Showing papers on "Rheology published in 1990"

Linear rheology of viscoelastic emulsions with interfacial tension

Abstract: Emulsions of incompressible viscoelastic materials are considered, in which the addition of an interfacial agent causes the interfacial tension to depend on shear deformation and variation of area. The average complex shear modulus of the medium accounts for the mechanical interactions between inclusions by a self consistent treatment similar to the Lorentz sphere method in electricity. The resulting expression of the average modulus includes as special cases the Kerner formula for incompressible elastic materials and the Oldroyd expression of the complex viscosity of emulsions of Newtonian liquids in time-dependent flow.

Polymer alloys and blends : thermodynamics and rheology

Abstract: Polymer alloys and blends: thermodynamics and rheology , Polymer alloys and blends: thermodynamics and rheology , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Studies on the steady-shear behavior of electrorheological suspensions

Abstract: The structures of nonaqueous suspensions subjected to electrical fields and continuous shear are described. Based on observations that particle strands and columns formed upon application of an electric field behave as elastic solids for small shear strains but yield above a critical strain, models for the yield stress and continuous-shear response of these electrorheological (ER) suspensions are developed. Comparison of model calculations with experimental results indicates that the yield stress is dominated by the work required to overcome electrically induced interparticle forces.

Shear thickening (dilatancy) in concentrated dispersions

Abstract: A new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.

Non-Newtonian Rheology of Igneous Melts at High Stresses and Strain Rates: Experimental Results for Rhyolite, Andesite, Basalt, and Nephelinite

Abstract: The stress-strain rate relationships of four silicate melt compositions (high-silica rhyolite, andesite, tholeiitic basalt, and nephelinite) have been studied using the fiber elongation method. Measurements were conducted in a stress range of 10–400 MPa and a strain rate range of 10−6 to 10−3 s−1. The stress-strain rate relationships for all the melts exhibit Newtonian behavior at low strain rates, but non-Newtonian (nonlinear stress-strain rate) behavior at higher strain rates, with strain rate increasing faster than the applied stress. The decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt. The decrease in viscosity observed at the high strain rates of the present study ranges from 0.25 to 2.54 log10 Pa s. The shear relaxation times τ of these melts have been estimated from the low strain rate, Newtonian, shear viscosity, using the Maxwell relationship τ = η s /G ∞. Non-Newtonian shear viscosity is observed at strain rates ( ɛ ˙ = time - 1 ) equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time. Brittle failure of the fibers occurs 2 log10 units of time above the relaxation time. This study illustrates that the occurrence of non-Newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate. High-silica rhyolite melts involved in ash flow eruptions are expected to undergo a non-Newtonian phase of deformation immediately prior to brittle failure.

Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel

Abstract: This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone—plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yeild stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log—log graph with typical shear stress-shear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.

The vane‐in‐cup as a novel rheometer geometry for shear thinning and thixotropic materials

Abstract: We have addressed the question of the suitability of the vane‐in‐cup as a rheometer geometry. A numerical simulation of this geometry was conducted for a power‐law fluid and the results compared with a similar study for a conventional bob‐in‐cup geometry. The comparison indicates that for a sufficiently shear‐thinning fluid (of shear‐thinning index less than 0.5) the fluid within the periphery of the vane blades is essentially trapped there and turns with the vane as a solid body. Calculation of the shear stress at the cup wall indicates that this quantity is equal in both geometries for a given rotational rate of the spindle. Thus the torque required to turn the spindle would be the same and identical flow curves would be predicted. This prediction was tested on two fluids thought to possess a yield stress: a 5.5% sodium carboxymethylcellulose (CMC) solution and a 4.2% Veegum PRO clay suspension. Equivalent flow curves were obtained at very low stresses/shear rates but a sudden, catastrophic viscosity loss was found for both fluids with the bob at shear rates which were still quite low. Such a loss was observed with the vane as well, but at much higher shear rates. It is suggested that this phenomenon is a form of apparent slip due to the formation of a thixotropic layer at the bob/vane surface. The much flatter stress profile obtained in the vane geometry is reasoned to postpone the formation of this layer. Rheological data obtained with the vane appear to be a faithful representation of these materials and show the absence of a true yield stress.

Modeling the rheology of polyisobutylene solutions

Abstract: Results are presented of a detailed rheological study of two different polyisobutylene systems in steady and transient shear flows. Shear‐thinning solutions are prepared by dissolving high molecular polyisobutylene in tetradecane, a low viscosity solvent. Nearly constant viscosity solutions are obtained by adding a third component of highly viscous polybutene. The linear viscoelastic behavior of these fluids is characterized in small‐amplitude oscillatory shearing flow and the nonlinear behavior is studied by means of steady and transient shear flows. The spectrum of relaxation times for both solutions has been determined. Temperature and concentration effects on the material functions are reported. Three differential constitutive equations with four relaxation modes were tested for fitting the experimental data in steady and transient shear flows: they are the quasilinear Oldroyd‐B model and the nonlinear Giesekus and Bird–DeAguiar models. The presence of nonlinear stress terms is found to be important f...

The onset of non-Newtonian rheology of silicate melts

Abstract: The viscoelastic behavior of silicate melts has been measured for a range of compositions (NaAlSi3O8, NaCaAlSi2O7, CaMgSi2O6, Li2Si4O9, Na2Si4O9, K2Si4O9, Na2Si3O7, K2Si3O7 and Na2Si2O5) using the fiber elongation method. A1l compositions exhibit Newtonian behavior at low strain-rates, but non-Newtonian behavior at higher strain-rates, with strain-rate increasing faster than the applied stress. The decrease in shear viscosity observed at the high strain-rates ranges from 0.3 to 1.6 log10 units (Pa s). The relaxation strain-rates, έrelax, of these melts have been estimated from the low strain-rate, Newtonian, shear viscosity, using the Maxwell relationship; έrelax=τ −1=(ηs/G∞)−1. For all compositions investigated, the onset of non-Newtonian rheology is observed at strain-rates 2.5+0.5 orders of magnitude less than the calculated relaxation strain-rate. This difference between the non-Newtonian onset and the relaxation strain-rate is larger than that predicted by the single relaxation time Maxwell model. Normalization of the experimental strain-rates to the relaxation strain-rate predicted from the Maxwell relation, eliminates the composition. and temperature-dependence of the onset of non-Newtonian behavior. The distribution of relaxation in the viscoelastic region appears to be unrelated to melt chemistry. This conclusion is consistent with the torsional, frequency domain study of Mills (1974) which illustrated a composition-invariance of the distribution of the imaginary component of the shear modulus in melts on the Na2O-SiO2 join. The present, time domain study of viscoelasticity contrasts with frequency domain studies in terms of the absolute strains employed. The present study employs relatively large total strains (up to 2). This compares with typical strains of 10−8 in ultrasonic (frequency domain) studies. The stresses used to achieve the strain-rates required to observe viscoelastic behavior in this study approach the tensile strength of the fibers with the result that some of our experiments were terminated by fiber breakage. Although the breakage is unrelated to the observation of non-Newtonian viscosity, their close proximity in this and earlier studies suggests that brittle failure of igneous melts, may, in general, be preceded by a period of non-Newtonian rheology.

On the rheology of filled polymers

Abstract: This paper develops a rheological model for flow of polymer compounds highly filled by small interacting (attractive) particles. The properties of the particle phase in the compound are described by introducing a viscoelastic ‘‘particle mode’’ with an additional kinetic equation which describes the processes of rupture and restoration of flocs of particles. The elastic deformations in the particle mode are assumed to be small enough to consider the geometrical nonlinearities as second‐order effects. The rheological behavior of the polymer matrix is modeled by means of nonlinear viscoelasticity, as in the case of pure polymer melts. It was shown that all the basic features of the yield‐like and thixotropic behavior of the filled polymer compounds, including the appearance of yield values in steady state flows, stress overshoots in start up flows and effects of ‘‘frozen memory’’ during relaxation could be described by this approach without using the mathematical yield criteria. Some preliminary comparisons of the theory with steady state data are also demonstrated.

The relationship between plate velocity and trench viscosity in Newtonian and power‐law subduction calculations

Abstract: The relationship between oceanic trench viscosity and oceanic plate velocity is studied using a Newtonian rheology by varying the viscosity at the trench. The plate velocity is a function of the trench viscosity for fixed Rayleigh number and plate/slab viscosity. Slab velocities for non-Newtonian rheology calculations are significantly different from slab velocities from Newtonian rheology calculations at the same effective Rayleigh number. Both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate. Non-Newtonian rheology eliminates the need for imposed weak zones and provides a self-consistent fluid dynamical mechanism for subduction in numerical convection models.

Transient and steady‐state rheology of a liquid crystalline hydroxypropylcellulose solution

Abstract: Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.

A Circular Non-Newtonian Fluid Model: Part I—Used in Elastohydrodynamic Lubrication

Abstract: A circular non-Newtonian fluid model associated with the limiting shear strength was considered. Using this model a modified Reynolds equation was developed which is almost the same as the classical Reynolds equation except for the viscosity term. The effects on performance of dimensionless load parameter, dimensionless speed parameter, slide/roll ratio, different oils, the limiting shear strength proportionality constant were studied

Principles of polymer engineering rheology

Abstract: Background introduction to polymers principles of continuum mechanics fabrication and flow in polymer processing experimental polymer rheology rheological characterization of polymer fluids experimental observations of rheological behaviour of polymer systems rheo-optics and flow birefringence rheological modelling of flow behaviour constitutive equations for non-Newtonian fluids applications to polymer processing.

Vibration Characteristics of a Composite Beam Containing an Electrorheological Fluid

Abstract: The complex moduli of a composite consisting of a hollow beam filled with an electrorheological (ER) fluid were obtained by analyzing the beam's motion in free oscillation. The beam was considered to be a uniform viscoelastic material and was modeled as a damped harmonic oscillator. Hydrous-based ER fluids consisting of either cornstarch-corn oil or zeolite-silicone oil mixtures with varying amounts of solids and water content were employed, along with electric field intensities ranging from 0.0 to 1.7 × 103 kV/m. Rheological data were also obtained for the fluids A linear correlation between the two components of the moduli and fluid strength was observed, which was ex plained in terms of fluid structure and particle polarization

Effect of oscillations during extrusion on rheology and mechanical properties of polymers

Abstract: An experimental study associated with the applications of sound and ultrasonic waves to extrusion is conducted. Two laboratory setups are developed for these purposes incorporating orthogonal and parallel superposition of oscillations on pressure flow during extrusion. The orthogonal superposition of sound oscillations is imposed in an annular die. The parallel superposition of ultrasound oscillations is imposed in a circular die. Several materials including unfilled and filled thermoplastics, thermoplastic elastomer, and elastomer are tested. Die characteristics, power consumption, and die swell are determined during extrusion. The rheological properties, molecular weight, tensile properties, and impact strength of materials with and without sound and ultrasound treatment are investigated, indicating permanent changes in these properties. The observed effects are found to depend on melt temperature, flow rate, nature of polymer system, and oscillating parameters such as frequency and deformation amplitude.

Rheology-morphology relationships in nylon 6/liquid-crystalline polymer blends

Abstract: Extrusion measurements have been carried out on blends of nylon 6 and a liquid-crystalline copolyesteramide (LCP). The flow curves at low temperature show a behavior similar to that of pure LCP with a rapid rise of the viscosity at low shear rates. At high shear rates the viscosity is lower than that for each of the two components. This minimum has been attributed to the lack of interactions between the two phases and to the formation of fibrils of the LCP phase. The SEM analysis shows, indeed, that fibrils of the LCP phase are produced in the convergent flow at the inlet of the capillary at high shear rates. These fibrils are lost during the flow in the long capillary.

Prediction of the nonlinear viscoelastic properties of a hard wheat flour dough using the Bird–Carreau constitutive model

Abstract: The Bird–Carreau model was used to predict the steady viscosity η, the primary normal stress coefficient ψ1, and the small amplitude oscillatory properties η’ and η‘/ω for a hard wheat flour dough containing 40% total moisture in the region of frequencies of 0.01 to 100 rad/s for the dynamic viscoelastic properties and a region of 10−5 through 103 s−1 for steady shear properties. The model accurately represented the experimental data in the shear rate/frequency region tested.

Rheology of filled polymers

Abstract: Published data, concerning the results of measuring rheological properties of polymer melts with rigid fillers, are discussed. The review is based mainly on the works of the 1980s. The effect of a filler's presence on three fundamental parameters of a material, namely yield stress, viscosity, and rubbery elasticity is considered. The analysis is based essentially on qualitative considerations with respect to physical phenomena appearing when a rigid filler is introduced into the melt. General rules associated with the effect of concentration, intermolecular interaction, size and form of filler particles on rheological properties of a composition are traced. The review does not mention hydrodynamic phenomena, specific for flows of liquids with rigid filler, and also rheology of non-flowing multi-component materials.

Experimental studies on the rheology of hard-sphere suspensions near the glass transition

Abstract: We have investigated the rheological behavior of sterically stabilized colloidal silica particles of three different sizes at volume fractions above 0.5. Despite a small surface charge, which elevated the intrinsic viscosity from the Einstein value of 2.5, the particles were found to behave essentially as hard spheres in the concentrated suspensions and to have properties highly reminiscent of molecular glasses. The zero shear rate viscosity, characteristic of disordered suspensions and present at all volume fractions, diverges as {phi} {yields} 0.6 and is well-described by the Doolittle equation for glassy flow. For suspensions with a relative zero shear rate viscosity greater than 5 {times} 10{sub 2}, shear thickening was observed. Characteristic time scales for particle rearrangement determined from critical shear rates for shear thinning and shear thickening were found to follow trends predicted for molecular glasses. A transition from a liquid like linear relaxation response to glassy stretched exponential behavior was observed as volume fraction was increased. The onset of the glassy relaxation response, indicative of nondecaying correlations, occurred near a volume fraction of 0.52.

Differential stress‐induced melt migration: An experimental approach

Abstract: A gradient in the dilatational component of a differential state of stress will cause migration of the melt phase in a texturally (quasi)equilibrated partial melt. An experimental approach to image such deformation-induced melt migration is presented. Two-phase, solid-liquid aggregate beams (prepared by a glass-ceramic technique) having a primary crystalline phase of MgSiO3 (orthoenstatite with a limited amount of clinoenstatite intergrowths) in chemical and textural equilibrium with a sodium aluminosilicate glass are subjected to four-point flexure; a first-order thermodynamic analysis, based on the energy balance between grain boundaries (solid-solid interfaces) and solid-liquid interfaces, indicates that the melt phase flows from that side of the specimen under a compressive principal stress to the specimen side under a tensile principal stress. When the solid-liquid aggregate is characterized by a Newtonian rheology (i.e., the deformation occurs via a solution-precipitation-enhanced diffusional creep mechanism), the melt migration is easily observed as a large deformation transient accompanying the flexural flow of a specimen. The melt migration is thus characterized as a completely recoverable, anelastic strain in the two-phase system; the rheology of the partially molten beams is well modelled by eT(t)=e0[1−exp(−Bt)]+e˙sst where eT is the total inelastic strain, e0 is the total anelastic strain due to melt migration, e˙ss is the steady-state strain rate for the two-phase aggregate, t is time and B is a function of either the viscosity of the liquid phase or of the rheology (viscosity) of the two-phase aggregate. In the experiments reported here, the melt migration is shown to be rate limited by the kinetics of compaction and/or dilation of the crystalline residuum. The impact of the experimental approach on compaction-based models of melt transport and segregation is discussed.

Surface chemistry effects on concentrated suspension rheology

Abstract: Two brown coals of different surface charge densities and ionic strengths were evaluated for their rheological properties. Rheological behavior ranging from low viscosity Newtonian to high viscosity pseudoplastic yield, depending on the surface chemistry, was observed. By appropriately varying the ionic strength or surface charge density it was possible to convert the rheological behavior of one brown coal to that of the other. Yield stress behavior occurs when the surface charge density is low or the ionic strength is high. Conversely, the suspension is Newtonian at high surface charge density or low ionic strength. A minimum viscosity occurs when the suspension is close to the point of transition from attractive to repulsive particle interaction (i.e., just dispersed). The knowledge gained from this fundamental investigation was subsequently exploited in the development of a coal-water suspension fuel.

Particle Packing in Ceramic Injection Molding

Abstract: The shrinkage of ceramic injection molding suspensions caused by pyrolytic removal of the organic vehicle was measured for initial ceramic volume fractions from 0.48 to 0.64. Shrinkage was inversely related to initial ceramic volume fraction V, and maximum volume fraction after pyrolysis V*,max was 0.65. The factors which restrict shrinkage are discussed. The maximum volume fraction, obtained from semiempirical equations relating the viscosity of suspensions to volume fraction of powder (Vmax) was 0.73 to 0.76. Both the viscosity of the suspensions and the shrinkage on removal of the organic vehicle can be interpreted in terms of a free volume of fluid over and above that needed to fill interstices between contacting particles. Thermomechanical measurements also show that the free-volume concept helps to interpret the transition from fluid to quasi-solid properties as the organic vehicle is removed by pyrolysis from a molded body.

Dynamic mechanical-properties of polymerically stabilized dispersions

Abstract: The rheological properties of suspensions, containing polymerically stabilized, monodisperse PMMA particles in nonaqueous media, have been measured in oscillatory flow. The contribution of the soft stabilizer shell has been investigated by changing the particle radius and keeping the shell thickness constant. The scaling laws for hard spheres can be used to reduce data taken at different temperatures, with a correction for the difference in thermal expansion between particle and suspending medium. At relatively low frequencies the suspensions relax approximately as Maxwell fluids. Comparison of the plateau storage moduli with results for hard spheres indicates a systematic deviation due to the softness of the stabilizer layer. This could provide a means for deducing the interparticle potential from a comparison of the experimental data with simulation results. For the smallest particles calculation of the relaxation times provides a direct measurement of particle mobility. In concentrated systems the concentration dependence of the relaxation times closely follows that of the zero shear viscosity but the Cox-Merz analogy between steady-state and oscillatory data does not hold.

Transport processes in concentrated suspensions: the role of particle fluctuations

Abstract: Transport of momentum in slow flows of concentrated suspensions may be strongly dependent upon the fluctuations of particles about their mean motion The intensity of the velocity fluctuations is an internal field that is the analog of temperature in classical kinetic theories This viscous temperature is governed by a balance law that includes flux, production, and dissipation terms We provide heuristic arguments to motivate the forms of the viscosity, conductivity, dissipation, and pressure in a theory that includes the viscous temperature The approach parallels previous developments for dry, granular materials Phenomena observed in flows of concentrated suspensions, including apparent normal stresses and shear-induced diffusion, are contained within the structure of this theory

Rheology of weakly interacting colloidal particles at high concentration

Abstract: The paper describes an experimental study of the rheology of concentrated latex under conditions where a secondary miminum in the interparticle pair potential is expected. Particular attention is given to the Bingham stress σB derived from steady-shear-flow data, which is taken as a measure of the additional or extra stress arising from the interparticle attraction. The data are used to deduce a correlation describing the dependence of the Bingham stress on the strength of attraction, S, the latter being taken a proportional to the calculated depth of the secondary minimum, with the result σB=kf(ϕ)S1.9d–2, where k is a constant and d is the particle diameter. A weak attraction between colloidal particles can also be induced by the addition of non-adsorbing polymer. Data for systems of this type are analysed similarly and shown to support this dependence. Earlier data illustrating the effect of particle size and concentration on the storage modulus are also discussed. The dependence upon volume fraction is shown to be very similar to that of the Bingham stress and the variation of both properties is described well by an equation deduced from liquid-state theory on the assumption that the attractive forces do not greatly perturb the structure, this reads G∞, σB∝(4ϕ2+ 2ϕ3–ϕ4)/(1 –ϕ)3. It is also shown, however, that for volume fractions ϕ > 0.2 or so this is virtually indistinguishable from the concentration dependence of the modulus observed for particulate gels formed by coagulation in the primary minimum. It would thus appear that the concentration dependence of the modulus is very insensitive to local structure and the strength of the attraction. In contrast, this is not the case for the particle-size dependence. The effect of attractive forces on the low-shear and high-shear viscosity coefficients is also described.

A temperature window of extrudability and reduced flow resistance in high‐molecular weight polyethylene; interpretation in terms of flow‐induced mobile hexagonal phase

Abstract: An unexpected feature of melt flow behavior has been identified in high-molecular weight (≥ 4105) polyethylenes such as are being considered unprocessable at conventional temperatures (> 160°C) and at practicable extrusion rates (> 1 cm/min) In addition to a lower temperature window of smooth extrudability, lying in the range of 138–155°C already observed in previous works, we now discovered within this window a narrow temperature interval (150–152°C) of minimum flow resistance (viscosity) The new effect has the attributes of being associated with a new phase of increased fluidity This, in turn, we attribute to a transient mesophase arising through the chain-orienting effect of the elongational flow within the extrusion orifice; from the experiments presented here, this mesophase depends critically on both molecular weight and strain rate The hexagonal form of polyethylene, known to exist under other circumstances, is suggested as this mesophase The relevance of the new findings for applications (extrusion, melt rheology) and for fundamentals (orientation-induced phase transformations, liquid crystals from flexible chains in particular) should be obvious and are accordingly highlighted