Onset Of Shear Flow Research Articles

Transient elasticity and the rheology of polymeric fluids with large amplitude deformations.

Transient elasticity is a systematic generalization of viscoelasticity. Its purpose is to give a coherent description of non-Newtonian effects displayed by soft-matter systems, especially polymer melts and solutions. Using the concept of transient elasticity we describe here a hydrodynamic model for polymeric fluids, which is applicable for large amplitude deformations. We present an energy density with only two independent parameters, which is compatible with all thermodynamic requirements and which reduces for small deformations to models studied previously. The expression discussed is simple enough to allow full analytic treatment and shows semiquantitative agreement with experimental data. This model is used to capture many of the interesting effects thought to be characteristic of polymer rheology for large deformations including viscosity overshoot near the onset of shear flow, the onset of elongational flows in situations for which there is no stationary solution as well as shear thinning and normal stress differences for a large range of shear rates. In addition, we analyze how well our model accounts for empirical relations including the Cox-Merz rule, the Yamamoto relation, and Gleißle's mirror relations.

Shear Pulses Nucleate Fibril Aggregation

We have studied the effect of shear flow on the formation of amyloid fibrils of the whey protein β-lactoglobulin. β-Lactoglobulin aggregates into long, thin, and semiflexible fibrils upon heating at low pH and low ionic strength. Solutions with a protein concentration of 0.5% (w/w) were used, and the formation of fibrils was quantified with flow-induced birefringence, a proportional measure of the length concentration of the fibrils. From the decay of the birefringence after cessation of the flow, a length distribution could be fitted. Pulsed and continuous shear treatment of the samples resulted in a comparable enhancement of the fibrillar growth as compared to the fibrillar growth under quiescent conditions. This indicates that the onset of shear flow is the key parameter for the enhancement of fibrillar growth and not the continuous shear flow itself. This behavior is comparable to a nucleation-like process, during which preaggregates of the fibrils are induced during the onset of the flow and orthokinetic coagulation is absent. However, a difference was present in the length distribution between the pulsed and continuously sheared samples, which can be explained by the homogenizing effect of shear flow.

Propagation of the interface in a fluid suspension after the onset of shear flow

A measurement is carried out to determine the propagation of interface after the onset of shear flow in the fluid suspension exhibiting yield stress held in the wide annular space between two coaxial cylinders. The position of a specific interface is evaluated either from the deformation of the solid-like zone in suspension or the velocity distribution in the liquid-like zone. The deformation is visualised by the tracer spread on the free surface and the velocity distribution is measured by the ultrasound technique. The proper interface position between the zones is related to the strain bound up with the end of plastic deformation and outset of viscous deformation in the solid-like zone.

Breakup of filaments in blends during simple shear flow

In this paper the breakup of droplets under shear in polymer blends is studied by means of linear conservative dichroism and small angle light scattering. More specifically breakup of long fibrils by interfacial instabilities is considered. Measurements are performed on dilute model systems containing nearly Newtonian components in transient flows that involve a sudden increase or decrease in shear rate. The experimental results are used to evaluate the Khakhar and Ottino theory [Khakhar and Ottino (1987)]. In this theory breakup times are calculated starting from the onset of the instability. It is demonstrated that the scaling derived from the Khakhar and Ottino theory also holds for the more readily accessible total breakup time, calculated from the onset of shear flow in a startup flow. The development of interfacial disturbances is studied in a flow history, which consists of generating fibrils by suddenly applying a shear rate followed by a sudden drop in shear rate during which the breakup process ...

The reptating rope model: Viscometric functions for concentrated polymer solutions and melts in shear flow

The viscometric functions for shear flow as predicted by the inextensible reptating rope model have been analysed numerically and analytically. The results obtained are compared with the predictions of the Curtiss—Bird theory. It is shown that if the correlation length of the rope is small as compared to its contour length significant deviations from the Curtiss—Bird theory are obtained. Results are presented for: (a) the onset of shear flow, (b) steady state shear flow and (c) small amplitude oscillatory shear flow.

A theory of nonlinear viscoelasticity based on a temporarily cross-linked network model

Abstract Two kinds of relaxation processes are introduced for the theory of nonlinear viscoelasticity of temporarily crosslinked network structures. One is the chain slip process and the other is the change in number of chains as function of time. The following assumptions are made for this theory: (1) chains are Gaussian; (2) the slip process is characterized by single relaxation time; (3)the chain breakage coefficient is proportional to the absolute value of the average force acting on a chain; (4) the rate of chain generation is constant; (5) the reformed chains have the same strain as that of unbroken chains. The results obtained by the application of this theory are as follows. Stress growth at the onset of shear flow shows the stress overshoot. Stress growth at the onset of elongational flow is monotonic. Stress relaxation at a large deformation is nonlinear. The storage modulus of periodic deformation superposed in parallel on a steady shear flow vanishes at a low frequency but not the loss modulus.