O. Manero

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

Abstract A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.

On the shear banding flow of elongated micellar solutions

Abstract Under steady shear flow, elongated micellar solutions show shear stress saturation above a critical shear rate due to the formation of shear bands that result in non-homogeneous flow. Long transients and oscillations accompany this stress plateau. When measurements are done with a controlled stress rheometer, frequently a metastable branch is observed. At higher shear rates, a second upturn is observed above a second critical shear rate, which indicates that homogeneous flow is recovered. Here, a model consisting of the codeformational Maxwell constitutive equation coupled to a kinetic equation to account for the breaking and reformation of the micelles is presented to reproduce the features described above in steady shear flow. The model also predicts a second metastable branch and long transients at higher shear rates and the existence of an inflexion point in stress-shear rate plots above which no shear banding behavior is detected.

The influence of rheological properties on the slow flow past spheres

Abstract The present communication is intended as part of a systematic investigation of the influence of rheological properties of various fluids upon the translational motion of a sphere. Drag measurements are presented for spheres of various diameters moving longitudinally with constant translational velocity, along the central axis of cylindrical containers of different radii. Four different types of fluids (based upon measurements of shear viscosity and first normal stress difference) are considered, namely Newtonian, viscoelastic, inelastic and constant viscosity-elastic liquids. The influence of each rheological property of these fluids upon the drag force is evaluated and analyzed. In the case of negligible wall effects, it is concluded that small perturbation theories provide an adequate description of the flow field for the creeping flow regime. The drag departure for viscoelastic liquids from the purely viscous Newtonian value has a quadratic dependence on the We number. Elastic effects are of primary importance at extremely low shear rates. For higher shear rates, shear thinning effects become predominant and an accurate drag prediction based on simple shear dependent viscosity values is presented. For non-Newtonian fluids, wall proximity effects are considerably reduced from the purely viscous Newtonian value. Both elasticity and shear thinning properties of the fluid contribute to that reduction. Elastic effects on the drag may be accounted for using Caswells wall correction term derived from small perturbation theories for creeping flow and its validity exceeds the limit of small We numbers; this is substantiated by numerical techniques. Fluid elasticity diminishes wall effects but is a rapidly decreasing function, valid only for small We numbers and large sphere/container ratios. Beyond this region, shear thinning effects are predominant both upon the drag force and upon the wall correction term. These inelastic effects may be evaluated quite accurately using a modified version of Caswells correction formula, which requires only a simple knowledge of the shear thinning viscosity function.

Dynamics of worm-like micelles: the Cox-Merz rule

Abstract The viscoelastic behaviour of worm-like micelles in small-amplitude oscillatory, steady simple shear and uniaxial extensional flows are analyzed with a model that couples the Oldroyd-B constitutive equation with a kinetic equation that accounts for the structural changes induced by the flow. In some cases, the constitutive equation predicts a viscoelastic behaviour that is consistent with the Cox–Merz rule. Departures from this rule are also predicted. Experimental data obtained for two worm-like micellar systems indicate that in these solutions, the Cox–Merz rule is not usually followed, in agreement with the predictions of our model. In uniaxial extensional flow, the model predicts a strain hardening in the extensional viscosity at low extensional rates and a strain-thinning at high extensional rates.

Complex flow of viscoelastic fluids through oscillating pipes. Interesting effects and applications

Abstract In the present communication, the flow of liquids through a straight pipe which is oscillating longitudinally about a mean position is examined. The basic flow is generated by a constant pressure gradient and the effect of the superimposed oscillations upon the flow is analyzed with particular attention to the flow rate. In the viscous case as well as for an elastic fluid with constant viscosity, no variation in the flow rate is present. This is in agreement with the theoretical analysis. Nevertheless, for viscoelastic fluids, increases in the flow rate of up to twenty times are possible when compared to purely rectilinear flow. This effect is examined for various viscoelastic fluids and relations are found with that basic properties of such fluids. The complex flow situation is analyzed using flow visualization techniques. As a result, the flow appears to be dominated by a shear-thinning effect. A numerical solution using a power-law fluid predicts increases in flow rate which agree qualitatively with the experimental data but are quantitatively different. It is therefore concluded that a more general model must be used for agreement between experiments and theory. In the light of the experimental results, applications are being presently undertaken for the flow of polymer melts in situations of industrial interest.

A study of velocity discontinuity for single air bubbles rising in an associative polymer

The motion of air bubbles in aqueous solutions of a hydrophobic alkali-swellable associative polymer is studied in this work. The associative nature of these polymer systems dictates their rheological properties: for moderate values of the shear rate, the formation of structure can lead to a shear-thickening behavior and to the appearance of first normal stress difference. For larger shear rates, the polymer associations can be broken, leading to shear thinning. In general, these fluids show a Newtonian behavior for small values of the shear rate, but behave as viscoelastic liquids for large shear rates. Experimental results show the appearance of a critical bubble volume at which a discontinuity in the relation velocity-volume occurs; however, the velocity increase found in this case is not as large as that previously reported for the case of shear-thinning viscoelastic fluids. The discontinuity is associated with a significant change of the bubble shape: before the critical volume, the bubbles are conve...

Modeling Wax Deposition in Pipelines

Abstract A multicomponent liquid-wax hydrodynamic model that incorporates phase equilibria and a full non-Newtonian behavior is proposed. In this model, molecular diffusion through the boundary layer induced by a temperature gradient between the liquid and the exterior pipe wall is assumed to be the dominant mechanism for deposition. Numerical solutions to the conservation equations for Newtonian and non-Newtonian flow regimes in a model pipe are presented, and results on calculated radial mass flux and wax deposition profiles as a function of time and position in a vertical pipeline are discussed in detail. The results are compared with predictions from a previous model developed by Svendsen (Svendsen, J. A. (1993). Mathematical modeling of wax deposition in oil pipeline systems. AIChE J. 39(8):1377–1388.) and with experimental flow data for a binary mixture reported by Cordoba and Schall (Cordoba A. J., Schall C. A. Application of a heat method to determine wax deposition in a hydrocarbon binary mixture. Fuel 2001 80:1285–1291). Good agreement in both cases is found.

Hierarchically Nanostructured Barium Sulfate Fibers

BaSO(4) nanostructures with controlled morphologies were successfully produced via one-step process through precipitation of BaSO(4) in aqueous and organic media. The synthesis is carried out by mixing solutions of BaCl(2) and Na(2)SO(4) in presence of EDTA (disodium ethylenediaminetetraacetic acid) at room temperature. The influence of the reaction conditions such as initial reactants concentration, pH, EDTA/[Ba(2+)] ratio and aging on the BaSO(4) nanoparticles organization is studied. Using EDTA in aqueous media, spherical secondary particles of 500 nm diameter are obtained, which are formed by 4 nm size primary particles. With dimethyl sulfoxide and small amounts of water (5%) and EDTA, the aging process allows the production of long homogeneous fibers, related to hierarchical organization of BaSO(4) nanoparticles. Direct observation of self-assembling of primary particles by HRTEM allows proposing a mechanism for fiber formation, which is based on multipolar attractions that lead to a brick-by-brick organization along a preferential orientation. Results evidence the role of EDTA as controlling agent of the morphology and primary and secondary mean particle size.

Inlet instabilities in the capillary flow of polyethylene melts

Inlet instabilities in the capillary flow of polyethylene melts were studied in this work. Extrudate distortions in branched polyethylenes, produced by unstable upstream flow, were found to be accompanied by pressure oscillations that do not have their origin in the slip phenomenon, but on polymer compressibility. The absence of slip was clearly evidenced in the experiments, and the differences between pressure oscillations occurring in linear and branched polymers are shown.Pressure oscillations in the capillary flow of branched polyethylenes were found to be made up of at least two components of different frequency and amplitude. These two components were identified with different bulk defects appearing in the extrudates. Information about the dynamics of vortices upstream of the contraction and extrudate distortions is obtained from the analysis of pressure oscillations.The influence of capillary entrance angle on flow curves was also investigated. From the results, it is concluded that the extensional component of the flow in the contraction is the main factor responsible for the slope change usually found in the log-log flow curves of both linear and branched polyethylenes.

Rheology of the Pluronic P103/water system in a semidilute regime: Evidence of nonequilibrium critical behavior

The linear and nonlinear rheological behaviors of semidilute aqueous solutions of the amphiphile triblock polymer Pluronics P103 in water are reported here. For C(surf) < or = 20 wt%, micelles are spherical at temperatures lower than ca. 27 degrees C and grow with increasing temperature to form long polymer-like micelles. These polymer-like micelles exhibit strong viscoelasticity and a shear-banding region that shrinks as the cloud point is approached. Master time-temperature-concentration curves were obtained for the dynamic moduli using traditional shifting factors. In the nonlinear regime, P103 polymer-like micellar solutions follow the master dynamic phase diagram proposed by Berret and colleagues, in which the flow curves overlap in the low-shear-rate homogeneous flow region. Within the nonhomogeneous flow region (confirmed by flow birefringence and small-angle light-scattering measurements), oscillations and overshoots are detected at the inception of shear flow, and two main relaxation mechanisms are apparent after cessation of steady shear flow. Evidence for nonequilibrium critical behavior is presented, in which the order parameter is the difference of critical shear rates that limit the span of the plateau stress. Most of the steady-state and transient features of the nonlinear rheology of the P103 polymer-like micelles are reproduced with the Bautista-Manero-Puig (BMP) model, including the predictions of nonequilibrium critical behavior under flow.