摘要:

A microstructural model for the rheology of large‐gas‐fraction foams and concentrated emulsions is developed. Large shear and extensional deformations of a two‐dimensional spatially‐periodic network consisting of monodisperse hexagonal cells are considered. The elastic response is determined by surface tension forces and the steric interaction of thin liquid films. Coalescence and disproportionation of Plateau borders result in the relative separation of cells and provide a basic mechanism for yielding and flow. The strain dependence of the macroscopic stresses and cell morphology is very sensitive to the initial cell orientation. The response is strain periodic for discrete values of the orientation angle; however, strain‐periodic orientations for simple shear and extension are mutually exclusive. The steady‐flow material functions are determined by averaging the instantaneous stress over the strain period. Three different physical interpretations of the yield stress are considered.

ISSN:0148-6055    

DOI:10.1122/1.549909

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

The angular and radial velocity components for 5–100 ppm solutions of poly(ethylene oxide) with a molecular weight of 2–3 million in 50% sugar water were measured in the laminar boundary layer of a rotating disk using laser Doppler velocimetry. The angular velocity profiles in the direction perpendicular to the disk show a small decrease in boundary layer thickness compared to a Newtonian fluid for low Weissenburg numbers. When the Weissenburg number defined as the product of the polymer time scale and the rotation rate of the disk exceeded a value near one, the boundary layer thickness was increased. Radial velocities were reduced at all Weissenburg numbers. Polymer‐induced velocity fluctuations of two types were observed. At a dimensionless distance from the disk ofz*=0.3,the angular velocity component contained fluctuations with frequencies above 10 Hz. These fluctuations were centered at a frequency that corresponded to the rotation of an ellipsoid in the shear flow. Atz*=1,the radial velocities contained large fluctuations at frequencies less than 10 Hz when the Weissenburg number was near or larger than one. These low‐frequency fluctuations are thought to be instabilities in the flow while the higher frequencies may be coherent polymer rotations.

ISSN:0148-6055    

DOI:10.1122/1.549903

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Transient and steady‐state recoverable strains in shear and uniaxial elongation were measured with rheometers of different type. Results on melts of linear and branched polyethylenes, polypropylene, polystyrene, polyamide 6, and a PIB solution are reported. To represent the linear viscoelastic behavior, the essential basis of the theoretical treatment, discrete relaxation time spectra have been determined from the dynamic moduli in oscillatory shear. Calculations of the primary normal stress and of the shear and elongational viscosities in the non‐Newtonian range are based on a single integral constitutive equation with strain‐dependent memory function. Recoverable strains observed after unloading the samples from steady‐state flow are calculated by means of a quasilinear treatment of the materials, making use of deformation rate dependent effective relaxation strengths. The agreement of predicted and measured recoverable strains over a very wide range of shear and elongation rates strongly supports this new concept, which does not involve an inversion of the integral constitutive equation. In addition, the applicability of two empirical equations that relate the primary normal stress coefficient and the recoverable strain in steady shear flow to the dynamic moduli are checked on the same materials.

ISSN:0148-6055    

DOI:10.1122/1.549855

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.549856

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A novel time‐dependent phenomenon has been found in the rheology of the disperse system of spherical particles. This is the phenomenon that the dynamic torque, i.e. the dynamic moduli, decrease with increasing dynamic shearing time to values lower than one‐thousandth of their initial ones and recover within a short time by the appropriate steady shear. This unsteady phenomenon can be repeated many times by repeating the shearing process. It is considered that the rigidity and the uniformity in shape and size of dispersing particles are important factors for this time‐dependent phenomenon.

ISSN:0148-6055    

DOI:10.1122/1.549857

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A steady laminar flow of an incompressible non‐Newtonian fluid (inelastic power law fluid,n<1) in the inlet region of rectangular ducts was studied by the theoretical analysis using finite difference methods and the experiment on pressure drops. Theoretical predictions were compared with experimental data and analytical results available and the agreement was shown to be satisfactory. Therefore, the axial pressure distributions and the velocity profiles in the inlet region were discussed on the basis of typical computations. It was shown that the velocity in the duct center and the pressure drop of a power law fluid were smaller than those of a Newtonian fluid; the inlet length of a power law fluid was larger than that of a Newtonian fluid and increased with the decreasing power law index. Also, the additional pressure loss in the inlet region was smaller in a power law fluid than in a Newtonian fluid and decreased with the decreasing power law index. In addition, when the effect of duct shape was considered, it was found that the velocity in the duct center and the pressure drop of a power law fluid decreased with the increasing aspect ratio of the duct, as in the case of a Newtonian fluid (they are at a maximum in the square duct).

ISSN:0148-6055    

DOI:10.1122/1.549858

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A phenomenological model describing the effects of non‐infinitely dilute polymer concentrations on average transport coefficients for polymer solutions in narrow pores is developed using gradient theory applied to transport processes. Average viscosities and diffusion coefficients are calculated and fit to experimental rheological data using the one adjustable parameter of the model. The concentration dependence of this parameter is examined and found to be in agreement with intuition and existing theories.

ISSN:0148-6055    

DOI:10.1122/1.549859

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

The problem of fluid element deformation behind an advancing flow front (fountain flow) which is encountered in injection mold filling, is addressed. The fluid element trajectories were calculated by integrating the velocity field, obtained by the finite element solution of the Navier‐Stokes equations. It is shown that fluid elements from the centerline stretch as they move toward the walls and form characteristic “V”‐shapes in a rolling‐type motion before they become fully extended. The results are in good agreement with available experimental observations.

ISSN:0148-6055    

DOI:10.1122/1.549860

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A solution of poly‐γ‐benzyl‐L‐glutamate (PBLG) in m‐cresol has been investigated. Viscoelastic properties during steady state shear flow and in oscillatory flow are presented. The existence of a stable negative first normal stress difference is confirmed. The dynamic Newtonian viscosity is considerably smaller than the corresponding steady state shear value. The transient behavior after cessation of flow is studied in detail using dynamic measurements. After cessation of flow, the dynamic moduli change over a time scale which is much larger than that for stress relaxation. Effects of frequency, temperature and previous shear rate are analyzed. The structural interpretation of the data is discussed. Available models for polymeric liquid crystals have to be developed further before a comparison with the present results becomes possible.

ISSN:0148-6055    

DOI:10.1122/1.549861

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A rheological study of a series of thermotropic liquid crystalline copolyesters containing varying proportions of hydroxybenzoic acid, isophthalic acid, and hydroquinone has shown unexpected behavior. Certain compositions were observed to exhibit a POSITIVE temperature coefficient of viscosity (viscosity increasing with shear rate) over the entire shear rate range. Other compositions showed flow curves which intersected at critical shear rates. This behavior is explained by a variation in chain stiffness with composition (high hydroxybenzoic acid content resulting in stiffer chains) which results in a biphasic melt. The unusual rheological behavior is a manifestation of the competition between isotropic and anisotropic phases, which is influenced by composition, temperature, and shear.

ISSN:0148-6055    

DOI:10.1122/1.549862

出版商:The Society of Rheology    

年代:1986

数据来源: AIP