摘要:

The yield stress of a magnetorheological suspension is calculated from two different approaches. The first one is based on a mesoscopic description of the structure taking only into account the shape anisotropy of the strained aggregates. The second one is based on a microscopic approach where the interparticle forces, due to the application of the field, are calculated numerically by taking into account the magnetostatics between the particles inside the aggregates. We show that the macroscopic description well applies to suspensions of nonmagnetic particles in a ferrofluid and that a layered structure, consisting of parallel slabs of magnetizable materials should have a yield stress much higher than a structure made of cylindrical aggregates. On the other hand the microscopic approach is appropriated for the description of suspensions of particles of high permeability. In this case, the yield stress is mainly determined by the rupture between pairs of particles and, consequently, it strongly increases with the angle between the line of centers of the pair undergoing the rupture and the field.

ISSN:0148-6055    

DOI:10.1122/1.550838

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The response of semi-concentrated model blends (10% disperse phase), consisting of slightly viscoelastic polymers, on a stepwise increase in shear rate is investigated. During the initial stage of the response droplets deform into fibrils. The shear and normal stress transients during the deformation process are modeled by combining the approach of Doi and Ohta with the affine deformation theory for single droplet behaviour. In the proposed equations the scaling relations of Doi and Ohta for transient stresses are preserved. They do not contain any fitting parameter. First, the model predictions are compared with experimental results on model blends. Good agreement is found under conditions for which affine deformation is expected. Second the applicability of the scaling relations of the Doi–Ohta theory is verified experimentally. Although the scaling laws should only apply for 50:50 mixtures of Newtonian liquids with equal viscosity, the experiments show that they hold as well for semiconcentrated systems containing slightly viscoelastic components with viscosity ratios deviating from unity.

ISSN:0148-6055    

DOI:10.1122/1.550870

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

An iterative model is described to predict isothermal and nonisothermal elongation of an extruded filament at a given force. The principle of the model allows a quite simple description of Rheotens tests with much flexibility regarding the shape of the elongational viscosity function, including true viscoelastic flow and thermal boundary conditions. The algorithm is compact and computing time on a PC is short. The application of an integral K-BKZ constitutive equation to predict Rheotens curves of the well characterized LDPE Melt I is demonstrated and the influence of the slope of the transient viscosity in the strain-hardening regime is discussed. A simple approximation to take into account the prehistory of the material in the die based on purely elongational strains is proposed which includes the irreversibility assumption of the damping function (Wagner model). Predicted Rheotens curves are compared with experimental results.

ISSN:0148-6055    

DOI:10.1122/1.550880

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The viscoelastic properties of a model binary polymer blend exhibiting an upper critical solution temperature phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements. A mixture ofunentangledmonodisperse polystyrene and poly(phenyl methyl siloxane), exhibiting Newtonian shear viscosity, was used, and its phase diagram was established by turbidity and dynamic light scattering measurements. In the miscible region, the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the surface fractions instead of volume fractions. Near the phase separation temperature and far from the glass transition, critical concentration fluctuations dominated the linear viscoelastic response and were responsible for the observed thermorheological complexity. An appropriate quantitative account of these fluctuations resulted in the accurate rheological determination of both the binodal and spinodal temperatures, extending thus the applicability of the relevant procedure earlier applied to lower critical solution temperature blends involving higher molecular weight entangled polymers. In the phase separated regime, the normal stress of the dispersed phase undergoing spinodal decomposition followed a recent scaling proposed for molecular mixtures with large viscosity difference.

ISSN:0148-6055    

DOI:10.1122/1.550830

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

An algorithm is demonstrated to yield accurate linear viscoelastic properties in a nonequilibrium stochastic simulation by variance reduction. For noninteracting polymer dumbbells, multiple orders of magnitude improvements in computational accuracy for nonequilibrium viscoelastic properties are easily realized. This simple and robust control variate scheme uses control variables defined as the viscoelastic properties obtained from a parallel, equilibrium simulation. Results for strongly interacting colloidal particles demonstrate some advantages as well as limitations of the method.

ISSN:0148-6055    

DOI:10.1122/1.550831

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Concentrated suspensions of charged stabilized colloidal particles exhibit very large viscosity at low shear rate, a strong shear-thinning behavior at intermediate shear rate, and a constant second Newtonian viscosity at high shear rate. This type of non-Newtonian behavior is affected by many factors such as the particle volume fraction φ, the particle diameter, the surface electric potentialψ0,salt concentration, etc. The generalized equation for the viscosity η of this system is proposed by applying Eyring’s transition state theory. The surface electric potentialψ0and the thickness of the electric double layerκ−1are determined by applying the theory to experimental data. Systematic experiments of η of the model colloidal dispersion systems are carried out as the function of φ and shear rate and the results are satisfactorily reproduced by the present theory. The effects of hydrodynamic diameterdhandψ0of the colloidal particle on η are also quantitatively explained.

ISSN:0148-6055    

DOI:10.1122/1.550875

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Computations of the flow of non-Newtonian fluids in the presence of a reentrant corner have a long history of convergence problems, which are believed to originate from a nonsquare-integrable stress singularity. Local flow analyses near such a corner have been inconclusive, due to the nonlinearity and the model dependence of the governing equations. We have used molecular dynamics simulations to compute the flow of both a Newtonian liquid and a model polymer melt through a channel with a reentrant corner, providing an unbiased and convergent calculation. The fluids interact via Lennard–Jones potentials, and for the polymer case we employ FENE chains of length up to 30. For the Newtonian fluid, the shear stress near the corner is found to agree with the Stokes flow prediction of Moffatt. In the non-Newtonian case, the shear stress has a stronger apparent divergence, increasing with velocity but not with chain length, which appears to saturate at an integrable value of approximately 0.8. The molecular origin of the stress enhancement is the additional elongation and rotation of the molecules near the reentrant corner.

ISSN:0148-6055    

DOI:10.1122/1.550832

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550862

出版商:The Society of Rheology    

年代:1997

数据来源: AIP