摘要:

The computational study of the rheology of two‐dimensional random cellular systems such as foam or concentrated emulsion systems is presented. The numerical simulations are performed under homogeneous shear using a computationally efficient model in which effects of viscous dissipation inside the liquid films are taken into consideration. It is shown that the system possesses a finite yield stress and behaves like Bingham plastics. The role of the topological change of the cellular structure is also examined.

ISSN:0148-6055    

DOI:10.1122/1.550383

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

An equation of the kinetic of the formation of reversible associations of macromolecules in flow is proposed. The results are compared with experimental data which were derived from viscosity measurements displaying a shear‐thickening behavior of dilute polymer solutions.

ISSN:0148-6055    

DOI:10.1122/1.550384

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

In dispersive mixing of immiscible liquids the minimum attainable dropsize is often deduced from the critical value of the Capillary number (the ratio of the shear stress to the interfacial stress) necessary for drop breakup under quasiequilibrium conditions. The critical Capillary number shows a minimum if the viscosity ratio between dispersed and continuous phase is about one. Hence, it is commonly accepted that the finest morphology is obtained if both viscosities match. In practical mixing devices, however, small drops are formed by a transient mechanism of thread breakup during extension rather than by stepwise breakup under equilibrium conditions. For Newtonian liquids, a comparison is made between the dropsizes resulting from a stepwise equilibrium and a transient breakup mechanism. Generally, the transient mechanism yields smaller drops and, more interestingly, a higher viscosity ratio between the dispersed and continuous phases results in a finer morphology, as already indicated by Tjahjadi and Ottino (1991). In the present paper the comparison is elaborated over a broad range of the relevant parameters while a compact illustrative presentation of the results is given to stress the possible consequences for practical blend morphologies.

ISSN:0148-6055    

DOI:10.1122/1.550385

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

A new rheometer was developed which can measure melt viscosity at constant strain rate elongation with superimposed sinusoidal oscillations (in short, superimposed elongations). The superimposed elongations were controlled by a pulse motor and a personal computer which drove rotating clamps in the rheometer. High density polyethylene (HDPE) with a bimodal molecular weight distribution and low density polyethylene (LDPE) melts were studied to assess structural changes in elongation. Linearity of stress response to the superimposed strain rate was confirmed below elongational strains of 2.8 for LDPE melt, and 2.0 for HDPE melt, when the constant strain rate was set to 0.08 s−1and the amplitude of the superimposed strain rate was set to below 0.02 s−1. Dynamic elongational moduli (E’,E‘) of HDPE and LDPE melts were calculated from stress response at the superimposed elongations by analogy to dynamic shear deformations. Strain dependence of dynamic elongational moduli of HDPE melt and LDPE melt showed steep increases at the inception of the nonlinear viscoelastic region. The dynamic elongational moduli were estimated in the linear viscoelastic region and they were found to be larger than the corresponding dynamic shear moduli.

ISSN:0148-6055    

DOI:10.1122/1.550386

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

It is demonstrated that weakly flocculated, concentrated colloidal dispersions show slip flow when sheared between smooth concentric cylinders. The precise pattern of behavior seen depends upon the stress and upon how long the flow is left to establish prior to measurement. With delay times of order hours, slip is not seen until a critical stress is exceeded (typically about 1 Pa) and, thus, the true low‐shear viscosity can be determined provided care is taken to ensure the stress does not exceed the critical level. With short delay times of order minutes, slip is seen irrespective of how small the stress is and the low‐shear viscosity can be underestimated by several orders of magnitude. Comparisons of flow curves obtained using smooth and roughened cylinders show that slip only occurs at the inner cylinder, and also that bulk flow is re‐established at higher stresses where the dispersions start to shear thin. The apparent low‐shear, relative viscosity measured in the presence of slip appears, to a first approximation, to depend only upon the concentration of particles, and not on particle size, medium viscosity, or the strength of the attractive forces causing the flocculation. In consequence the slip coefficient appears to depend primarily on particle concentration. In contrast, the true low‐shear, relative viscosity (RLSV) is found to increase exponentially with the interaction strength. For example, an attractive well in the interparticle potential of order 10kTgives rise to RLSV of order 106in the concentrated, submicron dispersions studied here.

ISSN:0148-6055    

DOI:10.1122/1.550387

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

Uniaxial elongation and compression data of linear and branched polymer melts as well as of crosslinked rubbers are analyzed with respect to their nonlinear strain measure. This is found to be the result of two contributions: (1) affine orientation of network strands, and (2) isotropic strand extension. Network strand extension is caused by an increasing restriction of lateral movement of polymer chains due to deformation, and is modeled by a molecular stress functionf, which in the tube concept of Doi and Edwards is the inverse of the relative tube diameter. Up to moderate strains,f2is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant volume of the tube. At large strains, melts show maximum molecular tension depending on the degree of long‐chain branching, while rubbers show maximum extensibility. The influence of crosslink density on the strain measure is considered.

ISSN:0148-6055    

DOI:10.1122/1.550388

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The linear viscoelastic behavior in the melt of PMMA/PS blends and various blends of PMMA containing rubbery latex particles has been characterized by dynamic shear rheometry. For the rubber‐toughened PMMA samples, the influence of rubber content, structure of the latex particles, and aggregation of the particles in the PMMA matrix have been investigated. Morphologies of the dispersed type or of the aggregated type were produced by performing the blending in the melt or in solution. The data forG’andG‘have been analyzed in the frame of a rheological emulsion model which is found to account for the behavior of the PMMA/PS blends and of rubber‐toughened PMMA at low rubber content. At high rubber concentrations the model does not predict the secondary plateau inG’which arises at low frequencies for these systems. Therefore, this plateau cannot be attributed to the deformability of dispersed inclusions as in PMMA/PS blends, but is shown to depend on the extent of aggregation of the dispersed particles, and to be most important in a well dispersed morphology where the particles form a network‐type structure.  

ISSN:0148-6055    

DOI:10.1122/1.550389

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The viscosity of semidilute bimodal suspensions of sterically stabilized silica particles was measured as a function of the total volume fraction and the size ratio of the dispersed particles, and of the composition of the mixtures. The unimodal components of which the bimodal suspensions were composed showed hard sphere behavior. In the mixtures, a minimum in the viscosity was observed at small size ratios (al/as<2) when plotted against the composition. For larger size ratios a maximum was found. The origin of the viscosity maximum is uncertain but might be explained by depletion forces.

ISSN:0148-6055    

DOI:10.1122/1.550390

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

The properties of aqueous solutions of model HEUR associative thickeners under dynamic and steady shear have been studied as a function of concentration, molecular weight, temperature, and hydrophobic end‐cap length. It is shown that solutions of AT behave as near perfect Maxwell fluids inasmuch that Cole–Cole plots of the dynamic moduli are almost exactly semi‐circular. An Arrhenius law temperature dependence of the static viscosity and relaxation time is also observed, providing confirmation of a single relaxation process. In certain other respects, AT solutions show more complex behavior, e.g., the Cox–Merz rule is not obeyed, with the steady shear viscosity showing a weaker dependence on shear rate than does the complex viscosity upon frequency. Furthermore, weak shear thickening is seen to precede shear thinning in steady shear. The above results are consistent with the predictions of a transient network theory presented recently by Tanaka and Edwards and Jenkins (generalized Green–Tobolsky theory). This does not however explain the strong effect of concentration on the various rheological coefficients. For example, the theory predicts a linear dependence of high‐frequency modulus and static viscosity on concentration, whereas in practice they are found to be more like quadratic and cubic, respectively, at low concentrations.In previous publications this strong dependence has been taken to mean that the network chains are entangled to the point where reptation dynamics determines the time scale of relaxation. This supposition has been tested by mixing solutions of AT with different relaxation times (achieved by means of different end‐cap lengths), on the basis that the mixed solutions should show an intermediate relaxation time if reptation is important. In practice, mixtures of two and three AT were found to show two or three sharp relaxation times, implying that the chains relax independently. It is shown that the true explanation of the strong concentration dependencies is connected with a different kind of change of network topology with concentration. An elementary statistical‐mechanical model, supported by Monte Carlo simulation, is used to argue for a gradual transition from, at low concentrations, micelles built predominantly from looped chains to, at high concentrations, a fully developed network comprising micelles linked by bridging chains. When the transient network theory is modified so as to take the presence of loops into account, it produces results in semiquantitative agreement with experiment.

ISSN:0148-6055    

DOI:10.1122/1.550391

出版商:The Society of Rheology    

年代:1993

数据来源: AIP

摘要:

We analyze the existing models of granular flow where the particle’s phase stress tensor is regarded as dominated by translational‐collisional stresses. We extend the commonly used concept of a granular phase pseudo‐temperature to develop a pseudo‐thermodynamic theory of such systems. This is accomplished by introducing the concept of pseudo‐entropy, a quantity which can actually be calculated from the existing theories. The systems considered are shown to exhibit entropic elasticity. The consistency of constitutive equations available in the literature is tested against the pseudo‐thermodynamic theory; the test is mostly successful, except for one term sometimes used in the constitutive equation for the flux of pseudo‐energy, which, according to our theory, should be identically zero.

ISSN:0148-6055    

DOI:10.1122/1.550392

出版商:The Society of Rheology    

年代:1993

数据来源: AIP