Showing papers on "Rheology published in 1973"

Field and Laboratory Studies of the Rheology of Mount Etna Lava

Abstract: Measurements have been made of viscosity and temperature for the 1971 and older lavas. Viscosities were estimated by two complementary methods: the classical measurements of the flow rate, and a ballistic method, in which the apparent viscosity of the surface layers was deduced from the depth of penetration of a steel spear, perpendicularly injected into the lava flow. Laboratory measurements on lava samples have been made with rotating cylinder viscometers. The principal result of the work indicates that between the bocca and the flow front the flow behaviour, and consequently the viscosity may vary considerably from lava to lava, even though the original magma varies little in composition. It is suggested that the different types of flow arises from the fact that the overall viscosity depends not only on the temperature and composition of the residual vitreous phase, but to an even greater extent on the lava s crystal and bubble content. The devitrification, which is a determinant factor of the rheological properties, is in turn highly influenced by the temperature of eruption and the rate of cooling of the lava.

Inversion of flow measurements for stress and rheological parameters in a valley glacier

Abstract: Methods are developed for determining the distributions of stress and effective viscosity in a glacier, under the assumptions: the ice is quasi-viscous, the flow is time independent, and acceleration forces are negligible. Measurements of the three-dimensional distribution of velocity are needed for their application. The differential equations of mechanical equilibrium, expressed in terms of viscosity, strain-rate components, mean stress, and their gradients, are viewed as equations to be solved for viscosity and mean stress subject to boundary conditions at the free upper surface. For certain rectilinear flow patterns, unique distributions of stress and effective viscosity can always be derived. For more complicated flow this is not necessarily so. However, it is still possible to choose the best values of rheological parameters in any trial flow law based on the requirement that the residuals to the equations of equilibrium be minimized in a mean-square sense. The techniques are applied to measurements of internal deformation made in nine bore holes on the Athabasca Glacier. At the center line the magnitude of the surface-parallel shear stress increases with depth more slowly than would be expected from a standard shape factor correction or the theoretical distribution of Nye. Correspondingly the lateral distribution of lateral shear stress shows the opposite relationships. In the lower one- to two-thirds of the depth corresponding to a range in effective stress from about 0.5 to 1.2 bars, the gross rheology of the ice is not distinguishably different from the experimentally determined flow law of Glen (n = 4.2, T = 0.02° C) as generalized by Nye. The results do not support the conclusion that the effective viscosity is higher than would be expected from Glen’s experiments as indicated by the more limited measurements of Paterson and Savage. Power-law parameters derived for the different bore holes considered separately show a spread, which suggests some rheological inhomogeneity. However, no definite conclusions can be drawn, because of direct measurement errors at the bore holes and less definable uncertainty in the interpolated distribution of velocity between the holes. The upper one- to two-thirds of the glacier constitutes an anomalous zone in which there is either a strong effect from a complex distribution of stress arising from longitudinal stress gradients or more complicated rheology than in a homogeneous power-law material.

Approach to generalization of concentration dependence of zero‐shear viscosity in polymer solutions

Abstract: The possibility of constructing zero-shear viscosity master curves valid for the entire range of concentration for a large number of polymer solutions in different solvents independent of molecular weight and the nature of the solvent is considered. The results obtained show that the parameters characterizing the individual macromolecular chain., viz., the dimensions of the polymer coil and the rheological effectiveness of segmental interactions, are significant in determining the viscosity of polymer solutions from very dilute to highly concentrated ones. The relation between the parameter of rheological interaction and characteristics of polymer solutions such as the flexibility of the polymer chain and the Flory-Huggins parameter is discussed. This permits one to separate the influence of the energy and entropy factors on the concentration dependence of zero-shear viscosity.

Elementary Concepts of Rheology Relevant to Food Texture Studies

Abstract: Rheology is a branch of physics which deals with the deformation and flow of materials, both solids and fluids (Reiner, 1960). In the case of food materials, their rheological (deformation and flow) behavior is directly associated with their textural qualities. Consumers, for example, estimate fruit firmness on the basis of the deformation resulting from physical pressure applied by the hand and fingers. The toughness or tenderness of meat is subjectively evaluated in terms of the effort required for the teeth to penetrate and masticate the flesh tissues.

Vibration-Induced Arterial Shear Stress

Abstract: The enhancement of arterial wall shear stress by mechanical vibration is examined from the viewpoint of its relationship to circulatory disorders found in those using vibrating hand tools. With the use of recent results, it is shown that certain arterial wall physiological processes are influenced by arterial wall shear and that for sufficiently large shear stresses, eg, approximately 1,000 dynes/sq cm, endothelial damage does occur. Calculations are presented that predict the influence of vibration on arterial wall shear. For typical arterial flow conditions, it is estimated that shear stresses in excess of 1,200 dynes/sq cm may be produced by a vibrating tool at 20 hertz with a half-amplitude of 2.5 cm and allowance for 50% attenuation of the vibration impulse.

Effect of molecular structure on polyethylene melt rheology. III. Effects of long-chain branching and of temperature on melt elasticity in shear

Abstract: The effects of long-chain branching and of temperature on the melt elasticity in shear of polyethylene were investigated using die swell measurements and relating them to recoverable shear strain, normal stress, and shear modulus. Die swell measurements, as a function of shear rate, were obtained for high- and low-density polyethylenes at temperatures ranging from 130° to 225°C. The samples were characterized by GPC and intrinsic viscosity for molecular weight distributions and degrees of long-chain branching. The importance of annealing the extrudates at temperatures above the polymer melting temperature to achieve equilibrium, or strain-free, values of die swell was demonstrated. The effect of long-chain branch was to decrease elastic deformation. At constant shear stress, the melt elasticity of both high- and low-density polyethylene was found to be essentially independent of temperature. Thus, at constant shear rate, elastic deformation decreased with increasing temperature, and it was demonstrated that this decrease could be quantitatively defined in terms of previously determined shear rate–temperature viscosity superposition shift factors.

On some rheological phenomena of amorphous polymers

Abstract: The molecular domain model for flexible macromolecules is presented in brief, and various observations are enumerated, indicating that the model holds true for the solid, melt, and solutions of different concentrations. The four forms in which the domains can exist under shear stress are then presented, and, by referring to them, such rheological phenomena as “die swell,” sudden jumps in flow rate, critical concentrations, and changes in viscosity are qualitatively explained.

Theory of rheology

Abstract: The structure of the modern theory of rheology is discussed to show the assumptions and limitations. Rheology is discussed as a branch of continuum mechanics to determine the relationships between stress, strain, and strain rate which will give a closer representation of lubricant properties than the Newtonian flow equation. Rheology is also investigated as a branch of chemical physics. Consideration is limited to those theories of nonpolymeric and polymeric fluids which can represent viscoelasticity in terms of identifiable and measureable molecular characteristics. The possibility that elastic liquids may rupture in shear and linear tension analogous to the failure of solids is proposed.

Dynamic rheological properties of bromine-terminated polybutadiene melts

Abstract: The viscous and elastic properties of bromine-terminated polybutadiene were investigated as functions of temperature, molecular weight, and molecular weight distribution. They were found to depend on these variables in a manner similar to other polybutadienes. It was shown that the effect on the viscous properties due to bromine terminal functionality is not as great as for carboxyl and/or hydroxyl terminal functionality. In this study it was shown that the viscosity resultant from the blending of two samples of the same polymer differing in molecular weight is dependent on the polydispersity of the polymers from which the blend is prepared. A form of the Ninomiya and Ferry theory was modified and compared with experiment. As a result of the experiments involving blending, it was shown that the temperature dependency of elasticity is less for polymers exhibiting greater polydispersity.

Effect of blending on the rheological properties of polystyrene

Abstract: Experimental non-Newtonian viscosity, primary normal stress difference, complex viscosity, and shear stress relaxation were taken for highly fractionated polystyrene in Aroclor 1248 (a chlorinated biphenyl) as well as for blends differing in molecular weight and concentration. The data are described by three parameters: a zero-shear rate value, the slope of the log–log plots in the high shear rate region, and a time constant defined as the inverse of the shear rate at the intersection of the low and high shear rate asymptotes. For the functions measured, the low shear rate region is characterized by a dependence on Mw and the high shear rate region by a dependence on Mn. Implications to polymer processing are discussed.

The Rheological Properties of Clay Suspensions, Latex and Clay-Latex Systems (1)

Abstract: The mechanism of the shear-thickening behavior of disperse systems is established on the basis that shear-induced collisions may lead to prolonged or permanent aggregation of particles, providing that the hydrodynamic compressive force between the colliding particles surpasses their repulsive force, but the subsequent shearing force is not sufficient to overcome the attractive force. By equating the hydrodynamic compressive force to the repulsive force between the colliding particles, the critical shear rate for shear-induced aggregation of particles was obtained in terms of particle size, colloidal stability, concentration, and the medium viscosity. The extent of shear-thickening behavior was also studied in terms of particle size, particle shape, colloidal stability, concentration, shear rate etc. The equilibrium and nonequilibrium rheograms of clay suspensions, latexes, and clay-latex systems were obtained using the Rotovisko and Hercules Hi-Shear viscometers, respectively.

Rheological properties of monodisperse linear polymers, their mixtures and solutions

Abstract: The paper considers the viscoelastic characteristics of linear polymers of a narrow molecular weight distribution at temperatures above the glass-transition region and the melting point. The viscoelastic properties of mixtures of narrow distribution polymers which model polydisperse polymers are discussed. In conclusion the specific features of solutions of narrow distribution flexible-chain linear polymers over a wide composition range are indicated. Attention is mainly focussed on high molecular weight polymers, since the peculiarities of their mechanical properties, which are of interest from the theoretical point of view and important for technology, presently remain unexplored. The problem is scrutinized predominantly on the basis of unpublished data from the Laboratory of Polymer Rheology (Institute of Petrochemical Synthesis).

Calculation of viscosity from immiscible displacement of highly viscous materials in capillary tubes

Abstract: This paper explains how immiscible displacement of a viscous liquid can be used to measure viscosity. Consistency variables and shear rate relations are developed that indicate when the effects of surface forces can be neglected and also provide a means of interpreting nonNewtonian behavior. While this simple technique has often been used incorrectly, it now appears to be able to obtain important rheological information on samples as small as ten microliters.

A Study on the Flows of Dilute Polymer Solutions : 2nd Report, On the Stability of the Laminar Boundary Layer Flow

Abstract: Considering the pronounced reduction of energy dissipation in turbulent flows of viscoelastic fluids, it is predicted that the elasticity of the fluids may affect the stability of laminar flows. In this report, the problem of the stability of the laminar boundary-layer flow along a flat plate with no pressure gradient is treated. In the analysis, the Denn model is used as the rheological model for viscoelastic fluids, and the stability equation is formed with the aid of the method of small disturbances. The analytical results show that the elasticity of the fluid destabilizes the flow and that the non-Newtonian viscosity of the fluid stabilizes the flow.