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We present results from a comprehensive investigation of the rheological responses of the gelled waxy crude oils during the yielding transition process by both stress-controlled and strain-controlled tests. If the deformation is within the linear viscoelastic region, G ′ increases slowly with increasing frequency following a power-law relationship. 80% of the total deformation accumulated during creeping is recoverable, and this recoverability remains largely independent of both imposed stress and testing time. The waxy gel relaxes the restored stress slowly with a time scale of ∼100 s. If the induced deformation exceeds the linear viscoelastic region, the elasticity of waxy structure starts to gradually decay with increasing creep stress and creep time, and the plastic contribution has come into play in the nonlinear region. A rapid reduction in the obtained time scale is observed with increasing applied strain. After the structure fractures, both G ′ and G ″ decrease with increasing strain following a power-law relationship. The ratio of the large amplitude shear-thinning exponents of G ′ and G ″ is around 2, indicating a more rapid decay of the elastic property than the viscous property. A brittleness index I is defined to quantitatively describe the brittleness of the waxy gel. The value of I for the waxy gel formed at lower temperatures is smaller, indicating that the waxy gel tends to have a more brittle fracture when yielding. The values of linear storage modulus G ′ ( γ 0 → 0 ) , critical linear strain γ 1 , critical linear stress σ 1 , and yield strain γ y obtained by different rheological testing methods are found to be well consistent. The yielding transition process is found to be strain determined rather than stress determined, and thereby γ 1 and γ y rather than yield stress are suggested to indicate the yielding transition. [ABSTRACT FROM AUTHOR]

A framework for determining the first nonlinear relaxation modulus of a viscoelastic fluid from a medium-amplitude oscillatory shear (MAOS) deformation is constructed. Knowledge of this "MAOS relaxation modulus" allows one to predict the weakly nonlinear stress response of a material under an arbitrary transient deformation via a memory integral expansion. Our framework is demonstrated by explicitly determining the MAOS relaxation modulus for a dilute suspension of Brownian spheroids subject to a dual-frequency oscillatory shear flow. Specifically, we first calculate the second normal stress difference for such a deformation from a corotational memory integral expansion. Second, the microstructural stress response of the model system of Brownian spheroids is determined via a regular perturbation expansion of the orientation distribution function at small dimensionless strain-rate amplitude, or Weissenberg number. An analytical expression for the MAOS relaxation modulus is resolved by comparing the second normal stress difference results of the memory integral expansion and microstructural stress calculation. Finally, using the MAOS relaxation modulus, we reconstruct the stress response of the model system for the start-up and cessation of simple shear and uniaxial extension. In summary, our work offers an approach to utilizing medium (and large) amplitude oscillatory shear results to predict stress dynamics of viscoelastic fluids in other transient, nonlinear flows. [ABSTRACT FROM AUTHOR]

Compression-assisted magnetorheological (MR) fluid strengthening at the nonconfined conditions was studied with the Anton Paar Physica MCR300 magnetorheometer adapted for measurements of the storage modulus of magnetized MR fluid after its exposure to compressive strains as well as for measurements of the shear stress during build-up of compression at different mutual field/compression orientations. It was found that the compression significantly increases the linear viscoelastic region, suggesting an essential enhancement in the elastic properties of the tested material. An elevated elasticity of MR fluid compressed normal to the field is observed. Build-up of compression of magnetized MR fluid results in a dramatic increase in the shear stress and occurs in two phases. It is suggested that in the first phase, at the low compressive strains, the increase in the stress is associated with the elastic deformation of the initial structure formed by a magnetic field. Subsequent increase in the shear stress in the second phase is attributed to the compression-assisted MR fluid strengthening, which takes place when the compressive strain increases beyond the elastic region and the initial chain structure gradually transforms into a structure of a thick and strong columns. Compression normal to the field results in significantly higher shear stress increment than imposed along it. [ABSTRACT FROM AUTHOR]

Planar contraction flow of a nematic solution of poly-paraphenylene terephthalamide (PPTA) in sulphuric acid (H2SO4) was studied experimentally and compared with the flow of an isotropic model fluid, a poly-ethyleneglycol-poly-ethyleneoxide (PEG-PEO) Boger fluid, in the same geometry. The dimensions and deformation rates in our experiments are relevant for industrial polymer extrusion processes, such as fiber spinning. Under these conditions, viscoelastic lip vortices arise at the re-entrant corners in the nematic flow. The experimental results show that the vortex growth behavior in the nematic PPTA solution closely resembles that in isotropic viscoelastic fluids, such as the PEG-PEO Boger fluid that was used as a reference. The velocity field in the nematic flow is characteristic of Hele-Shaw flow of a shear-thinning power-law fluid, although locally, the influence of the nematic defect structure was observed, especially for the first minutes after starting the flow. Furthermore, the lateral movement of the extruded jet, downstream of the contraction, was shown to be coupled with velocity fluctuations in the upstream flow. Our experiments give detailed insight in the complex flow behavior of a nematic polymer solution under flow conditions that are relevant to practical applications. The results can be applied in the optimization of nematic polymer extrusion processes. [ABSTRACT FROM AUTHOR]

The rheology of a semidilute dispersion of colloids having short-ranged attractions is studied. A depletion potential is chosen as a model for the attractive interaction and arises from nonadsorbing polymer dispersed with the colloids. The complex viscosity of these materials can be calculated by investigating their response to weak oscillatory shear. A first order expansion in small rates of deformation is used to solve for the microstructure and stress in the dispersion. Additionally, the effect of hydrodynamic interactions is studied via the excluded annulus model, which views the radius at which hard-sphere interactions occur as a barrier that resides beyond the hydrodynamic radius of the particles. This treatment allows a continuous variation of the hydrodynamic interaction strength. The viscoelastic response exhibits a sharp transition when going from weak attraction to strong attraction. Below a critical strength, increasing the interparticle attraction reduces the low frequency viscosity. Strong attractions increase the viscous response and delay the onset of an elastic plateau at high frequencies. An asymptotic analysis shows that the stress response is a result of the interplay of two length scales: The range of attraction and the diffusive boundary layer around particles. Independent of the strength of hydrodynamic interactions, the complex viscosity can be described with high accuracy by two well characterized viscoelastic models: At low frequencies, a Maxwell mode; at high frequencies, the purely repulsive hard-sphere response. Our results demonstrate that beyond hydrodynamic interactions the strength and range of the interaction potential between particles plays a central role in setting the viscoelasticity. [ABSTRACT FROM AUTHOR]

A stochastic constitutive model has been developed that explicitly acknowledges the nanometer size dynamic heterogeneity of glassy materials, where the distribution of the viscoelastic relaxation times emerges naturally as a result of the dynamic heterogeneity. A set of stochastic differential equations for local stresses and entropy describing behavior of a mesoscopic domain are developed, and the observed macroscopic response of the material is obtained as an average of an ensemble of domains. The stochastic constitutive model naturally predicts and provides a mechanism for the postyield stress softening and its dependence on physical aging that is observed during constant strain rate uniaxial deformations. [ABSTRACT FROM AUTHOR]

The discrete slip-link model (DSM) was developed to describe the dynamics of flexible polymer melts. The model is able to predict linear viscoelasticity of monodisperse linear, polydisperse linear, and branched systems. The model also shows good agreement with dielectric relaxation experiments, except for the single data set available for bidisperse linear systems with a small volume fraction of long chains. In this work, both shear and elongational flow predictions obtained using the DSM without parameter adjustment are shown. Model predictions for shear flow agree very well with experimental results. The DSM is able to capture the transient response as well as the steady-state viscosity. However, for elongational flow, agreement is unsatisfactory at large strains. The DSM captures the onset of strain hardening, but after a Hencky strain between 2 and 3, it predicts transient strain softening, whereas experiments show only monotonic growth. We explore a number of assumptions and approximations of the model and their effect on flow predictions. The approximations are related to the neglect of these phenomena, which are expected to be more sensitive in elongational flow: finite extensibility, convective constraint release, and deformation of dangling ends. We indeed find that shear flow predictions are insensitive to these approximations, but elongational flow is affected. However, none of these effects is able to bring prediction in line with experiments. We conclude that the currently accepted view of entanglement dynamics is incomplete. [ABSTRACT FROM AUTHOR]

It has recently been shown that the dynamics of complex interfaces can be treated by hyperelastic finite deformation, and the Cauchy stress tensor can be written as a tensor derivative of the interfacial energy. In the present study, the analytical nature of this tensor derivative was further explored, resulting in a more useful constitutive relation between the stress tensor and the Finger strain tensor. To accommodate the interfacial relaxation effects, an energy balance principle was adopted in developing a nonlinear relaxation model which was then used in a visco-hyperelastic formulation for stress determination. Case studies on this visco-hyperelastic model were performed, and validations against known results were attempted. While at small deformation rates, the new model agrees with the linear viscoelastic calculations, as well as Doi and Ohta's scaling predictions, high nonlinearity was observed at large deformation rates. [ABSTRACT FROM AUTHOR]

The main aim of this work is to study the flow, failure, and rupture dynamics of a benchmark low-density polyethylene (BASF Lupolen 1840H) in true tensile creep conditions in both the viscoelastic and elastic deformation regimes. For this, we used a novel extensional rheometer that for the first time allows real controlled-stress conditions to be applied to the materials (as opposed to nominally controlled-stress) until physical rupture occurs (as opposed to being limited by maximum Hencky strain). We observed that constant strain rate was achieved for all flow conditions, and depending on the level of applied tensile stress, one or two states were obtained. In the former case, the observed mode of rupture was ductile, or liquid-like, and in the latter was cohesive, or elastic-like. The coupling at the molecular level between these flow and rupture mechanisms is not yet fully understood, although some recent studies in the literature may be able to offer at least partial explanations. [ABSTRACT FROM AUTHOR]

A rheological model for a short fiber suspension in polymeric liquid is proposed according to irreversible thermodynamics of viscoelastic deformation of polymers with the effect of fiber orientation taken into account. In this model, the evolution of elastic deformation tensor, namely, reversible Finger strain tensor, is governed by not only the reversible Finger strain tensor but also the fiber orientation tensor in a manner of a positive entropy production. The final form of stress tensor combines the viscoelasticity of polymeric matrix of the Leonov model and the fiber contribution of the Dinh-Armstrong model. The Folgar-Tucker model is employed for fiber orientation kinematics along with an invariant-based optimal fitting closure approximation. The rheological behavior of the model is comprehensively studied using steady and transient shear flows. [ABSTRACT FROM AUTHOR]

A new method for measurement of the viscoelastic properties of the liquid with the torsion resonator on forced resonance is presented in this paper. Both theoretical analysis and experiments of different glycerol-water mixtures with our homemade torsion pendulum apparatus demonstrate that this new method avoids the restriction of the common method that the resonator should have the high quality factor both in air and in liquid sample and has the prominent advantage of measuring the concentrated and large-viscous liquid in the low frequency range (≤100 Hz) with a better precision, which is usually unavailable by the common method. Employment of the new method can thus extend the accessible ranges of viscosity and frequency of the torsion resonator apparatus and close the gaps between conventional rheometers and the torsion resonators. [ABSTRACT FROM AUTHOR]

This paper reports finite-element simulations of drop deformation in converging flows in an axisymmetric conical geometry. The moving interface is captured using a diffuse-interface model and accurate interfacial resolution is ensured by adaptive refinement of the grid. We have explored the effects of viscoelasticity on drop deformation when either the drop or the matrix is a Giesekus fluid. Contrary to the popular belief that viscoelasticity in the drop hinders deformation and that in the matrix enhances deformation, we predict a more complex picture in which viscoelasticity in either component may suppress or promote drop deformation depending on the capillary number Ca and the drop-to-matrix viscosity ratio β. Smaller Ca and β are conducive to the behavior mentioned above, while large Ca and β may produce the opposite effect. Both trends are explained by the reaction of the polymer stress to the inhomogeneous and transient deformation in the converging flow field. Finally, this understanding reconciles contradictory results in the literature as opposite limits in the parameter space. [ABSTRACT FROM AUTHOR]

Dispersions of multiarm star polymers in an athermal solvent are studied as a model system to explore the effects of soft colloidal interactions on the dynamics of colloidal glasses. Linear viscoelastic measurements in the glassy state are congruent with Mode Coupling Theory predictions for hard sphere glasses at moderate frequencies, indicating similarities in the relaxation processes of hard and soft colloidal glasses near equilibrium. On the other hand, distinct features of the star relaxation (related to arm disengagement) are observed to affect the nonlinear behavior associated with shear melting of the glass, which exhibits transitions previously unreported for hard sphere systems. Whereas a single maximum in G″(γ) is evident under large amplitude oscillatory shear at frequencies near the beta relaxation, secondary transitions between yielding and the onset of macroscopic flow are observed at higher and lower frequencies. The latter are annealed out upon complete fluidization of the star polymer suspension. The overall nonlinear behavior of the soft glassy suspension is interpreted in terms of two primary mechanisms: (i) deformation and yielding of the cage-like glassy microstructure, as in the hard sphere case, and (ii) local stretching and disengagement of interpenetrating arms of the colloidal star polymers. [ABSTRACT FROM AUTHOR]

The creep compliance and relaxation functions used in characterizing the mechanical response of linear viscoelastic solids are traditionally found by conducting two separate experiments. Alternatively, one of the functions may be determined from a single experiment while the other is obtained through interconversion. All direct interconversion methods, however, require the solution of an ill-posed problem. The goal of this paper is to present the theoretical framework for developing a new apparatus, based on “spring loading,” which facilitates the determination of both creep and relaxation functions from a single experiment. There is no need for interconversion. Questions of stability with respect to the measured data are discussed and a stable numerical algorithm is presented. [ABSTRACT FROM AUTHOR]