Showing papers on "Rheology published in 1982"

A thin viscous sheet model for continental deformation

Abstract: Summary. For the purposes of describing its large-scale and long-term deformation, the continental lithosphere is regarded as a continuum, obeying a Newtonian or a power law rheology. The flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible. A numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent-continent collision. The material flows in response to forces applied to its boundaries (for example, the indenting of one continent by another) and to forces in its interior arising from gradients in crustal thickness. The horizontal divergence of the flow produces changes in the crustal thickness and hence a time-dependent form to the flow itself. For a given set of boundary conditions, the flow depends on the stress exponent in the power law rheology, n, and on the Argand number& which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates. When the effective viscosity of the medium is very high (Ar-+O), crustal thickness variations do not influence the flow. If the material is Newtonian (n = l), the deformation associated with an influx of material (approximating an indenter) is of much greater lateral dimension than the width of the indenter, whereas when material has a power law rheology (n = 3, 5 are used), the deformation is confined to a region of lateral extent comparable to that of the indenter. As Ar increases, the forces arising from crustal thickness contrasts exert more influence on the flow, and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates. In the limit of a very weak medium (Ar> 10) the lithosphere is unable to sustain appreciable crustal elevation contrasts. The results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

Introduction to rheology

Abstract: Common to liquids, solids and substances in between the former two is that if a stress is applied to them, they will strain. Stress may be visualized by placing a small amount of fluid between two parallel plates. When one plate slides over the other, forces act on the fluid dependent upon the rate of the plate movement. This causes a shear stress on the liquid. Recall laminar flow of fluids through a tubular vessel. Strain is the response to the stress. If solids are elastic, they deform and return to their original shape. Since fluids are not elastic and, hence, viscous, their deformation is irreversible.

Rodlike Polymer Solution Flow through Fine Pores: Influence of Pore Size on Rheological Behavior

Abstract: Variations of apparent viscosity with pore size in both the Newtonian and shear‐thinning regimes have been studied in noninertial flow of a rodlike polymer solution through well‐calibrated cylindrical pores, small glass bead beds, sharp‐edged particle packs, and a sandstone under conditions where flow properties are not influenced by polymer adsorption or retention. The bulk shear viscosities of microgel‐free xanthan solutions were accurately measured over a wide range of shear rates and polymer concentrations in order to determine macromolecular dimensions, confirm rodlike behavior, and know the effects of shear rate on intermolecular interactions. In fine pores, both the Newtonian apparent viscosity and the shear‐thinning index were found to decrease with pore size as long as pore diameters were larger than rod length. Such behavior is interpreted by the existence near the wall of a depleted layer in which steric hindrances decrease monomer concentration and thus viscosity near the wall. The proposed tw...

Melt rheology of polymer blends

Abstract: The rheological behavior of polymer blends is compared with that reported for emulsions, block polymers and homologous polymer blends. It has been shown that the properties of polymer alloys frequently differ from those of these “model” systems—the principal difference being the lowering of at least one rheological function below the value predicted In the log-additivity rule. Primarily, the most recent (post 1977) results are reviewed. The data obtained by the authors on the flow of poly(ethylene terephthalate)/Polyamide-6,6 are discussed in detail.

Non-Newtonian Fluids in Circular Pipe Flow

Abstract: Publisher Summary This chapter focuses on heat transfer behavior of viscoelastic fluid in turbulent pipe flow Although the asymptotic values of the heat transfer and friction factor can be calculated, there exist no firm criteria for determining whether asymptotic conditions exist Predictions of the intermediate values of the friction and heat transfer are not yet possible, even if the rheology and the thermal properties of the aqueous polymer solution are known To deals with the problems, the Weissenberg or Deborah number has to be taken into account The behavior of viscoelastic fluids flowing turbulently in noncircular channels or over external surfaces represents a relatively unexplored area of fluid mechanics Open channel flow of viscoelastic fluid is another interesting field currently being investigated The chapter concludes that to approach turbulent heat transfer behavior analytically, the usual and simplest method is to solve the uncoupled energy equation using the empirically determined velocity profile Hence, it is essential to understand the fluid mechanics of non-Newtonian fluids as well as the rheology

Rheology of a thermotropic polyester in the nematic and isotropic states

Abstract: The rheological properties of a thermotropic polyester were determined in the nematic and isotropic states. In the isotropic state, the viscosity is almost constant and the polymer is only slightly elastic. The nematic phase has a lower viscosity than the isotropic, except at low frequencies or shear rates, where the viscosity increases as though the polymer had a yield stress. There is a marked dependence of the rheology on shear history. The effects of shearing can be erased by returning the material first to the isotropic state and then back to the nematic state. The results are discussed with reference to analogous observations in small-molecule liquid crystals and in thermotropic aromatic co-polyesters.

Rheology of apricot puree: characterization of flow

Abstract: Complete characterization of flow of apricot puree is obtained by evaluation and quantification of thixotropy, measurement and calculation of yield stress, and application of selected mathematical models. Additionally, a structural model is tested. Apricot puree is slightly thixotropic, exhibiting a definite yield stress and obeying the Herschel-Bulkley equation, Information related to structural characteristics is obtained by applying the Michaels and Bolger model to experimental rheological data.

Texture, Viscosity, and Food

Abstract: This chapter discusses texture, viscosity, and food. The four principal quality factors in foods are appearance, flavor, texture, and nutrition. The importance of texture in the overall acceptability of foods varies widely, depending upon the type of food. Texture of foods has the following characteristics: (1) it is a group of physical properties that derive from the structure of the food; (2) it belongs under the mechanical or rheological subheading of physical properties; (3) it consists of a group of properties, not a single property, (4) texture is sensed by the feeling of touch, usually in the mouth, but other parts of the body may be involved; (5) it is not related to the chemical senses of taste or odor; and (6) objective measurement is by means of functions of mass, distance, and time only. Viscosity is defined as the internal friction of a fluid or its tendency to resist flow. Both gases and liquids have viscosity. At first sight, the distinction between texture and viscosity seems simple— texture applies to solid foods and viscosity applies to fluid foods. Unfortunately, the distinction between solids and liquids is so blurred that it is impossible to clearly demarcate between texture and viscosity. While rock candy can definitely be considered as a solid and milk a liquid, there are many solid foods that exhibit some of the properties of liquids and many liquid foods that exhibit some of the properties of solids.

The Rheology and Processing of Olefin-Based Thermoplastic Vulcanizates

Abstract: Olefinic thermoplastic vulcanizates (OTV's) comprise rubber in the form of discrete, completely crosslinked particles uniformly dispersed in an olefinic thermoplastic matrix. By their nature, they flow like filled polymer melts: die swell is low, and shows evidence of a particle association at low shear rate and high temperature; viscosity obeys a power law relationship with shear rate over a wide range. In place of a Newtonian plateau at low shear rate, the viscosity of OTV's begins to rise precipitiously in building toward a yield stress. As with other olefinic thermoplastic elastomers, their viscosity is more highly sensitive to shear rate than to temperature. Both shear viscosity and die swell were measured for evidence of morphological character and definition of fabricability. Due to their high shear sensitivity, these OTV's can be easily processed by injection molding and extrusion. Processing temperatures are not critical, but lower ranges are preferred to optimize appearance and promote ...

Melt rheology of blends of semicrystalline polymers. I. Degradation and viscosity of poly(ethylene terephthalate)–polyamide‐6,6 mixtures

Abstract: Melt rheology of poly(ethylene terephthalate)–polyamide-6,6, and their blends was studied between 240°C and 300°C, in capillary and rotational rheometers. The flow curves were determined in the range of rate shear from about 10−2to 105 (s−1). The results indicate a considerable degree of compatibility, presence of associations between the two types of macromolecules, and cocrystallization. A new mechanism of flow for the blends has been proposed. The study also considers the kinetics of thermal degradation.

Damped Oscillations in the Viscosity of Suspensions of Rigid Rods. I. Monomodal Suspensions

Abstract: The viscosity of suspensions of nearly uniform rigid rods has been shown to exhibit transient non‐Newtonian behavior. Damped oscillations of frequency approximately twice that of the rotation of the suspended particles occur. The attenuation of the oscillations is shown to be mainly due to polydispersity and particle interactions. Rotary Brownian motion which can also contribute to the damping was considered theoretically but shown to be negligible in our experiments. Effects of the axis ratio of the particles and the rate of shear on the damping were studied in detail, and the results were found to be in general agreement with earlier theoretical predictions.

Rheological properties of disperse systems at low shear stresses

Abstract: Three-dimensional network structures can be built up in disperse systems due to long-range colloidal interactions between the dispersed particles. The rheological behaviour of such coagulation structures has been studied by means of creep and recovery experiments at low shear stresses, i.e. by measuring the shear strain as a function of time under constant stress and after removal of stress. Measurements of this type give insight into the elastic and viscous deformations and the retardation times necessary to reach equilibrium or steady-state conditions. Results obtained with dispersions of pigments in polymer solutions and with monodisperse polymer latexes indicate the existence of an equilibrium state at low shear stresses with a predominant elastic deformation and a high viscosity suggesting that the disperse systems investigated do not behave exactly as rigid gels but apparently exhibit a dynamic equilibrium of structural break-down and formation under applied stress. This behaviour is approximately described by a 4-parameter-model with an instantaneous and a steady-state compliance, one retardation time, and a viscosity. At higher shear stresses thixotropic structural break-down occurs resulting in a transition from the rheological behaviour described here to a liquid-like state with a comparatively low viscosity. In this stress range the viscoelastic properties become strongly time-dependent. These measurements give evidence of the presence of two types of deformation: an instantaneous, purely elastic deformation attributable to the unperturbed coagulation structure and the creep-recovery behaviour of an elastic liquid apparently related to the breaking and re-forming of bonds.

CHAPTER 1 – Texture, Viscosity, and Food

Abstract: Publisher Summary This chapter discusses texture, viscosity, and food. The four principal quality factors in foods are appearance, flavor, texture, and nutrition. The importance of texture in the overall acceptability of foods varies widely, depending upon the type of food. Texture of foods has the following characteristics: (1) it is a group of physical properties that derive from the structure of the food; (2) it belongs under the mechanical or rheological subheading of physical properties; (3) it consists of a group of properties, not a single property, (4) texture is sensed by the feeling of touch, usually in the mouth, but other parts of the body may be involved; (5) it is not related to the chemical senses of taste or odor; and (6) objective measurement is by means of functions of mass, distance, and time only. Viscosity is defined as the internal friction of a fluid or its tendency to resist flow. Both gases and liquids have viscosity. At first sight, the distinction between texture and viscosity seems simple— texture applies to solid foods and viscosity applies to fluid foods. Unfortunately, the distinction between solids and liquids is so blurred that it is impossible to clearly demarcate between texture and viscosity. While rock candy can definitely be considered as a solid and milk a liquid, there are many solid foods that exhibit some of the properties of liquids and many liquid foods that exhibit some of the properties of solids.

A Three-Layer Semi-Empirical Model for Flow of Blood and Other Particular Suspensions Through Narrow Tubes

Abstract: A three-layer mathematical model has been developed and tested for calculating the velocity profile and wall layer thickness for the flow of blood and other particulate suspensions in narrow tubes, from measurements of apparent viscosity and mean concentration in tube. The model consists of a thin cell-free layer, accounting for wall exclusion effect, a cell-depleted one due to radial cell migration and a central core with uniform cell concentration. It is shown that this model contains several previously published models as particular cases and that it gives more realistic predictions of velocity profiles from the same data than less refined models. With proper adjustment of its parameters it is applicable satisfactorily to a wide variety of suspensions. The model results confirm the importance of cell deformability on blood rheological properties in capillaries less than 100 mu.

Melt rheology of ion-containing polymers. I. Effect of ionic content in a model polyesterionomer.

Abstract: The effects of copolymerizing small amounts of an ionic moiety (0–7 mol %) into the backbone of a polycondensation polymer have been investigated by a combination of linear viscoelastic, steady-shear, and extensional testing. At a constant viscosity in dilute solution, both the zero-shear-rate viscosity and the maximum relaxation time in the melt increase monotonically with an increase in the ionic character. These effects are present after the data are normalized to remove the increase in the glass transition temperature Tg with increased ionic content. Large-strain, steady-shear flow experiments showed that the viscoelastic memory of the melt is affected by ionic content. An increase in ion content causes a decrease in the effect of strain on the memory function. This effect is not apparent in extensional flow experiments, which indicate that the ionic content does not affect the response if the tests are conducted at a constant temperature above Tg.

Rheology of polymer blends

Abstract: Rheological properties of blends of amorphous and crystalline polymers were studied for a broad range of compositions and temperatures. It was established that below the melting pointTm the viscoelastic properties of blends of crystalline polymers are similar to those of polymers filled with mineral fillers. In both cases these properties are influenced by the existence, in such systems, of a temporary structural network formed by mineral or polymeric particles and its subsequent breakdown under the action of shear stresses. It was found that an anomalous decrease in the melt viscosity of the main component on addition of a small amount of a second polymer depended on deformation conditions. The comparison of data on viscoelastic properties and thermodynamic interaction between the components in the melt, estimated from the parameterχ23 of a new Flory theory, shows that the sharp drop of viscosity takes place in the region of microphase separation due to the appearance of an excess free volume in the interphase region. Calculation of the relaxation spectra for various blends also revealed marked changes when various amounts of a second component were added to the main polymer.

A rheological expression of coagulation rate theory

Abstract: In rheology, the viscosity reduction of suspensions is frequently attributed to the break-down of structure of agglomerates by the shear stress applied. It is also widely known that the addition of dispersant reduces the viscosity. Both phenomena are considered to be closely related via change in number of junctions between the particles. Starting from the stability theory of suspensions by Verwey and Overbeek22, and separating the time-dependent viscosity from the time-independent viscosity, an equation (12) expressing the increase of junction number as a function of time was derived. By equation (12), calculation of the viscosity of unagitated suspensions, which has not been possible by rheological equations containing the rate of shear as one of the parameters, became possible.

Effect of Rheological Properties of Compounds on Fiber Formation in Mixtures of Incompatible Polymers

Abstract: Mixtures of linear polyethylene with polystyrene were used in studying the fiber formation pattern of mixtures of incompatible polymers whose melts were forced through dies. Variables included the viscosity of the components, mixture preparation conditions, and shear stress. It has been established that the process of fiber formation occurs within a definite range of shear stresses, dependent on the viscosity ratio of the fiber-forming polymer and the polymer serving as the dispersion medium. When this ratio is equal to or less than unity, mixtures containing the fiber-forming component in the form of a finely dispersed phase yields continuous fibers several micrometers in diameter within a broad range of shear stresses. At greater values of the component viscosity ratio (tens of units), no fiber formation takes place in the mixtures.

Rheological Properties of Liquid-crystalline Polymer Solutions

Abstract: An analysis is presented of the principal rheological factors inherent in the flow of nematic solutions of rigid-chain aromatic poly(para-amides), viz, viscosity anisotropy, yield points, and viscosity peaks in the region of phase transition on the temperature and the concentration scale, thereby making it possible to use the rheological characteristics to plot phase diagrams

Flocculation and rheology of kaolinite/quartz suspensions

Abstract: Rheological properties of suspensions of Na-kaolinite and colloidal quartz (Min-U-Sil) at constant overall volume concentration of 2% are determined with a Weissenberg Rheogoniometer using a combined Couette and cone-and-plate geometry. The results are interpreted in terms of the flocculation behaviour of the constituent particles in the presence of high salt concentrations (0.1–0.75m NaCl) at pH 6, 7 and 8. In these chemical environments these suspensions are pseudoplastic for much of the range of mixture compositions becoming Newtonian for suspensions containing only quartz. These properties reflect the dominant influence of interactions between kaolinite particles on the flocculation behaviour of the mixture.

Thermoplastic-Guayule Rubber Blends-Rheological Properties

Abstract: An experimental study of the flow characteristics of blends of natural Guayule rubber with high-density polyethylene was carried out, and the effect of crosslinking and carbon black loading of the rubber phase was examined. It was observed that (1) an increase in shear rate decreases the viscosity and increases the extrudate swell of the blends; (2) an increase in temperature decreases the viscosity of the blends; (3) the viscosity and extrudate swell of the blends appear to be nonadditive functions of the viscosity and extrudate swell of the homopolymers and are lower than predicted by simple additivity; (4) the viscosity increases with increasing degree of crosslinking of the rubber phase; (5) the viscosity increases with increasing carbon black content in the rubber phase, the increase being more pronounced when the rubber-black compound is the continuous phase; (6) when crosslinking was effected, extrudate swell varied in a very peculiar manner, which requires further study. Based on the data...

Damped Oscillations in the Viscosity of Suspensions of Rigid Rods. II. Bimodal and Polydisperse Suspensions

Abstract: The transient non‐Newtonian viscosity of suspensions with a (slightly polydisperse) bimodal particle size distribution and of rather highly polydisperse suspensions has been studied. For bimodal systems the dominant frequencies can be determined from the frequency spectra of the oscillations. They correspond to the rotational frequencies of the two species constituting the bimodal suspension. For polydisperse suspensions, the damping in viscosity is very rapid and the dominant frequency is shifted toward values corresponding to the period of rotation of the larger particles present.

Determination of the plastic and viscoelastic properties of a metallic soap by a hot hardness measurement

Abstract: An analysis of an indentation test in the case of mixed plastic—viscoelastic behaviour, as is met in metallic soaps, is described. The viscoelastic part of the strain is described in terms of a Burgers model whose five rheological parameters are then a yield stress,σ0, two elasticity moduli,G0 andG1, and two viscosities,η0 andη1. This analysis is applied to a hot hardness experiment on a compacted metallic soap, calcium stearate, between 20 and 130° C. By fitting the model to the experimental curves, values ofσ0,η0,η1,G0 andG1 as a function of temperature, have been derived. It is shown that plastic strain is much greater than viscous strain at low temperature, sinceη0 is very high. The material is therefore a solid. It is also shown that, as temperature increases, viscous strain increases and plasticity vanishes (above a transition temperature,σ0=0, and only viscoelasticity remains). The curvesη0(T),η1 (T),G0(T) andG1 (T) have marked slope changes at about 90° C (which is the crystal—crystal phase transition temperature of calcium stearate), and the viscosities fall at 123° C, which is the first crystal mesomorphic phase-transition temperature. This test seems to be a good simple rheological measurement for bodies exhibiting simultaneous plasticity and viscosity or viscoelasticity.