring level of the microstructure. In contrast to previous models of this kind, the structure parameter varies from zero to a positive and typically large number. The lower limit corresponds to a fully unstructured material, whereas the upper limit corresponds to a fully structured material. When the upper limit is finite, the model represents a highly shear-thinning, thixotropic, and viscoelastic liquid that possesses an apparent yield stress. When it tends to infinity, the behavior of a true yield-stress material is achieved. Predictions for rheometric flows such as constant shear rate tests, creep tests, SAOS, and large-amplitude oscillatory shear (LAOS) are presented, and it is shown that, in all cases, the trends observed experimentally are faithfully reproduced by the model. Within the framework of the model, simple explanations are given for the avalanche effect and the shear banding phenomenon. The LAOS results obtained are of particular importance because they provide a piece of information that so far is absent in the literature, namely a quantitative link between the Lissajous–Bowditch curve shapes and rheological effects such as elasticity, thixotropy, and yielding.

This paper reports on geometry dependent viscosity curves of the biaxial thermotropic LC-Polymer Vectra B 950 measured with rectangular slit dies. The geometry dependence of the flow behaviour originates from the flow of two layers. The layer near the wall is highly flow oriented and free from defects. The layer in the bulk has an ordered texture. The thickness of the wall layer decreases with wall shear stress. Based on the Frank theory and the assumption that the core of a moving disclination is the smallest radius of distortion, the wall layer thickness is predicted and compared with experimental data.

A plea for greater attention to rheological characteristics of paints, especially by the smaller producers with limited resources, and for rheologists to devise simpler and cheaper methods of instrumentation (perhaps impossible), or at least of presentation of results.

A constitutive equation for rubber-like materials is developed using the left stretch tensor. This process starts with a model for hyperelastic solids based on a separable energy function. This model accurately fits extensional data for vulcanized natural rubber until the onset of hysteresis at intermediate strains. Better predictions outside the hyperelastic range are obtained by directly modifying this constitutive equation to describe limited extensibility. The resulting model accurately fits biaxial, planar, and uniaxial extension data for a variety of rubber-like materials using three constants. This model also predicts simple shear results derived from planar extension data and characterizes inflation of spherical membranes for elastomers and soft tissue. A final modification accurately describes hardening associated with crystallization at large tensile strains.

Die bei viskosen und viskoplastischen Stoffen beobachtete Druckabhängigkeit der Zähigkeit kann in die Stoffgesetze dadurch aufgenommen werden, daß der die Zähigkeit der Stoffe beschreibende Stoffparameter als vom örtlich herrschenden mittleren Normaldruck abhängig angenommen wird. An Strömungsfällen, die einen ebenen Fließzustand ermöglichen, werden die Auswirkungen untersucht, die die Einführung eines druckabhängigen Zähigkeitsparameters in die Stoffgesetze derNewtonschen und derBinghamschen Flüssigkeit besitzt. Die erweiterten Stoffgesetze führen auf Differentialgleichungssysteme, aus denen durch Reihenansätze bei geringer Abhängigkeit der Zähigkeit vom Druck lösbare Rekursionssysteme linearer, partieller Differentialgleichungen gewonnen werden können. Mit den so gewonnenen Gleichungssystemen werden zwei Beispiele, nämlich die Strömung der zähen Flüssigkeit mit druckabhängiger Zähigkeit infolge eines Druckgefälles zwischen parallelen Platten und die überlagerte Scher- und Druckströmung einer zähplastischen Flüssigkeit mit druckabhängiger Zähigkeit zwischen parallelen Platten untersucht.

Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina. The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strain-softening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick–slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent.