Emerging applications for models of molecular rheology

Our understanding of the rich and complex motion of macromolecules has developed rapidly in recent years, particularly in the entangled regime. In this paper we highlight examples of research that has exploited our detailed picture of macromolecular motion. These studies encompass a broad range of areas in polymer physics, including neutron scattering under strong flow, tear properties of oriented polymers, flow-induced crystallisation and single molecule rheology of biopolymers. In each case molecular modelling plays a key role in understanding the observed phenomena.     

Shear behaviour of biopolymer suspensions with spheroidal and cylindrical particles

The rheological behaviour of suspensions is influenced by many parameters, one of which is the particle shape. For rigid particle suspensions a number of studies demonstrate the effects of the particle aspect ratio. Indeed, fibres are widely used as rheology modifiers in different materials such as synthetic polymers. This work is concerned with testing the hypothesis that regularly shaped particles with aspect ratios larger than one that are made of gelled biopolymers could be used as rheology modifiers for biopolymer solutions. Biopolymers, and mixtures thereof are a widely used ingredient in foods and other products with structure functionality. Tailoring rheology modifiers by morphology offers an alternative to using different biopolymers. It is demonstrated how biopolymer suspensions with regular spheroidal, or cylindrical particle shapes can be produced by gelling the droplet phase of a liquid two phase biopolymer mixture in a shear field. Biopolymers were chosen such that gelation is initiated by cooling. Shear-cooling at constant stresses leads to the formation of ellipsoidal particles. Cylindrical particles can be generated by stepping up the shear stress prior to gelation, i.e., stretching the droplet phase into fibrils, and trapping the shape prior to break-up through gelation. Morphologies and steady shear rheological data for suspensions of the two biopolymers gellan and κ-carrageenan with an internal phase volume of 0.2 are reported. The influence of particle shape on relative viscosity is pronounced. At high shear stresses particle orientation leads to decreased viscosity with increasing particle aspect ratio. In the low shear region, higher aspect ratio suspensions show higher viscosities. Additionally, the material properties, including the interfacial tension, which influence the suspension morphology are reported. Received: 3 March 2000 Accepted: 22 August 2000     

Structure and orientation dynamics of sepiolite fibers–poly(ethylene oxide) aqueous suspensions under extensional and shear flow,probed by in situ SAXS

The structure and orientation dynamics of sepiolite clay fibers about 1,000 nm long and 10 nm thick, suspended in an aqueous poly(ehtylene oxide) matrix of 10⁵ g/mol molecular mass, have been studied under control extensional and shear flow. A new extensional flow cell developed at the “Laboratoire de Rhéologie” and the combined rheology and small angle X-ray scattering (Rheo-SAXS) setup available at the European Synchrotron Radiation Facility have allowed access to in situ and time-resolved fiber orientations and structure properties in the volume of suspensions under flow. In the volume fractions and shear rate domains for which the suspensions exhibit shear-thinning properties, two regimes of orientation separated by a critical strain rate have been identified under extensional flow.     

Orientation behavior of AB and ABC block copolymers under large amplitude oscillatory shear flow

Flow alignment in a large amplitude oscillatory shear field (LAOS) of a lamellar AB and a lamellar ABC block copolymer (A,B are lamellae, C forms cylinders in B-lamellae) has been studied. 2D-small angle X-ray scattering (2D-SAXS) and scanning electron microscopy were used for morphological characterization, and flow birefringence and Fourier-Transform rheology were used to monitor the orientation. The diblock copolymer shows the known frequency-dependent orientation behavior, i.e., a perpendicular or a parallel orientation of the lamellae, while under all conditions for the ABC block copolymer only a perpendicular orientation after a long induction period was found. Due to the introduced third block C the AB lamellar structure with a high viscosity contrast between the A and B domains cannot adapt a parallel orientation of sliding phases. Dynamic mechanical analysis indicates shear induced improvement of the microphase separation of the short C block.     

Evaluating rheological models for human blood using steady state,transient, and oscillatory shear predictions

The rheological characterization of human blood, through modeling and analysis of steady state, transient, and oscillatory shear flows, has made tremendous progress recently. Due to the aggregation of red blood cells at low shear rates, many recent models for blood rheology include a structural, thixotropic component with one of the most recent attempts unifying this approach with a viscoelastic formulation. We will show how these models, along with proposed modifications to another recent structural, kinetic thixotropy model, can improve modeling predictions. Results are compared to the Maxwell-like Bautista-Manero-Puig model, the Oldroyd-8 inspired viscoelastic Anand-Kwack-Masud model, a viscoelastic-thixotropic model from Blackwell and Ewoldt, and the Herschel-Bulkley model. We explore the weaknesses of the legacy blood models and then demonstrate the efficacy of the newly improved models for modeling human blood steady state and transient shear rheology to predict oscillatory shear flow.     

Effects of anionic additives on the rheological behavior of aqueous calcium montmorillonite suspensions

Three different experimental measurements, namely, rheology, particle sizing, and x-ray diffraction (XRD), were used to study the effect of anionic additives on the properties of bentonite suspensions. The three additives were sodium carboxymethylcellulose, xanthan gum, and sodium dodecyl sulfate. Flow curves were obtained from shear stress–shear rate measurements, and the viscoelastic properties were determined from oscillatory and transient measurements. Mineralogical data were evaluated by XRD and the particle size analysis performed by light scattering technique. The presence of the surfactant modifies the face-to-face interactions and yields changes of the mixtures rheological behavior at low deformation rates. Polymers act by coating each clay particle and prevent their agglomeration. Therefore, the additives are responsible for the mechanisms of destructuration and structure reorganization as well as the mixtures viscous and viscoelastic behavior.     

Anomalous rheology of polypyrrole nanoparticle/alginate suspensions: effect of solids volume fraction, particle size, and electronic state

The rheology of dispersions of polypyrrole (PPY) nanoparticles (nPPY) is compared to that of micron-sized PPY particles (CPPY), each suspended in aqueous sodium alginate. With increasing PPY volume fraction, the Newtonian viscosity of the CPPY/alginate suspensions exhibits a ??normal?? increase, whereas that of the nPPY/alginate suspensions decreases to a minimum and then increases again. Enhanced elasticity, indicative of agglomerate formation via bridging interactions with the alginate, is observed only in the CPPY rheology. By comparing doped versus dedoped nPPY particles, and investigating the effect of nPPY particle size, we conclude that the negative viscosity change of the nPPY dispersions is due to adsorption of a dense layer of alginate, resulting in a decrease in bulk alginate concentration. The viscosity upturn at higher nPPY volume fractions indicates the onset of particle agglomeration via bridging interactions with alginate. The results demonstrate improved dispersability of both doped and dedoped nPPY over CPPY particles.     

Lubricated compression of BMC, a concentrated and fibre-reinforced granular polymer suspension

Bulk moulding compounds (BMCs) are thermoset polymer composites widely used in electric and automotive industries. During their processing by injection, BMCs look like fibre-reinforced granular suspensions in the form of pastes, the rheology of which is not well known. For that purpose, lubricated compression tests were performed on BMC samples with various formulations. Firstly, results show that samples flow without sticking to the rheometer’s plates, validating the efficiency of the lubricant. A correction, which requires few assumptions on the rheology of BMC, is then proposed to account for its contribution to the overall axial stress. Thereby, the influences of the axial strain rate, the loading path, the polydispersity of the mineral filler and the fibre content on the BMC flow are analysed. A 1D elementary non-linear viscoelastic model is proposed to capture the main observed trends.     

Extended irreversible thermodynamics,generalized hydrodynamics,kinetic theory,and rheology

The gist of extended irreversible thermodynamics and generalized hydrodynamics is presented within the context of rheology of complex molecules (e.g., polymers) in this paper. Then, the constitutive equation for stress developed for polyatomic fluids in a previous paper is applied to rheology of polymeric fluids. This constitutive equation is fully consistent with the thermodynamic laws. It is shown that the collision bracket integrals appearing in the constitutive equation can be recast in terms of friction tensors of beads and equilibrium force-force correlation functions if the momentum relaxation is much faster than the configuration relaxation and there exist such relaxation times. The force-force correlation functions reduce to those related to the mean square radius of gyration of the polymer if the Hookean model is taken for forces. By treating the recast collision bracket integrals in the constitutive equation as empirical parameters, we analyze some experimental data on shear rate and elongation rate dependence of polymeric melts and obtain excellent agreement with experiment. We show that the empirical parameters can be related to the zero shear rate viscosity and the ratio of the secondary to the primary normal stress coefficient. Therefore, for the plane Couette flow geometry considered in the paper, the constitutive equation is completely specified by the limiting material functions at zero shear rate and relaxation times.Work supported in part by the Natural Sciences and Engineering Research Council of Canada and Fonds FCAR, Quebec. This paper was presented at the Symposium on Recent Developments in Structured Continua II held at Magog, Quebec, Canada, May 23–25, 1990.     

The changing face of rheology

By looking back to the formal beginning of rheology (1929) and studying more recent offerings at congresses, we see that there is a drift of content in the subject. In the beginning rheology was seen as an extension of classical continuum mechanics, but more recently one has seen the withering of offerings in solids rheology and the start of some new branches of the subject. Some opinions on possible future directions are given.     

Current trends in suspension rheology

The complex rheological behaviour of suspensions is very strongly affected by the details of the microstructure and the interparticle forces by which it is controlled. The difficulties involved in characterizing the microstructure, in particular during flow, caused the development of suspension rheology to lag behind that of polymer rheology. Progress in theory, experiments and simulation is rapidly changing the picture, providing opportunities for further significant developments. Possible evolutions in six areas of suspension rheology are discussed here.     

The cyber infrastructure initiative for rheology

A new cyber infrastructure for rheology (CIR) has the potential of integrating the diverse rheological knowledge of experts around the world. In a multi-disciplinary effort, experts in specialized topics of rheology began to write CIR-modules that seamlessly merge into a general code so that it can be used by a wide range of engineers and scientists. At the center of CIR is a platform operating system that connects a wide range of dedicated software modules. These CIR-modules perform calculations and return the corresponding results to a central graphics screen. The computer platform allows the detailed analysis of experimental data, the communication of data, and the prediction of rheological material functions from a wide range of theories in rheology. Rheologists can access each other’s experimental results, make predictions with each other’s theories and simulate with each other’s computer codes. Through such collaboration, seemingly disparate theories and experimental observations can be linked and taken to their limits, thereby leading to unexpected insights and new questions. Beyond the pool of experts, CIR will draw industrial users into the rheology discussion. Easy-to-use CIR-tools will allow industrial rheologists to adjust rapidly to the changing needs and the pressure to obtain short-term solutions in a competitive environment. CIR has the potential of generating ideas for novel materials and novel manufacturing methods. At the same time, it will supply the tools to examine ideas quantitatively and to push these ideas even further. User-friendly methods are essential not only for research and application, but also for the teaching of rheology. We envision tools that allow a student to move seamlessly and rapidly between experimental data and the most advanced rheological theories, simulations, and modeling of applications. In-depth data analysis and evaluation of theory should become easy enough to be performed after reasonable training and without relying on over-simplifications. This will enable the student to reach a deeper understanding of rheology and to appreciate the significance that rheology has in technical applications. Even untrained talent may get introduced quickly into advanced concepts of rheology.Presented at the Annual Meeting of European Society of Rheology, Grenoble, April 2005     

Light scattering by bubbles in liquids: Mie theory, physical-optics approximations, and experiments

Angular structures in the far-field scattering from bubbles are observed and modeled. Mie theory supports a model of diffraction and interference near the critical scattering angle. A new expression for the angular spacing of fine structure is derived. Photographs of scattering show some of the predicted features. Application of these structures to bubble sizing and detection are summarized and the theoretical extinction coefficient in water is plotted.Mie computations for bubbles in water also reveal backward and forward glory effects. These are partially manifested as cross-polarized scattering. Observed scattering from bubbles in the near backward direction is found to have a strong cross-polarized component.     

采动岩体的流变与控制技术

指出采矿工程中岩体的稳定性研究焦点是破裂岩体的流变与控制技术,这给固体流变学研究带来严峻挑战和发展良机,本文较为全面地 采矿工程中巷道围岩变形,采场覆岩运动,岩层移动与围岩突出中的流变现象,以及与之相应的传统理论与控制技术,并为岩石流变学研究指出了新的发展方向。     

Gelation kinetics in elastomer/thermoset polymer blends

The chemical gelation of polymer blends involving a thermoset component has been investigated utilizing multiple waveform dynamic rheology, complemented by differential scanning calorimetry and small angle x-ray scattering measurements. Three epoxy/rubber mixtures, with different degrees of pre-reacted material and catalyst, were used. The curing temperatures were determined with simple temperature ramps, whereas the characteristic gel times were quantified using the well-known self-similarity of the gel structure, which yielded a very low gel exponent (0.05), indicative of the high molecular weights and additives used in the samples; this exponent was unchanged with temperature, suggesting the same gelation mechanism. These times decreased with increasing temperature. Gelation was not affecting the phase-separated morphology. The presence of pre-reacted material and catalyst were found to reduce the gel times (for different temperature steps). Finally, the shear history of the samples did not affet the curing temperatures and kinetics. These results can serve as a useful guide for appropriately blending thermosetting and thermoplastic compounds in order to obtain structural materials for different applications.     

Maurice Couette,one of the founders of rheology

This article presents a biography of Maurice Couette, whose name is associated with a type of flow, of viscometer, and with a correction method for end effects in capillary flows. His life and work are described, with special mention being made of the cylinder apparatus that he designed. The relevance of his work to present day rheology is stressed.     

A review of microstructure in concentrated suspensions and its implications for rheology and bulk flow

An overview of present understanding of microstructure in flowing suspensions is provided. An emphasis is placed on how the microstructure leads to observable bulk flow phenomena unique to mixtures. The bridge between the particle and bulk scales is provided by the mixture rheology; one focus of the review is on work that addresses the connection between microstructure and rheology. The non-Newtonian rheology of suspensions includes the well-known rate dependences of shear thinning and thickening, which have influence on bulk processing of suspensions. Shear-induced normal stresses are also measured in concentrated suspensions and include normal stress differences, and the isotropic particle pressure. Normal stresses have been associated with shear-induced migration, and thus have influence on the ultimate spatial distribution of solids, as well as the flow rate during processing; a second focus is on these uniquely two-phase behaviors and how they can be described in terms of the bulk rheology. An important bulk fluid mechanical consequence of normal stresses is their role in driving secondary flows.     

Nanofluids: Stability, phase diagram, rheology and applications

There is no doubt about the potential technological significance of nanofluids. The promising application areas have been identified as effective heat transfer fluids, contrast agents in magnetic resonance imaging, magnetohyperthermia treatment, precursors to high performance nanocomposites and ordered nanostructures. However, commercial applications are rare, in part due to the limited understanding of the nanofluid fundamentals such as colloid stability, phase diagrams and rheology. This paper intends to provide a brief overview of the scientific disciplines that are important to nanofluids, and the interconnection among different disciplines in order to gain a perspective on the future development of this intriguing area.     

Rheology for deposition control of polymer-amended oil sands tailings

Mineral slurries may be dewatered to the point that they manifest non-Newtonian behavior. Many such slurries exhibit both thixotropic and hysteric behavior in their rheology, which has important implications for managing their deposition in tailings impoundments. This paper characterizes the rheology of a mineral slurry with relatively high clay content, which is treated with a high molecular weight anionic polymer to induce flocculation. The rheology exhibits viscosity bifurcation behavior similar to pure clay, including shear history-dependent apparent yield stress values. Rheometry results are presented including stress growth, controlled stress tests, and oscillatory rheometry, all using a vane fixture. The measured rheology is modeled using a previously published viscosity bifurcation model that accounts for hysteresis in the apparent yield stress. The rheology results are used semi-quantitatively to explain deposition rate-dependent behavior seen in flume tests. The geometry of tailings in flume tests with relatively slow deposition is affected by the presence of deposited tailings that have come to rest sufficiently to manifest the yield stress of initially fully structured material, rather than the lower value yield stress that characterizes when the material first comes to a stop. This full recovery of the yield stress seems to be particularly important to managing surface deposition, as zones of tailings that have stopped moving substantially steepen the slope of deposits near the deposition point.     

Surface rheology of sorbitan tristearate and β-lactoglobulin: Shear and dilatational behavior

Proteins and surfactants behave very differently under shear and dilatational deformation. In this work, we compare specifically their surface properties by evaluating their rheological response. Oil-soluble surfactant, sorbitan tristearate (Span 65), and globular protein, β-lactoglobulin, were spread and adsorbed onto the surface, respectively. A 2D searle-type measuring geometry with a biconical bob was used for measuring the surface shear rheology, and a pendant drop film balance was used for measuring the dilatational rheology. Both equipments provided the viscoelastic properties (surface shear and dilatational complex moduli) of interfacial layers. Also, the linear and non-linear rheology of these systems was studied by increasing the amplitude of the oscillation. Linear rheology showed that dilatational deformation is mostly affected by the nature of the molecular structure at the interface, whereas shear deformation is affected by the strength of the surface film due to the intermolecular interactions. Furthermore, large-amplitude oscillatory shear rheology indicated that the non-linearity increases with the surface concentration, and is higher for insoluble Span 65 spread films than for soluble protein adsorbed layers. Dilatational and shear deformation provide complementary information about interfacial layers that can be optimized so as to fully characterize the surface depending of the type of film (spread or adsorbed) and the technique used (shear or dilatational rheology under linear or non-linear regimes). This information is useful to correlate the structure and the mechanical properties of interfacial systems.