摘要:

New experiments on the orientations of a cylinder settling in viscoelastic and pseudoplastic fluids are described in an attempt to identify the main mechanisms which control the orientation of the cylinder as it falls.

ISSN:0148-6055    

DOI:10.1122/1.550380

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

A noninertial Hookean dumbbell with internal viscosity (or internal friction) is studied in transient and steady shear flows by use of a Gaussian closure on the second moment equation of the configuration of the dumbbell. The model predicts shear thinning for the viscosity and first normal stress coefficients for all values of the relative internal viscosity parameter ε. A second Newtonian region is observed for the viscosity. Qualitative, but not quantitative, agreement is found with optically determined orientation angles of polymer coils in steady shear flows for dilute polymer solutions. The model greatly overestimates the amount of relative stretching of the polymer coil in steady shear flow. In startup flows, large, but finite, values of ε show shear stress overshoot at high shear rates, and oscillatory behavior at the highest shear rates studied. Transient negative values of the first normal stress difference are also predicted. The maximum in stress is attained at much lower values of strain than for the predictions at small ε. The oscillations are shown to be caused primarily by oscillations in the orientation of the polymer coil, rather than by oscillations in the size of the polymer coil. Instantaneous jumps in the shear stress att=0 are observed in agreement with Manke and Williams. Cessation of shear flows shows a jump in stress in agreement with data on xanthan gum. The decay upon cessation is nonexponential, but does follow the Lodge–Meissner relation. The width of the polymer coil is predicted to go through a maximum during this decay. Also, the addition of internal viscosity to the dumbbell satisfactorally gives a positive asymptotic value for η’−ηs(in‐phase complex viscosity minus solvent contribution) in small amplitude oscillatory shear flow.

ISSN:0148-6055    

DOI:10.1122/1.550406

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The elongational behavior of two polyethylenes with different structures, namely, a low‐density polyethylene and a linear low‐density polyethylene, has been investigated by isothermal melt spinning. The length along the spinline has been rescaled according to time. The evolution of the calculated elongational viscosity versus time has been compared to the transient elongational viscosities measured after imposition of a constant extension rate. The data demonstrate that these two situations, which are in many ways similar, give nearly the same results. Consequently, the viscosity which can be obtained from a fiber spinning experiment is a transient elongational viscosity. Differences in the elongational behavior between the long branched and the linear polyethylene are presented such as displayed in the spinning experiment. The strong influence on practical parameters such as melt strength or breaking stretch ratio is demonstrated.

ISSN:0148-6055    

DOI:10.1122/1.550408

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

A filament stretching device for measuring the extensional viscosity of low‐viscosity liquids is presented. The fluid sample is held between two disks which move apart at an increasing velocity so that the extension rate, based on the filament midpoint diameter, is constant. The device was used to measure the extensional stress growth coefficients of three ideal elastic solutions, including the model fluid M1 and a shear‐thinning model fluid A1. The results indicate that all solutions containing high molecular weight polymer exhibit significant strain hardening as the fluid is extended. For the ideal elastic fluids, steady state in extensional stress was observed at strain above 4.5 and the steady Trouton ratio obtained for the fluids range from 2 to 5×103. For the fluid M1 the extensional viscosities obtained are higher than the apparent extensional viscosity obtained by other methods. This is the first time that the steady extensional viscosity has been measured for polymer solutions. The results obtained enable one to evaluate the numerous constitutive equations that have been proposed for polymer solutions.

ISSN:0148-6055    

DOI:10.1122/1.550372

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Determination of the retardation line spectra from data on both the storage and the loss compliances of a rheodictic material may yield an acceptable estimate of the zero‐shear‐rate stead‐state fluidity (the reciprocal of the zero‐shear‐rate steady‐state viscosity), even when the loss compliance data do not extend sufficiently far into the flow region to determine it from the loss asymptote. The same calculations also furnish an estimate of the difference between the steady‐state and the glassy compliance. The zero‐shear‐rate steady‐state viscosity can be obtained from the storage and loss moduli by first converting them to the compliances. This route is preferable to the traditional ways of estimating it via summation or integration, because those routes yield an acceptable value for the viscosity only when the experimental window is ‘‘pseudoinfinite,’’ i.e., it spans a sufficiently large portion of the frequency scale to approach effectively the ‘‘infinite’’ window extending on the logarithmic frequency scale from minus to plus infinity.

ISSN:0148-6055    

DOI:10.1122/1.550373

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Simple shearing flow of a ‘‘dry,’’ perfectly ordered, three‐dimensional foam composed of planar films is considered. The undeformed spatially periodic cell structure is formed by regular tetrakaidecahedra, which have six square surfaces and eight regular hexagonal surfaces. The elastic–plastic response of the foam is modeled by assuming that all surfaces remain planar and that the angle between connected surfaces does not change during elastic deformation. An explicit expression for the stress tensor that is valid up to the elastic limit is determined. Past the elastic limit, the foam structure and macroscopic stress are piecewise continuous functions of strain. Discontinuities in structure and stress are associated with topological (T1) changes in the film network structure that occur when the area of an individual film vanishes. These T1 changes, which reduce surface energy and result in the switching of cell neighbors, are essential mechanisms for yield behavior in foam flow. The foam structure is determined for all values of shear strain by choosing initial cell orientations that lead to periodic behavior with strain. The shear stress evaluated from a strain energy method differs from that obtained by volume averaging the local surface tension forces; this inconsistency arises because a foam with planar films cannot satisfy the equilibrium requirement that three films meet at equal angles of 120°.

ISSN:0148-6055    

DOI:10.1122/1.550463

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

In this paper we use the constitutive equation of Bhaveetal. (1993) for rod‐like, liquid‐crystalline polymer solutions to analyze the isothermal, steady‐state spinning of these liquids in order to understand the evolution of microstructure, predict the velocity and normal stress distributions in the filament, and examine the effect of different upstream microstructural conditions. Our analysis shows that in contrast to fiber spinning models of isotropic liquids, the velocity, structure, and stress profiles are sensitive to the choice of initial conditions. In addition we have investigated the impact of the closure approximation used in the constitutive equation of Bhaveetal. on the fiber spinning problem by solving the equation for the distribution function directly; only slight changes are seen in the velocity and stress profiles. An apparent elongational viscosity defined as the ratio of normal stress difference to strain rate at the takeup compares very well with the true elongational viscosity η̄ for the model, thereby suggesting that fiber spinning flows can be used to determine η̄ for liquid‐crystalline polymer solutions. Model predictions of the velocity and stress agree well with data obtained by Prilutski (1984) for HPC/acetic acid solutions. Finally, we present a linear stability analysis of the spinning problem to show the impact of viscoelasticity, inertia, gravity, and surface tension on the onset of draw resonance instabilities. The neutral stability curves obtained for dominant viscoelastic forces reflect trends in the apparent elongational viscosity. Model predictions are in qualitative agreement with the draw resonance data reported by Prilutski.

ISSN:0148-6055    

DOI:10.1122/1.550374

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Jet swelling is shown to exist when a concentrated oil‐in‐water emulsion is ejected from a capillary tube into a stagnant fluid. The jet swells up to ten times the diameter of the capillary tube depending on the shear rate in the tube and the density difference between the ejected and the stagnant fluid. The relative jet swelling increases with silicone oil concentration, decreases in silicone oil viscosity, decreases in tube diameter, and decreases in tube length. The diameter ratio (jet/tube) grows as the 1/3 power of the wall shear stress in the capillary tube. The analogy between the swelling behavior of these emulsions and that observed in dilute and semidilute polymer solutions is discussed.

ISSN:0148-6055    

DOI:10.1122/1.550375

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

In this paper and its sequel, we attempt to understand the rheological differences between lyotropic and thermotropic liquid‐crystalline polymers by contrasting the rheology of three different liquid‐crystalline polymers at concentrations ranging from moderate (12%–30%) to highly concentrated (30%–50%) to densely packed (50%–100%). This first paper presents the steady‐state first and second normal stress differencesN1andN2as functions of the shear rate γ̇ for solutions of poly(γ‐benzyl‐glutamate) with molecular weight 238 000, in the solvent metacresol, at concentrationsCranging from 12.5% to 40% by weight. Predictions ofN1andN2for this range of concentration are obtained from the Doi molecular theory for rod‐like nematics, using both an approximate, and a nearly exact, method for solving the Doi equation. The predictions ofN1, and to a lesser extentN2, agree qualitatively with measured values for all concentrations. In particular, the range of shear rates over whichN1is negative shifts upward with increasing concentration: if we define γ̇maxas the shear rate for whichN1reaches a positive maximum valueN1max, then we find that theory agrees with experiment in that both γ̇maxandN1maxincrease monotonically with concentrationC. These increases occur because the strength of the nematic interaction increases withC, which implies that for high concentrations,N1becomes negative only at high‐shear rates.

ISSN:0148-6055    

DOI:10.1122/1.550377

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Fumed silica suspensions in low molecular weight solvents are used in many photonic and microelectronic applications. The rheology of these thixotropic systems plays a major role in the effectiveness of their usage. In this study, we use dynamic rheological measurements to examine the particle–particle and particle–solvent interactions of fumed silica with hydrophilic and hydrophobic surface groups dispersed in both polar and nonpolar solvents, polypropylene glycol and mineral oil, respectively. We find the mineral oil‐based suspensions to have a frequency‐independent elastic modulus (G’) for all solids concentration, whereas the polypropylene glycol‐based systems exhibit a ‘‘sol–gel’’ transition to a frequency‐independentG’at high concentrations. The results are explained in terms of different solvent particle mechanisms present in the two systems. The behavior of the mineral oil suspensions are dominated by particle–particle interactions through hydrogen bonds, resulting in a gel structure. The polypropylene glycol systems, on the other hand, are dominated by the interactions of the polar solvent with the fumed silica thereby preventing the formation of a 3D gel network. Static light‐scattering experiments are used to probe the microstructure of both suspensions. We find the presence of a gel‐like network in mineral oil but not in polypropylene glycol, corroborating the rheological results. In addition, both rheology and light‐scattering data for the mineral oil suspensions are consistent with the prediction of a diffusion‐limited cluster–cluster aggregation model.

ISSN:0148-6055    

DOI:10.1122/1.550378

出版商:The Society of Rheology    

年代:1993

数据来源: AIP