摘要:

The flow of concentrated suspensions of noncolloidal particles in viscous Newtonian fluids gives rise to shear‐induced particle diffusion which can generate a nonuniform particle concentration profile within the sample being sheared even under creeping flow conditions. This diffusive process owes its existence to the fact that the motion of a given particle within the suspension acquires a random component, as a result of the hydrodynamic interactions among the particles the positions of which are also random. We distinguish three such diffusive phenomena; (a) self‐diffusion of a marked particle in a suspension of uniform concentration; (b) diffusion from regions of high concentration to low; and (c) diffusion from regions of high shear to low. As a consequence of (b) and (c), when a well‐mixed suspension of neutrally buoyant particles is made to flow in a long tube or channel, a fully developed particle concentration profile is attained in which the concentration increases monotonically from the wall to the center. The effective viscosity of the suspension adjacent to the wall is thereby reduced and hence, for a given volumetric flow rate, the pressure drop in such a system is significantly lower than if the particles were uniformly distributed. These diffusive effects are also responsible for the phenomenon of viscous resuspension whereby an initially settled bed of heavy particles in contact with a clear fluid above it can be resuspended under the action of a shear flow. Several additional consequences of shear‐induced particle diffusion in flowing concentrated suspensions are presented and then discussed.

ISSN:0148-6055    

DOI:10.1122/1.550619

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A device was developed to measure the rheological properties of coarse suspensions exhibiting zone settling properties. The device is an elongated double gap cup and bob arrangement that can be attached to most rotational viscometers. It was tested on magnetite aqueous suspensions which are used by industry in beneficiation processes as dense media. In the design, the bob is positioned in an elongated cup so that it is completely within the constant density zone of the settling suspension for the duration of the measurement. Other design features include vertical grooves in the shearing surfaces of the cup and bob to reduce wall slip errors, and narrow gap sizes to lessen non‐Newtonian shear rate effects. Tests were conducted to confirm theoretical aspects of the design and to establish measurement constraints such as the maximum shear rate at which laminar flow is maintained. Finally, measurements obtained using the new device are compared to those acquired using a conventional fixture.

ISSN:0148-6055    

DOI:10.1122/1.550620

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Stokesian dynamics computer simulations were performed on monolayers of equally sized spheres. The influence of repulsive and attractive forces on the rheological behavior and on the microstructure were studied. Under specific conditions shear thickening could be observed in the simulations, usually together with a change in the microstructure from ordered layers to a state with large clusters. These clusters are responsible for the high viscosities in shear thickening conditions. The shear thickening results show qualitative agreement with the theoretically expected behavior and with experiments. Van der Waals forces are found to sharpen the shear thickening transitions by inducing even stronger clusters and thus higher viscosities.

ISSN:0148-6055    

DOI:10.1122/1.550621

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We demonstrate that the model of [Polymer22, 1010 (1981)] has the property of forming a model of a necking polymer. Numerical models of necked bodies are generated by incorporating the theory into a finite element scheme. Large multiaxial extensions are applied to polypropylene sheets at a high temperature to produce necked specimens. Values of the model parameters are chosen such that the observed necks—in uniaxial, planar, and equibiaxial extension—are predicted realistically. The drawing forces observed are also shown to be consistent with the predictions.

ISSN:0148-6055    

DOI:10.1122/1.550622

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A new approach is described to obtain dynamical equations for the conformation of a liquid crystalline polymer (LCP). The starting point is kinetic theory, which yields an unwieldy evolution equation for the orientation distribution of molecules associated with a material point of the liquid crystal. The usual approach is to average this integro‐partial‐differential evolution equation for the distribution function and to make use of moment closure approximations in order to obtain a direct moment tensor evolution equation. In the approach proposed herein, the evolution equation for the orientation distribution function is specialized to a particular class of deformations of the LCP. The resulting ordinary differential equation for the remaining degrees of freedom of the deformation is self consistent, in the sense that deformations remain in the designated class. This deformation tensor model is carefully constructed to yield physically sensible dynamics in any flow; however, in tests of the model particular emphasis is placed on the behavior in shear flow. The model displays the complex dynamics of flow aligning, tumbling, log rolling, wagging, etc. that are observed in experiments. Moreover, the model captures the subtle distinctions between flow aligning and tumbling dynamics based on molecular aspect ratio.

ISSN:0148-6055    

DOI:10.1122/1.550717

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The flow‐induced structure and dynamics of concentration fluctuations in a near‐critical polymer solution have been investigated using the technique of polarimetry. Specifically, this article presents scattering dichroism of a single‐phased, semidilute solution of polystyrene in dioctyl phthalate (PS/DOP) subjected to small amplitude oscillatory shear flow. The measured frequency response is compared to results obtained using the Onuki–Doi theory, which relates dichroism to structure factor, and the Helfand–Fredrickson model, from which the structure factor is calculated. The predictions are in good qualitative agreement with the experimental data when stress relaxation is taken into account in the constitutive equation relating stress to strain in the system. Our results demonstrate that the strength of the coupling between viscoelastic stress and flow governs the behavior of concentration fluctuations in semidilute polymer solutions under oscillatory shear flow.

ISSN:0148-6055    

DOI:10.1122/1.550623

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Steady and unsteady state velocity and concentration profiles are presented for a 40% by volume suspension of polymethyl methacrylate (PMMA) spheres in polyalkylene glycol (PG). The profiles were obtained by using the noninvasive technique of nuclear magnetic resonance imaging (MRI) in three experimental geometries: coaxial rotating cylinders (i.e., for generating wide‐gap Couette flow), coaxial cylinders in which a straight flight rotates with the inner cylinder and spans the annulus between the surfaces, and a single screw extruder. Concentration profiles document the presence of particle migration from high shear to low shear regions in the concentric cylinder apparatus and in the extruder. However, concentration gradients across the gap in the straight‐flight cylinder are not exhibited, indicating the relative importance of mixing in that geometry. Velocity profiles for the pure PG fluid and for suspension flows which remain well‐mixed agree quantitatively with profiles predicted for Newtonian fluids. Deviations from Newtonian behavior that are present as a result of inhomogeneities in the suspension are interpreted in light of the diffusive‐flux model of particle migration. In particular, in the extruder and straight‐flight geometries it is shown that the importance of migration in causing variations in the particle volume fraction can be predicted from time scales describing the relative rates of circulation and migration. The results obtained from this work provide insight into time scales of particle mixing and migration which can be applied to industrial equipment such as scraped‐surface heat exchangers, static mixers, and commercial extruders.

ISSN:0148-6055    

DOI:10.1122/1.550724

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

We have characterized the rheology of two concentrated, liquid crystalline solutions of poly(γ benzyl–L–glutamate) of molecular weight 238 000 inm–cresol. Comparing these results to previous work on less concentrated, liquid crystalline solutions separates the effects of concentration from the direct influence of defect texture on liquid crystal polymer rheology. This work also defines the limitations of current polydomain models and suggests improvements. The solution atC=37wt% PBLG behaves similarly to moderately concentrated nematic solutions (12wt%

ISSN:0148-6055    

DOI:10.1122/1.550624

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The derivation of the expression of the Brownian contribution to the stress tensor in a dispersion of spherical particles is discussed in relation to the level of description. The result obtained by Batchelor at the configuration space level is compared with a similar result of Bossis and Brady, derived at the underlying stochastic level from a Langevin equation. It is shown that the result of Bossis and Brady may be derived also at the configuration space level. This clarifies the connection between both approaches and provides a generalization of Batchelor’s result.

ISSN:0148-6055    

DOI:10.1122/1.550625

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

In this work we perform an experimental characterization of the rheology of hydroxypropyl guar (HPG) solutions. The flow situations investigated are shear rheometry, elongational flow in opposed jets, and flow through porous media. The results of the experiments show that hydroxypropyl guar behaves as a semiflexible molecule in aqueous solutions under elongational flow situations. In shear flows and under semidilute conditions, the HPG solutions have a shear‐thinning behavior, whereas in opposed‐jets flow they exhibit extension thickening. In porous media flows, semidilute solutions behave as shear thinning, but there is evidence that extension thickening is present. We have developed a technique in which a linear charged‐coupled device (CCD) array is used as a retardation detector to measure birefringence profiles in opposed‐jets flow. This technique has shown that HPG molecules are stretched in the flow field, which confirms that the HPG molecules are semiflexible. The addition of a cross‐linking agent to HPG solutions was found to have a pronounced effect on their flow behavior in extensional flows. Filtered dilute solutions exhibit intramolecular cross links that reduce the apparent viscosity of the solution in both opposed jets and porous media flows. On the other hand, semidilute solutions (as well as filtered solutions of low concentrations) exhibit intermolecular cross links that, in porous media flows, result in a sizable increase in pressure drops.

ISSN:0148-6055    

DOI:10.1122/1.550626

出版商:The Society of Rheology    

年代:1995

数据来源: AIP