摘要:

A novel three-dimensional flow birefringence technique has been developed to measure the real time, three-dimensional, optical anisotropy in flowing polymer solutions. Full tensor optical rheometry utilizes three polarization-modulated beams to probe all independent elements of the refractive index tensor and is, thus, used as a quantitative probe of time-dependent, three-dimensional average molecular orientation. We present results for shear flow of polystyrene in a 6% semidilute solution of tricresyl phosphate over a range of shear rates. Although the time-dependent behavior ofN2has not been widely reported, probably because of experimental difficulties, it can be a powerful means of testing rheological models. Experimental measurements of the transient behavior ofN2and −N2/N1on inception and cessation of simple shear flow are compared with the predictions of the Doi–Edwards and Giesekus models. The Giesekus model is in qualitative agreement with most of the time-dependent features observed forN2and −N2/N1. On-flow inception overshoots inN2and −N2/N1are observed. Further, on-flow cessation,N2relaxes more slowly thanN1and −N2/N1shows a strong nonexponential increase to a constant value of ∼0.9 at long times. The Doi–Edwards model is less successful in predicting transient results but agrees quite well with observed steady-state dependence ofN2with shear rate.

ISSN:0148-6055    

DOI:10.1122/1.550806

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Micromechanical models for the instantaneous elastic stress tensorτof “film fluids,” i.e., fluids with internal interfaces such as foams, emulsions, and immiscible blends, yield formulas that are similar to each other and are well approximated simply byτ∝C1/2, whereCis the Cauchy strain tensor. This expression can be derived as an approximation by assuming that the interfaces deform affinely and that the tension in the interface is independent of the deformation. The expressionτ∝C1/2is, therefore, the analog of the classical expressionτ∝C−1for the instantaneous elastic stress in polymeric “line fluids.” The second normal stress difference of a “film fluid” is predicted to be negative and of comparable magnitude to the first normal stress difference.

ISSN:0148-6055    

DOI:10.1122/1.550857

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

The flow properties of aqueous suspensions of silica particles with diameters ranging from 117 to 780 nm were measured as a function of volume fraction, ionic strength, and continuous phase composition. Dense, aqueous suspensions of all sizes displayed similar behavior with a yield stress in the limit of low shear rate, shear thinning as the stress was raised and, above a critical volume fraction, shear thickening. Master flow curves which are weakly dependent on particle size, volume fraction, surface potential, magnitude of the decay length for the electrostatic forces, or particle size distribution are produced when stress is scaled on the suspension’s elastic modulus,G, and shear rate onG/ηc.Hereηcis the continuous phase viscosity. For suspensions which ordered at rest and of sufficiently high volume faction, shear thickening occurred as a discontinuous decrease in shear rate as the stress increased. This behavior was not observed in suspensions altered so that no order occurred at rest. When the continuous phase viscosity was increased by suspending the particles in a glycerin solution, no evidence for a yield stress or ordering were observed. These suspensions thinned with increasing stress to a minimum viscosity where thickening occurred as a smooth transition. The elastic scaling that successfully produced master curves for the aqueous suspension failed for the glycerin suspensions.

ISSN:0148-6055    

DOI:10.1122/1.550876

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

In the literature there exist many theories that provide predictions for the rheological properties of concentrated colloidal suspensions, some in excellent agreement with existing experimental data. However, the manner in which hydrodynamic interactions are included differs greatly among the various approaches. Here we incorporate hydrodynamic interactions into the formalism developed previously to account for many body thermodynamic interactions in concentrated suspensions. A conservation equation involving two particle conditional averages of the hydrodynamic functions is derived and solved with different approximations for the functions, including the dilute limit, the rescaling due to Brady, and a lubrication approximation from our previous work. Through our previous calculations without hydrodynamic interactions we have a well characterized approximation for the thermodynamic couplings in concentrated suspensions. The addition of approximations for the hydrodynamic interactions allows prediction for hard sphere suspensions of the low shear viscosity, linear viscoelasticity, long-time self-diffusion coefficient, dichroism, and the nonequilibrium structure for comparison with extensive experimental data. We demonstrate that the form of the nonequilibrium structure is relatively insensitive to the model for the hydrodynamic interactions, while its magnitude is increased by the overall slowing down of relaxations. The calculation of the stress is dominated by the structure near contact at volume fractions approaching random close packing while at intermediate volume fractions it is more sensitive to the model for the interaction functions over the entire range of separations. Comparison of the predictions with a wide range of experimental measurements and simulation results suggests that existing hydrodynamic approximations are of the correct magnitude, while existing thermodynamic approximations are correct in form but underestimate the magnitude of the nonequilibrium structure.

ISSN:0148-6055    

DOI:10.1122/1.550873

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration,c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependence of the zero shear viscosity, withη0∼c8. Evidence for shear-induced structuring in steady shear, large amplitude oscillatory shear, and parallel superposed steady and dynamic shear is presented. Such structuring is more pronounced at lower polymer concentrations, consistent with the formation of intermolecular associations at the expense of intramolecular. In contrast to the simple linear telechelic associative polymers considered in a number of previous studies, the network dynamics of the polymer are no longer represented by a single characteristic time. This deviation from a classical Maxwellian response in oscillatory shear is interpreted as a broadening of the relaxation spectrum, arising from the coexistence of both hydrophobic associations and topological entanglements. Mechanistically, stress relaxation is better envisaged in terms of “hindered reptation” [Liebler et al. (1991)] of the chains, rather than Rouse-like behavior moderated purely by the hydrophobe disengagement rate [Annable et al. (1993)].

ISSN:0148-6055    

DOI:10.1122/1.550807

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

摘要:

Starting from a nonlocal description of the stress in a slender, rigid fiber suspension [Schiek and Shaqfeh (1995)], we calculate the dynamic properties of a free-fiber suspension under oscillatory shear in a very narrow gap. For a fiber suspension, three field quantities including the fluid velocity, the fiber concentration, and the fiber configuration strongly affect the suspension’s behavior. When the width of the gap confining the suspension is of the same scale as a suspended fiber’s length, then all three of the important field quantities change rapidly on that scale. The nonlocal stress equation is coupled to the momentum conservation equation for the fluid velocity and a Fokker–Plank equation for the fiber’s probability density function resulting in a closed set of nonlinear, integrodifferential equations. These equations were solved in the limit of a small Péclet number, where Brownian motion dominates, for arbitrary gap widths and oscillation frequencies. From the calculated stress fields, we obtained the real and imaginary viscosities that one would measure in flow experiments. The dependence of all four dynamic properties on gap width was investigated and we find that below a critical gap width (equivalent to one full fiber length) all dynamic properties undergo dramatic changes. Additionally, as the gap width shrinks, the relaxation time of the suspension was found to decrease, approaching the relaxation time of the pure Newtonian solvent in the limit of zero gap width. Scalings for the relaxation time as a function of small gap width are also presented.

ISSN:0148-6055    

DOI:10.1122/1.550808

出版商:The Society of Rheology    

年代:1997

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550810

出版商:The Society of Rheology    

年代:1997

数据来源: AIP