摘要:

The recoil of anisotropic hydroxypropylcellulose solutions was studied by optical microscopy and light scattering. Different stresses were applied before stopping the flow and observing the recoil. At low stress levels, the scaling of the recoverable strain versus scaled relaxation time applies. A change of texture is noticeable only after most of the strain has been recovered. At a high stress level, a band texture appears during recoil. The appearance of this band texture stops the recovery process until the band texture disappears, and the recovery process then continues. The presence of a band texture during recoil is shown to promote a larger recoil.

ISSN:0148-6055    

DOI:10.1122/1.550949

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Continous flow and oscillatory measurement results of a series of highly pigmented inks, containing a bimodal distribution of solid particles and dissolved polymer in the ink vehicle, are presented. Viscosity versus shear rate data is adjusted to a “Bingham Exponential Decrease” model, showing that the viscosity at high shear rates decreases with temperature following an Arrhenius-like equation. However, yield stress, viscosity taken at a shear stress of 250 Pa, and thixotropy, increase with temperature displaying an unusual behavior. It is assumed that a reinforcement of the cohesive interactions between components takes place on heating. Time, temperature, and frequency effects on dynamic viscoelastic functions are analyzed, confirming the existence of flocculation due to interparticle attractions. The results are characterized by the following features: (i) Contrary to what is usually observed in liquids, both storage and loss moduli increase with temperature. (ii) Both viscoelastic functions increase in a convex way with time, following an adapted Cheng–Evans model. (iii) The increase of viscoelastic functions is not altered by periods of resting time. The experimental results indicate that at rest the dispersions flocculate to a greater extent at higher temperatures (typically at 40 and 60 °C) than at 20 °C. To explain these results we assume that heteroflocculation, through bridging of small particles, is promoted by phase separation of polymer solution forming the ink vehicle, which takes place at 36 °C.

ISSN:0148-6055    

DOI:10.1122/1.550948

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

We describe a shear device designed to allow the visualization of suspensions or emulsions under an optical microscope. The optical axis of the microscope is perpendicular to the plane of shear. The detailed flow profile is determined by numerical simulation with two different aspect ratios of the flow cell. We use this shear device to investigate the structure of dilute suspensions of colloidal magnetic particles submitted to a magnetic field and to a flow.

ISSN:0148-6055    

DOI:10.1122/1.550972

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

While the molecular weight distribution (MWD) model of Bersted and Slee, and later Malkin and Teishev, has been widely used in viscosity to MWD transformation, a fundamental assumption in the model—the empirical nonlinear mixing rule for homologous blendsη1/α=∑wiηi1/α—has not been examined as a function of deformation rate and MWD. In this study of the nonlinear mixing rule, we evaluated the influence of deformation rate and MWD, both experimentally and theoretically, by using real polymers. Direct experimental results indicate that the nonlinear mixing rule exponent can be a function of deformation rate. According to the theory-based quadratic mixing rule, the exponent α depends on both the deformation rate and the MWD of blends, suggesting that the nonlinear mixing rule is fundamentally invalid. However, for moderately wide distributions(Mw/Mn<2),the nonlinear mixing rule with a constant α of 3.4 for all shear rates and MWDs, is almost equivalent to the quadratic mixing rule. Thus the simplified mixing-rule assumption of Bersted and Slee should be valid for many practical applications.

ISSN:0148-6055    

DOI:10.1122/1.550953

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Nonequilibrium Brownian dynamics simulations (NEBD) are used to model the dynamics of semiflexible macromolecules undergoing shear flow. The mathematical model utilizes a discretized version of the Kratky–Porod wormlike (or persistent) chain as the building block, generalized to include flow. The Fokker–Planck equation resulting from such an analysis is converted to a stochastic differential equation from which the simulation algorithm for the NEBD is obtained. Various conformational and rheological quantities are monitored, under both steady-state and transient conditions, with the primary independent variable being theflexibility parameterβ, the bending constant of the chain. The model qualitatively describes many of the experimentally observed effects in such systems, most notably birefringence overshoots, cessation effects, and various steady-state effects. In addition, many of the qualitative rheological features of both rigid rod (or flexible) polymers are captured as β is made very large (or small). The advantage of the NEBD over an analytical treatment is its ability to incorporate (analytically intractable) effects such as hydrodynamic interactions and its (natural) ability to obtaintransientinformation, a facet useful in comprehending the differing dynamics of rigid and less rigid macromolecules.

ISSN:0148-6055    

DOI:10.1122/1.550937

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Suspensions comprised of neutrally buoyant spheres in Newtonian fluids undergoing creeping flow in the annular region between two rotating, coaxial cylinders (a wide-gap Couette) display a bulk migration of particles towards regions of lower shear rate. A series of experiments are performed to characterize this particle migration, including the influence of particle size, surface roughness, and volume fraction. Little, if any, effect of particle surface roughness is observed. An existing continuum diffusive-flux model [Phillips et al. (1992)] for predicting particle concentration profiles in monomodal suspensions is evaluated using the current series of experimental data. This model predicts a dependence of the migration rate on the square of the suspended particles’ radius,a2; whereas the present experiments indicate that systems with average particle volume fractions of 50% display a rate that scales witha3. Previous use of the diffusive-flux model has assumed constant values for diffusion coefficients which serve as tuning parameters in the phenomenological equation. Here the experimental data are used to investigate variations of the model in which the diffusion coefficients depend upon either the local or global particle volume fraction. For initially uniform suspensions, the coefficients are found to be best modeled as functions of the local particle volume fraction.

ISSN:0148-6055    

DOI:10.1122/1.550954

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Existing nonisothermal thin-filament melt spinning models treat temperature as a one-dimensional (1D) quantity, varying only with axial distance. Such models accurately simulate processes in which the melt has high thermal conductivity relative to the surface heat loss (i.e., small Biot number), or the ambient air temperature is near the melt temperature. However, in industrial melt spinning processes, where neither of these characteristics is true, a leading-order transverse temperature variation exists, no matter how slender the filament. Alternative models implement a hybrid 1D mass and momentum, 2D energy computation. Here we extend existing 1D models to include radial as well as axial resolution of temperature, thereby also providing a 2D resolution of stress, while retaining the computational advantages of 1D models. The model’s predictions depend on the thermal conductivity of the melt, a property absent from existing 1D models. In addition to demonstrating the 2D resolution afforded by the new model, we also demonstrate significant differences between the new model and existing 1D models in predictions of fundamentally 1D quantities such as average axial stress, the quantity from which performance properties of the spun fiber are typically inferred.

ISSN:0148-6055    

DOI:10.1122/1.550899

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Rheological measurements on concentrated colloidal dispersions at high frequencies probe interactions at small separations and provide the most direct link between the shear modulus and the pair potential. For colloids stabilized with a grafted polymer layer, others have extracted the pair potential from measurements of the high frequency shear modulus via a molecular theory, which does not account for hydrodynamic interactions. Here we apply a nonequilibrium theory that includes the repulsion between polymer layers and its effect on the equilibrium structure, while accounting for flow of the suspending fluid within the layer in evaluation of the hydrodynamics. Predictions with a simple approximation for the hydrodynamic interactions, which interpolates between the lubrication and far field limits, are compared with the molecular theory. We also demonstrate qualitative agreement with experimental data and assess over what range of effective volume fractions neglect of hydrodynamic interactions permits data to be inverted to obtain an interaction potential.

ISSN:0148-6055    

DOI:10.1122/1.550940

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

In situx-ray scattering techniques are used to measure molecular orientation and investigate the coexistence of hexagonal and nematic phases in highly concentrated PBG/cresol solutions under shear flow. Beyond a threshold concentration, the diffuse nematic lobe in x-ray scattering patterns is accompanied by sharp reflections indicating lateral packing of molecules in a hexagonal arrangement. The threshold concentration for formation of this hexagonal phase depends on molecular weight, but occurs near 35 wt % PBG and corresponds directly to the onset of region I shear thinning. Applied shear acts to reduce the amount of hexagonal phase and ultimately, at high enough shear rates, return the solution to a fully nematic state. Relaxation experiments at high shear rates indicate that the final state of the solution is highly sensitive to the continued presence of the hexagonal phase during shear. Specifically, a much higher final orientation results if the hexagonal phase—even a very small amount—persists during the preceding shear. We postulate a nucleation mechanism to explain this behavior, in which regrowth of the hexagonal phase during relaxation is promoted in such a way that molecular orientation is favored in a direction parallel to the residual hexagonal phase in the sample at the onset of relaxation. This results in increased molecular orientation in both the hexagonal and nematic phases. Finally, we show examples of erratic relaxation behavior observed in PBG/cresol at concentrations immediately below the threshold of hexagonal phase formation. The degree of this erratic behavior appears to depend on the amount of strain applied before flow cessation.

ISSN:0148-6055    

DOI:10.1122/1.550956

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The three-dimensional deformation of an isolated drop in an immiscible liquid phase undergoing simple shear flow was investigated by using a parallel-plate apparatus. The drop was observed by video-enhanced contrast optical microscopy, either along the vorticity direction or along the velocity gradient direction of the shear flow. An experimental methodology based on image analysis was especially developed to study in a quantitative way the three-dimensional shape of the deformed drop, both under steady-state flow and during transients. Up to moderate deformations, the steady-state drop shape was well described within experimental error by an ellipsoid having three different axes. The deviation of drop shape from an ellipsoid at higher deformations was also characterized in a quantitative way. Good agreement was found between the experimental results of this work and numerical simulations reported in the literature.

ISSN:0148-6055    

DOI:10.1122/1.550942

出版商:The Society of Rheology    

年代:1998

数据来源: AIP