摘要:

A technique is presented in which approximate values of the elongation stress can be measured at large elongation rates by using opposed orifices to generate a stagnation flow similar to uniaxial elongation. The elongation rate is related to the volumetric flow rate of fluid into the orifices and the apparatus geometry. The first law of thermodynamics is simplified to relate the pressure drop due to fluid entering the orifices with the first normal stress difference in uniaxial, elongation flow. Thus only the flow rate and pressure drop are needed to determine the elongation viscosity as a function of elongation rate. Results for elongation rates up to 1000 s−1for three polymer melts, linear low‐density polyethylene, low‐density polyethylene, and polypropylene, are presented. The technique can also be used at low elongation rates and results are presented in which the elongation viscosity approaches 3× the zero shear viscosity.

ISSN:0148-6055    

DOI:10.1122/1.550681

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

An investigation is made of the flow properties of a suspension of uniform 197 nm diameter spheres at a volume fraction of 0.50 dialyzed to equilibrium against 10−3MKCl which displayed long‐range orientational order at rest. In wide gap rheometers (i.e., tool gaps ≥0.5 mm), microstructures and flow properties are similar to those seen in many suspensions which are ordered at rest. Low shear rate response is found to be independent of rheometer tool gap down to 4.5 μm. However, the shear rate marking the onset of thickening is a decreasing function of rheometer tool gap for gaps smaller than 15 μm. These results are consistent with abrupt shear thickening seen in dense suspensions containing uniform particles arising from the formation of percolating clusters produced during shear melting.

ISSN:0148-6055    

DOI:10.1122/1.550697

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The rheological and microstructural properties of dense suspensions of uniform, charge stabilized colloidal spheres with diameters greater than 200 nm are investigated at volume fractions just below the ordering transition up to 0.6. Shear stresses marking static and dynamic yielding, discontinuous shear thinning, and shear thickening are weakly dependent on volume fraction and particle size when scaled on the crystal’s elastic modulusG0. As shear stress is increased microstructures evolve through similar states independent of volume fraction. As rest, presheared suspensions exhibit long‐range orientational order. Above the dynamic yield stress, the suspensions deform with a polycrystalline microstructure which, at intermediate shear rates, evolves to hexagonally close‐packed planes lying parallel to the rheometer walls. At higher shear rates the suspensions melt. Thickening is only observed above a volume fraction of 0.4–0.5, depending on particle size and at shear rates above that where the hexagonal close packing has been degraded. Shear rates characteristic of changes in flow properties and microstructure increase exponentially with volume fraction in the same manner asG0. As a consequence, a volume fraction independent constitutive response is observed when stress is scaled onG0and the shear rate is scaled onG0/μ. Here μ is the continuous phase viscosity.

ISSN:0148-6055    

DOI:10.1122/1.550687

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A multimode interfacial constitutive equation is presented to describe the steady‐state and dynamic behavior of polymer/metal interfaces under flow conditions. The polymer melt is considered as a network of different types of polymer segments, modeled as Hookean springs with different relaxation times. In this way a set of convection equation results; these are solved numerically by using a direct stochastic interpretation method originally developed by Petruccione and Biller [J. Chem. Phys.89, 577 (1988)]. The stress tensor and the slip velocity are calculated as functions of time in various simple shear flows and the numerical results are compared with available experimental slip velocity data in both steady and transient shear flows.

ISSN:0148-6055    

DOI:10.1122/1.550688

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The behavior of the Doi model for liquid crystalline polymers, using a closure approximation due to Hinch and Leal (HL1), is studied and compared to the behavior using the simple quadratic closure. It is found that the HL1 closure gives some aphysical results for simple shear flow, but describes a much richer solution structure for this flow than the quadratic closure, including tumbling and wagging phenomena that are found both experimentally, and in solutions of the Doi theory without closure approximations. However, there is a very sharp transition in the predicted behavior as the flow is changed very slightly to one in which the strain rate exceeds the vorticity: the orientation distribution becomes flow aligning for all values of the nematic potential, and the phenomena of tumbling and wagging are suppressed completely. Although the quadratic closure model does not exhibit tumbling in simple shear flow, and thus has been sharply criticized as an approximation of the Doi model, it does exhibit tumbling for flows that are only very slightly more rotational than simple shear flow, and the sharp transition to flow aligning solutions with added strain rate occurs just prior to simple shear, instead of just after as in the HL1 model. Finally, we find that the transition from flow‐aligning to periodic solutions occurs via two distinct mechanisms, one occurring for Pe≳20 where there is a strong coupling between the director orientation and the degree of alignment and the other for smaller Pe where the degree of orientation is unchanged by the flow.

ISSN:0148-6055    

DOI:10.1122/1.550689

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

When one wants to study the behavior of coarse suspensions directly, one needs a special large‐scale rheometer with a wide gap. As a first step towards the understanding of coarse suspension behavior a wide‐gap coaxial cylinder rheometer (1.2 m diameter, 0.2 m gap, sample volume: 0.5 m3) was built. This apparatus, which should be cheap and strong, was designed to be towed and used in the field to test fluids including particles whose diameters do not exceed 1 cm. The validity of hypotheses and the various theoretical problems of the rheometrical calculations in the case of a wide‐gap rheometer when used to test a coarse suspension are then examined. The main advantage of this geometry appears to be that edge effects should not be too important. Its main disadvantage, especially for coarse suspensions with a complex behavior (yield stress, thixotropy, presence of coarse particles), is the heterogeneity of shear rate from one tool to the other. Experiments with various natural materials (water–clay mixtures and water–debris flow sample mixtures) at different concentrations were carried out. Some experimental problems (settling, slip, fracture, etc.) were encountered but they could be mastered reasonably well. These results show that when the clay fraction is high enough, a Herschel–Bulkley model can well represent the flow curve of these mixtures in the range of shear rates (0.5; 50 s−1). When the clay fraction is low and the solid particle fraction is high enough, suspensions behave in a very peculiar way: the flow curve has a minimum.

ISSN:0148-6055    

DOI:10.1122/1.550693

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Flow experiments with a capillary rheometer using 0.3% polyacrylamide and 0.75% polyox aqueous solutions were performed to study flow development and end effects generated by polymer solutions of flexible macromolecules. SeveralL/Dcapillary ratios were employed, and a method proposed recently was used to evaluate the corrections due to end effects. The corrections obtained are of the same order of magnitude than those obtained using the Bagley method, but the Bagley corrections have much larger uncertainties. The recently proposed method is superior to the Bagley method when accurate end effects are required, as for correlations with elasticity or to evaluate numerical predictions for the flow behavior of these fluids.

ISSN:0148-6055    

DOI:10.1122/1.550694

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The slip velocity of potato granule pastes as a function of wall shear stress was determined in capillary flow at several temperatures and aqueous concentrations. The Mooney technique was used together with a variation of the method in which the shear rate for each die was subtracted from that for a no‐slip flow, approximated using rough dies. General agreement between the two methods was found, despite some unrealistic results in applying the Mooney technique. A critical wall shear stress for slip was found, consistent with published results for other materials. The exponent in the wall shear stress–slip velocity power law was consistently higher than that in the wall shear stress–apparent wall shear rate relationship. This is in keeping with the observation that the ratio of slip flow rate to total flow rate fell with increasing wall shear stress.

ISSN:0148-6055    

DOI:10.1122/1.550695

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

At small stresses or strains, polymeric liquids show a linear viscoelastic behavior. This can be demonstrated in creep followed by creep recovery. If the creep period 0

ISSN:0148-6055    

DOI:10.1122/1.550696

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Suspensions of electrostatically stabilized calcium halophosphate particles of four unimodal sizes were studied under steady shear Couette flow conditions. The particle size ranged from 8.4 to 16.8 μm. The suspensions were prepared in an aqueous polyethylene oxide solution containing an ammonium neutralized polyacrylic acid dispersant. Shear thinning, followed by shear thickening at higher shear rates, was observed for suspensions containing either the individual unimodal fractions or their blends. The polyelectrolyte dispersant is a necessary component for shear thickening and also for reduction in viscosities below the high shear limit. At a constant total volume fraction of 0.38, the dependence of viscosity on shear rate changed with blend composition. In the shear thinning region at low shear rates, the viscosity increased linearly with decreasing median particle diameter, with no observable polydispersity effect over the range of size distribution investigated by blending the halophosphate fractions. At shear rates within the shear thickening transition, a broader size distribution significantly reduces the viscosity and delays the onset of shear thickening, even for blends containing size ratios as low as 1.64:1. A scaling analysis of the relative importance of forces acting on the particles in the shear thinning and shear thickening regimes is presented.

ISSN:0148-6055    

DOI:10.1122/1.550698

出版商:The Society of Rheology    

年代:1995

数据来源: AIP