摘要:

We investigate the stability of a periodic unidirectional two‐dimensional flow of viscoelastic fluid. An exact value of the critical Reynolds numberRcis found for two models of a viscoelastic material: the upper convected Maxwell fluid and the Oldroyd B or Boger fluid.

ISSN:0148-6055    

DOI:10.1122/1.550230

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

When the shear stress magnitude of a Bingham material exceeds the yield shear stress, quasi‐Newtonian flow results, otherwise the material is rigid. The Bingham model has been used in tribology to describe the behavior of greases, but may also be used for electrorheological fluids proposed as ‘‘smart’’ lubricants. For two‐dimensional flow, different modified Reynolds’ equations are obtained, depending on the possible local formation of a rigid core which may be attached to either surface, or float between the surfaces. From the modified Reynolds’ equation it is straightforward to predict bearing behavior, patching together the different core formation cases. Results are presented for two geometries: the squeeze film damper and the journal bearing.

ISSN:0148-6055    

DOI:10.1122/1.550231

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Experiments were performed in a sliding plate rheometer with a high density polyethylene to determine the conditions for the onset of slip and the relationship between slip velocity and shear stress. It was found that melt slip occurs at a critical shear stress of approximately 0.09 MPa in both steady and transient shear tests. The effect of the presence of a layer of fluorocarbon at the interface on both the slip velocity and the critical shear stress for the onset of slip, was also studied. Exponential shear was used to study the effect of shear history on slip. Both steady state and dynamic models for the slip velocity are proposed that are consistent with the experimental observations. Results of oscillatory shear experiments suggest that melt slip is a physicochemical process in which the polymer–wall interface undergoes continuous change during successive cycles.

ISSN:0148-6055    

DOI:10.1122/1.550178

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

In many multicomponent systems, a transition from a single phase of uniform composition to a multiphase state with separated regions of different composition can be induced by changes in temperature and shear. The density difference between the phase and thermal and/or shear gradients within the system results in buoyancy driven convection. These differences affect kinetics of the phase separation if the system has a sufficiently low viscosity. This investigation presents more preliminary developments of a theoretical model in order to describe effects of the buoyancy‐driven convection in phase separation kinetics. Polymer solutions were employed as model systems because of the ease with which density differences can be systematically varied and because of the importance of phase separation in the processing and properties of polymeric materials. Isopycnic polymer solutions were used to determine the viscosity and density difference limits for polymer phase separation. From these methods, it was possible to examine polymer–polymer interactions in a θ solvent. The rheological measurements were extended to detect the phase separation and to define the critical temperature.

ISSN:0148-6055    

DOI:10.1122/1.550179

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Here we present a mesoscopic theory of the low‐flow‐rate rheological properties of textured or polydomain samples of liquid crystalline polymers. In this theory, the Leslie–Ericksen equations are assumed to apply to each domain; these equations are averaged over a spatial region, large compared to a single domain, yet small compared to bulk dimensions. Along with these averaged equations, phenomenological expressions are postulated that allow us to obtain a relatively simple set of coupled equations for the domain size and the mesoscopic orientation and stress tensors. The values of the Leslie–Ericksen viscosities that appear in the equations are obtained from the Doi theory for nematic polymers. We apply the theory to several shear flows, namely recoverable shear after cessation of steady shearing, and step reversal and step increase of shear rate. In each case promising agreement is found between the predictions of the mesoscopic theory and measurements on lyotropic liquid crystalline polymers.

ISSN:0148-6055    

DOI:10.1122/1.550180

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

A theoretical analysis is developed to compute the extensional force exerted on a fully stretched macromolecule in an elongational flow of a given extensional strain rate. The long chain molecules are described by a bead‐on‐a‐string model. The dynamic problem in the external flow field is solved by the boundary collocation truncated series method. The total stretching force is then computed from the viscous stresses on the surfaces of each bead. This approach permits calculation of the magnitude of interbead shielding effects. Current practice for estimating stretching and fracture forces on macromolecules in elongational flow experiments involves application of Stokes law to each bead as though the flow around it were independent of that around the others. In the present analysis, an analytical expression for the shielding factor is developed. An approximate formula relating shielding effects to molecular weight is extracted from the results of numerical computation. Using this formula it is proved that the free‐draining Rouse model and the non‐free‐draining Zimm model represent opposite extremes with respect to shielding effects. Also, a more accurate power index in the relationship between relaxation time and molecular weight is given. A calculation is also carried out for spectrin, the protein molecule responsible for the elasticity of the red blood cell membrane, as an application example. On the basis of a chain of 40 beads separated by 50 Å and having a radius 12.5 Å, a shielding factor of 37% was calculated. Thus, even for this relatively short polymer, the neglect of shielding would overestimate the drag on its surface by a factor of 2.7.

ISSN:0148-6055    

DOI:10.1122/1.550181

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Equations are derived for analyzing the flow of inelastic, non‐Newtonian lubricants in short journal bearings. A realistic bearing geometry is used along with the power‐law fluid model to study the influence of lubricant shear thinning on the load‐bearing capacity under isothermal conditions. The results are useful for designing journal bearings to achieve desired values of the friction coefficient or the minimum oil film thickness for steady as well as time‐varying loads. These results show that the idea of using the viscosity corresponding to the shear rate in the bearing for evaluating engine oil flow characteristics is a reasonable one.

ISSN:0148-6055    

DOI:10.1122/1.550182

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The flow of a shear‐thinning viscoelastic fluid (Separan AP‐30) through a periodically constricted tube (PCT) is simulated using a mixed pseudospectral finite difference numerical method and a variety of constitutive equations. The results obtained with viscoelastic models show, at best, a small increase in the flow resistance beyond the value computed for a purely viscous inelastic fluid. The largest increase was observed with the modified Phan‐Thien–Tanner model. This model, in addition to fitting the standard viscometric data (viscosity and first normal stress difference), predicts a nonzero second normal stress difference and allows for dynamic behavior characterized by nonaffine deformation and stress overshoot in start‐up of shear flow. All the calculated values, although they show the correct tendency, are well below the experimentally measured values of Deiber and Schowalter (1981).

ISSN:0148-6055    

DOI:10.1122/1.550183

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

A nonlinear rheological model combining elastic, viscous, and yielding phenomena is developed in order to describe the rheological behavior of materials which exhibit a yield stress. A key feature of the formulation is the incorporation of a recoverable strain; it has a maximum value equal to the critical strain at which the transition from an elastic solid‐like response to a viscous shear thinning response occurs. An analysis is presented to enable determination of all the model parameters solely from dynamic measurements which are easily accessible experimentally. A rigorous correlation, analogous in form to the Cox–Merz rule, is shown to exist between the steady shear viscosity and the complex dynamic viscosity in terms of a newly defined ‘‘effective shear rate.’’ Experimental data obtained for a 70 vol % suspension of silicon particles in polyethylene indicate agreement with theoretical predictions for both the dynamic and steady shear behavior.

ISSN:0148-6055    

DOI:10.1122/1.550184

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550185

出版商:The Society of Rheology    

年代:1991

数据来源: AIP