D. De Kee

FLOW OF VISCOPLASTIC FLUIDS ON AN INCLINED PLANE: EVALUATION OF YIELD STRESS

Within the general category of so called generalized Newtonian fluids, there exists a class of materials which do not deform until the applied shear stress exceeds a critical value called yield stress. Conversely, such a material behaves like a solid as long as the shear stress is less than the yield stress. This class of materials is called viscoplastic fluids. The question whether the true yield stress exists or not is indeed far from settled (1), but the notion of a yield stress has proved to be quite useful in practice in describing the steady shear rheological behaviour of a range of materials especially of particulate suspensions. Consequently, yield stress appears as a parameter in all constitutive relations (which purport to describe the steady shear behaviour of viscoplastic materials), and its evaluation is important before an engineering flow problem can be solved. Conversely, there are some simple hydrodynamic situations which allow the value of the yield stress to be extracted from macroscopi...

A photographic study of shapes of bubbles and coalescence in non-Newtonian polymer solutions

This communication reports (photographically) on the shapes of bubbles of different gases in several Newtonian and polymer solutions, encompassing a wide range of rheological behavior. Effects of a surface active agent are mentioned and bubble coalescence in viscoelastic solutions is visualized.

Sorption and permeation of organic environmental contaminants through PVC geomembranes

The effect of penetrant diffusion on the barrier properties of PVC geomembranes have been determined for the following organic environmental contaminants: benzene, dichloromethane, and trichloroethylene. The membranes experienced swelling to a degree which depends on the type of penetrant used. Some dissolution may occur, and in general we are dealing with Case II transport. The experimental work was performed with an ASTM-cell, a Cahn balance, and an in-house built gravity (G-) cell. Those instruments generated comparable data on breakthrough times. The obtained diffusion coefficients as well as the breakthrough times obeyed an Arrhenius-type relation over the temperature range studied. Liquid sorption of the various penetrants modified the geomembrane structure. Membrane surface pretreatment with different contaminants influences the subsequent transport of organic penetrants through PVC geomembranes. That is to say: the induced swelling, as result of membrane contact with one penetrant, is likely to alter the system free volume, allowing for a different rate of mass transport for subsequent penetrants.

Elastic modulus and yield stress of suspensions

A recent technique, developed to measure yield stress has here also been used to determine the elastic modulus of a suspension. Temperature effects have been measured.

THE SLOW MOTION OF A SPHERICAL PARTICLE IN A CARREAU FLUID

The drag on a spherical particle is studied for two limiting cases, namely for the rigid sphere and for the bubble. An approximate solution is found for creeping flow around a particle suspended in...

Non-Markovian diffusion process in polymers and stretched exponential relaxation

In this contribution, we model the “long-time” behaviour of the desorption from an LDPE sheet, using non-Markovian random walks. It is shown that the mass of penetrant in the final stage of desorption decays as t−m, where m is proportional to the exponent of the probability distribution ψ(t) ∼ t−(1+u), 0 < v < 1. Furthermore, it is shown that this model may lead to the so-called mechanical stretched exponential relaxation, and that Wagners memory function can be obtained as a special case.

Fractal-time stochastic processes and dynamic functions of polymeric systems

Dynamic material functions of polymeric systems are calculated via a defect-diffusion model. The random motion of defects is modelled by a fractaltime stochastic process. It is shown that the dynamic functions of polymeric solutions can be approximated by the defect-diffusion process of the mixed type. The relaxation modulus of Kohlrausch type is obtained for a fractal-time defect-diffusion process, and it is shown that this modulus is capable of portraying the dynamic behavior of typical viscoelastic solutions.The Fourier transforms of the Kohlrausch function are calculated to obtainη′ andη″. A three-parameter model forη′ andη″ is compared with the previous calculations. Experimental measurements for five polymer solutions are compared with model predictions.

Shear stress from rate-dependent and nonaffine network models for melts in complex shear histories

Analytical expressions of shear stress for arbitrary multi-rate-step flows are presented for a rate-dependent network model and for a nonaffine network model. For both models the linear spectrum is modified to account for large deformations. Predictions of both models are evaluated for the following cases: concave steps, reversed steps, and large amplitude oscillations. Model predictions are compared to experimental results on a polydimethylsiloxane melt. Quantitative agreement is obtained for both models, which justifies the usefulness of the proposed equations for the prediction of shear responses in complex transient shear flows. The performance and limitations of the two models are also discussed in terms of physical considerations.

DESORPTION CURVES AND TRANSPORT PROPERTIES OF POLYMERS

Abstract The desorption of hydrocarbons in LDPE film is analyzed with the help of a continuous-random-walk model. It is shown that for obtaining the mechanical relaxation modulus of the sample, one has to know the complete desorption curve. The existence of a universal desorption is suggested by the normalized desorption curves for the investigated systems.

Relations between stress growth and stress relaxation functions

The stress relaxation function after steady shear flow and the stress growth function at inception of steady flow are derived for several constitutive equations of the integral and differential types. Relationships between these functions are deduced and discussed.