Showing papers on "Rheometer published in 2011"

Rheology of globular proteins: apparent yield stress, high shear rate viscosity and interfacial viscoelasticity of bovine serum albumin solutions

Abstract: Globular proteins influence the flow, microstructure, phase behavior and transport of biofluids and biomolecules in the mammalian body. These proteins are essential constituents of food, drugs and cosmetics, and their dynamics determine the physical properties and application of these multicomponent materials. In conventional rheological studies conducted using typical geometries on torsional rheometers, solutions of globular proteins are commonly reported to have a solid-like response at concentrations as low as 0.03% by weight. Typical explanations invoke the presence of long-range repulsions that are stronger than electrostatic interactions. In this study, we probe the bulk and the interfacial viscoelasticity of surfactant-free bovine serum albumin (BSA) solutions using a stress-controlled torsional rheometer, augmented by microfluidic rheometry and interfacial rheometric measurements. We demonstrate that the origin of this yield-like behavior, which is manifested as a highly shear-thinning bulk rheological response, lies in the formation of a film of adsorbed protein, formed spontaneously at the solution/gas interface. We provide direct interfacial rheometric measurements to study the concentration-dependent viscoelasticity of the adsorbed protein and we describe a simple, but quantitative, additive model useful for extracting the interfacial viscosity contribution from bulk viscosity measurements over a wide range of shear rates.

Stress overshoot in a simple yield stress fluid: An extensive study combining rheology and velocimetry

Abstract: We report a large amount of experimental data on the stress overshoot phenomenon which takes place during start-up shear flows in a simple yield stress fluid, namely a carbopol microgel. A combination of classical rheological measurements and ultrasonic velocimetry makes it possible to get physical insights on the transient dynamics of both the stress σ(t) and the velocity field across the gap of a rough cylindrical Couette cell during the start-up of shear under an applied shear rate . (i) At small strains (γ w. Finally, by changing the boundary conditions from rough to smooth, we show that there exists a critical shear rate s fixed by the wall surface roughness below which slip at both walls allows for faster stress relaxation and for stress fluctuations strongly reminiscent of stick-slip. Interestingly, the value of s is observed to coincide with the shear rate below which the flow curve displays a kink attributed to wall slip.

Rheological non-Newtonian behaviour of ethylene glycol-based Fe2O3 nanofluids

Abstract: The rheological behaviour of ethylene glycol-based nanofluids containing hexagonal scalenohedral-shaped α-Fe2O3 (hematite) nanoparticles at 303.15 K and particle weight concentrations up to 25% has been carried out using a cone-plate Physica MCR rheometer. The tests performed show that the studied nanofluids present non-Newtonian shear-thinning behaviour. In addition, the viscosity at a given shear rate is time dependent, i.e. the fluid is thixotropic. Finally, using strain sweep and frequency sweep tests, the storage modulus G', loss modulus G″ and damping factor were determined as a function of the frequency showing viscoelastic behaviour for all samples.

Extensional flow of blood analog solutions in microfluidic devices.

Abstract: In this study, we show the importance of extensional rheology, in addition to the shear rheology, in the choice of blood analog solutions intended to be used in vitro for mimicking the microcirculatory system. For this purpose, we compare the flow of a Newtonian fluid and two well-established viscoelastic blood analog polymer solutions through microfluidic channels containing both hyperbolic and abrupt contractions/expansions. The hyperbolic shape was selected in order to impose a nearly constant strain rate at the centerline of the microchannels and achieve a quasihomogeneous and strong extensional flow often found in features of the human microcirculatory system such as stenoses. The two blood analog fluids used are aqueous solutions of a polyacrylamide (125 ppm w/w) and of a xanthan gum (500 ppm w/w), which were characterized rheologically in steady-shear flow using a rotational rheometer and in extension using a capillary breakup extensional rheometer (CaBER). Both blood analogs exhibit a shear-thinning behavior similar to that of whole human blood, but their relaxation times, obtained from CaBER experiments, are substantially different (by one order of magnitude). Visualizations of the flow patterns using streak photography, measurements of the velocity field using microparticle image velocimetry, and pressure-drop measurements were carried out experimentally for a wide range of flow rates. The experimental results were also compared with the numerical simulations of the flow of a Newtonian fluid and a generalized Newtonian fluid with shear-thinning behavior. Our results show that the flow patterns of the two blood analog solutions are considerably different, despite their similar shear rheology. Furthermore, we demonstrate that the elastic properties of the fluid have a major impact on the flow characteristics, with the polyacrylamide solution exhibiting a much stronger elastic character. As such, these properties must be taken into account in the choice or development of analog fluids that are adequate to replicate blood behavior at the microscale.

Shear thickening and jamming in densely packed suspensions of different particle shapes.

Abstract: We investigated the effects of particle shape on shear thickening in densely packed suspensions. Rods of different aspect ratios and nonconvex hooked rods were fabricated. Viscosity curves and normal stresses were measured using a rheometer for a wide range of packing fractions for each shape. Suspensions of each shape exhibit qualitatively similar discontinuous shear thickening. The logarithmic slope of the stress vs shear rate increases dramatically with packing fraction and diverges at a critical packing fraction ${\ensuremath{\varphi}}_{c}$ which depends on particle shape. The packing fraction dependence of the viscosity curves for different convex shapes can be collapsed when the packing fraction is normalized by ${\ensuremath{\varphi}}_{c}$. Intriguingly, viscosity curves for nonconvex particles do not collapse on the same set as convex particles, showing strong shear thickening over a wider range of packing fraction. The value of ${\ensuremath{\varphi}}_{c}$ is found to coincide with the onset of a yield stress at the jamming transition, suggesting the jamming transition also controls shear thickening. The yield stress is found to correspond with trapped air in the suspensions, and the scale of the stress can be attributed to interfacial tension forces which dramatically increase above ${\ensuremath{\varphi}}_{c}$ due to the geometric constraints of jamming. Using this connection we show that the jamming transition can be identified by simply looking at the surface of suspensions. The relationship between shear and normal stresses is found to be linear in both the shear thickening and jammed regimes, indicating that the shear stresses come from friction. In the limit of zero shear rate, normal stresses pull the rheometer plates together due to the surface tension of the liquid below ${\ensuremath{\varphi}}_{c}$, but push the rheometer plates apart due to jamming above ${\ensuremath{\varphi}}_{c}$.

New Low-Temperature Performance-Grading Method: Using 4-mm Parallel Plates on a Dynamic Shear Rheometer

Abstract: Mechanical measurements on asphalt binders to determine the performance grade of the low-temperature specification are typically carried out with a bending beam rheometer (BBR). The BBR test requires considerable material to fabricate a specimen (approximately 15 g per beam). The relatively large amount of asphalt binder required for the BBR limits its applications. The BBR is difficult to apply to extracted asphalt binder or to other situations in which there is a limited amount of binder, for example, residue from emulsions. This paper proposes an alternate mechanical test to the BBR to determine low-temperature PG. Only approximately 25 mg of asphalt binder are required to perform a test. The test employs 4-mm-diameter parallel plates on a dynamic shear rheometer (DSR) and includes a correction for machine compliance. This correction allows testing to -40°C. A low-temperature specification from the 4-mm rheometry is suggested by the establishment of a correlation between BBR creep stiffness data and DS...

Extensional rheology of human saliva

Abstract: We have developed an oscillatory cross-slot extensional rheometer capable of performing measurements with unprecedentedly small volumes of test fluids (∼10–100 μL). This provides the possibility of studying exotic and precious or scarce bio-fluids, such as synovial fluid. To test our system, we have looked at a relatively abundant and accessible biological fluid, namely human saliva; a complex aqueous mixture of high molecular weight mucin molecules and other components. The results represent our first attempts to by this technique and as yet we have only sampled a small dataset. However, we believe we have produced the first successful quantitative measurements of extensional viscosity, Trouton ratio, and flow-induced birefringence made on saliva samples. The results significantly add to the scant literature on saliva rheology, especially in extension, and demonstrate the important role of saliva extensibility in relation to function.

Cone-partitioned-plate geometry for the ARES rheometer with temperature control

Abstract: A cone-partitioned-plate fixture for the ARES rheometer (TA instruments, DE) has been designed, implemented, and validated. This geometry allows measuring the nonlinear shear flow properties of samples, which display edge fracture in regular cone-and-plate geometries, such as polymer melts and concentrated solutions. Reliable bulk shear flow data can be obtained with these systems at high rates and strains, using very small sample quantities. Measurements can be performed at temperatures ranging from at least − 50 °C up to over 200 °C in a controlled nitrogen environment. An extensive set of start-up shear measurements on moderately entangled linear monodisperse polyisoprene (60 kg/mol) and polystyrene (182 kg/mol) melts to validate the design are presented and discussed with focus on the Cox–Merz rule and the characteristics of the stress overshoot. With this new geometry, the range of artifact-free data is extended by a decade in Weissenberg number (WiD). It is shown that the obtained results compared w...

Practical food rheology: an interpretive approach.

Abstract: Preface. Contributors. 1 Introduction - Why the Interpretive Approach? (Niall W. G. Young). 1.1 Rheology - What is in it for me? 1.1.1 Case study. 2 Viscosity and Oscillatory Rheology (Taghi Miri). 2.1 Introduction. 2.2 Food rheology. 2.3 Directions of rheological research. 2.3.1 Phenomenological rheology or macrorheology. 2.3.2 Structural rheology or microrheology. 2.3.3 Rheometry. 2.3.4 Applied rheology. 2.4 Steady-state shear flow behaviour: viscosity. 2.4.1 Rheological models for shear flow. 2.4.2 Wall slip. 2.5 Viscoelasticity and oscillation. 2.5.1 Oscillatory testing. 2.6 Process, rheology and microstructural interactions. 2.7 Rheology of soft solids. 2.7.1 Capillary rheometer. 2.7.2 Squeeze flow rheometer. 2.8 Measuring instruments - practical aspects. 2.8.1 Choosing the right measuring system. 3 Doppler Ultrasound-Based Rheology (Beat Birkhofer). 3.1 Introduction. 3.1.1 Overview. 3.1.2 History of ultrasonic velocimetry. 3.1.3 Existing literature on UVP-based rheometry. 3.2 Ultrasound transducers. 3.3 Flow adapter. 3.3.1 Doppler angle. 3.4 Acoustic properties. 3.4.1 Propagation. 3.4.2 Attenuation. 3.4.3 Sound velocity. 3.4.4 Scattering. 3.4.5 Backscattering. 3.5 Electronics, signal processing and software. 3.5.1 Electronics. 3.5.2 Signal processing and profile estimation. 3.5.3 Software. 3.6 Pipe flow and fluid models. 3.6.1 Gradient method or point-wise rheological characterisation. 3.6.2 Power law fluid model. 3.6.3 Herschel-Bulkley fluid model. 3.6.4 Other models. 3.7 Rheometry. 3.7.1 Averaging effects at the pipe wall. 3.7.2 Fitting. 3.7.3 Gradient method. 3.8 Examples. 3.8.1 Carbopol solution. 3.8.2 Suspension of polyamide in rapeseed oil. 3.9 Summary. 4 Hydrocolloid Gums - Their Role and Interactions in Foods (Tim Foster and Bettina Wolf). 4.1 Introduction. 4.2 Behaviour of hydrocolloid gums in solution. 4.3 Hydrocolloid gelation and gel rheology. 4.4 Hydrocolloid-hydrocolloid interactions. 4.5 Hydrocolloids in foods - role and interactions. 5 Xanthan Gum - Functionality and Application (Graham Sworn). 5.1 Introduction. 5.2 Xanthan molecular structure and its influence on functionality. 5.3 The conformational states of xanthan gum. 5.4 Food ingredients and their effects on xanthan gum functionality. 5.4.1 Salts. 5.4.2 Acids (pH). 5.4.3 Xanthan and proteins. 5.4.4 Xanthan and starch. 5.5 Food processing and its impact on xanthan gum functionality. 5.5.1 Thermal treatment. 5.5.2 Homogenisation. 5.5.3 Freezing. 5.6 Food structures. 5.6.1 Emulsions. 5.6.2 Gels. 5.7 Applications. 5.8 Future trends. 6 Alginates in Foods (Alan M. Smith and Taghi Miri). 6.1 Alginate source and molecular structure. 6.2 Alginate hydrogels. 6.3 Alginic acid. 6.4 Alginate solutions. 6.5 Enzymatically tailored alginate. 6.6 Alginates as food additive. 6.6.1 Gelling agent. 6.6.2 Thickening agent. 6.6.3 Film-forming agent. 6.6.4 Encapsulation and immobilisation. 6.6.5 Texturisation of vegetative materials. 6.6.6 Stabiliser. 6.6.7 Appetite control. 6.6.8 Summary. 7 Dairy Systems (E. Allen Foegeding, Bongkosh Vardhanabhuti and Xin Yang). 7.1 Introduction. 7.2 Fluid milk. 7.2.1 Rheological properties of milk. 7.2.2 Measurements of the rheological properties of milk. 7.2.3 Factors influencing milk rheological properties. 7.2.4 Correlating rheological properties of milk to sensory perceptions. 7.2.5 Process engineering calculation. 7.3 Solid cheese. 7.3.1 Small amplitude oscillatory tests. 7.3.2 Large strain rheological analysis. 7.3.3 Creep and stress relaxation. 7.4 Rheological properties of semi-solid dairy foods. 7.4.1 Flow properties. 7.4.2 Yield stress. 7.4.3 Viscoelastic properties of semi-solid dairy products. 7.5 Effect of oral processing on interpretation of rheological measurement. 8 Relationship between Food Rheology and Perception (John R. Mitchell and Bettina Wolf). 8.1 Introduction. 8.2 Rheology and thickness perception. 8.3 Rheology and flavour perception. 8.4 Mixing, microstructure, gels and mouthfeel. 8.4.1 Mixing. 8.4.2 Microstructure. 8.4.3 Mouthfeel. 8.4.4 Gels. 8.5 Beyond shear rheology. 8.6 Conclusions. 9 Protein-Stabilised Emulsions and Rheological Aspects of Structure and Mouthfeel (Fotios Spyropoulos, Ernest Alexander K. Heuer, Tom B. Mills and Serafim Bakalis). 9.1 Introduction. 9.2 Processing and stability of emulsions. 9.2.1 Instabilities in emulsions. 9.2.2 Protein functionality at liquid interfaces. 9.2.3 Protein-stabilised oil-in-water emulsions - Effect of aqueous phase composition. 9.2.4 Effect of processing. 9.3 Oral processes. 9.3.1 Different stages and phenomena during oral processing. 9.3.2 Fluid dynamics during oral processing. 9.3.3 Interactions with saliva. 9.3.4 Interaction with oral surfaces. 9.4 In vitro measurements of sensory perception. 9.5 Future perspectives. 10 Rheological Control and Understanding Necessary to Formulate Healthy Everyday Foods (Ian T. Norton, Abigail B. Norton, Fotios Spyropoulos, Benjamin J. D. Le Reverend and Philip Cox). 10.1 Introduction. 10.2 Design and control of material properties of foods inside people. 10.2.1 Oral perception of foods. 10.2.2 Food in the stomach. 10.2.3 Food in the intestine. 10.3 Reconstructing foods to be healthy and control dietary intake. 10.3.1 Use of emulsions as partial fat replacement. 10.3.2 Duplex emulsions. 10.3.3 Fat replacement with air-filled emulsion. 10.3.4 Sheared gels (fluid gels). 10.3.5 Water-in-water emulsions. 10.3.6 Self-structuring systems. 10.4 Conclusions. References. Index.

Microrheology of Complex Fluids

Abstract: Many of the diverse material properties of soft materials (polymer solutions, gels, filamentous proteins in cells, etc.) stem from their complex structures and dynamics with multiple characteristic length and time scales. A wide variety of technologies, from paints to foods, from oil recovery to processing of plastics, all heavily rely on the understanding of how complex fluids flow (Larson, 1999). Rheological measurements on complex materials reveal viscoelastic responses which depend on the time scale at which the sample is probed. In order to characterize the rheological response one usually measures the shear or the Young modulus as a function of frequency by applying a small oscillatory strain of frequency ω. Typically, commercial rheometers probe frequencies up to tens of Hz, the upper range being limited by the onset of inertial effects, when the oscillatory strain wave decays appreciably before propagating throughout the entire sample. If the strain amplitude is small, the structure is not significantly deformed and the material remains in equilibrium; in this case the affine deformation of the material controls the measured stress, and the time-dependent stress is linearly proportional to the strain (Riande et al., 2000). Even though standard rheological measurements have been very useful in characterizing soft materials and complex fluids (e.g. colloidal suspensions, polymer solutions and gels, emulsions, and surfactant solutions), they are not always well suited for all systems because milliliter samples are needed thus precluding the study of rare or precious materials, including many biological samples that are difficult to obtain in large quantities. Moreover, conventional rheometers provide an average measurement of the bulk response, and do not allow for local measurements in inhomogeneous systems. To address these issues, a new methodology, microrheology, has emerged that allows to probe the material response on micrometer length scales with microliter sample volumes. Microrheology does not correspond to a specific experimental technique, but rather a number of approaches that attempt to overcome some limitations of traditional bulk rheology (Squires & Mason, 2010; Wilson & Poon, 2011). Advantages over macrorheology include a significantly higher range of frequencies available without time-temperature superposition (Riande et al., 2000), the capability of measuring material inhomogeneities that are inaccessible to macrorheological methods, and rapid thermal and chemical homogeneization that allow the transient rheology of evolving systems to be studied (Ou-Yang & Wei, 2010). Microrheology methods typically use embedded micron-sized probes to locally deform the sample, thus allowing one to use this type of rheology on very small volumes, of the order of a microliter. Macro-

Flow of low viscosity Boger fluids through a microfluidic hyperbolic contraction

Abstract: In this work we focus on the development of low viscosity Boger fluids and assess their elasticity analyzing the flow through a microfluidic hyperbolic contraction. Rheological tests in shear and extensional flows were carried out in order to evaluate the effect of the addition of a salt (NaCl) to dilute aqueous solutions of polyacrylamide at 400, 250, 125 and 50 ppm (w/w). The rheological data showed that when 1% (w/w) of NaCl was added, a significant decrease of the shear viscosity curve was observed, and a nearly constant shear viscosity was found for a wide range of shear rates, indicating Boger fluid behavior. The relaxation times, measured using a capillary break-up extensional rheometer (CaBER), decreased for lower polymer concentrations, and with the addition of NaCl. Visualizations of these Boger fluids flowing through a planar microfluidic geometry containing a hyperbolic contraction, which promotes a nearly uniform extension rate at the centerline of the geometry, was important to corroborate their degree of elasticity. Additionally, the quantification of the vortex growth upstream of the hyperbolic contraction was used with good accuracy and reproducibility to assess the relaxation time for the less concentrated Boger fluids, for which CaBER measurements are difficult to perform.

Consistently Inconsistent: Commercially Available Starch-Based Dysphagia Products

Abstract: Individuals with dysphagia may be prescribed thickened fluids to promote a safer and more successful swallow. Starch-based thickening agents are often employed; however, these exhibit great variation in consistency. The aim of this study was to compare viscosity and the rheological profile parameters complex (G*), viscous (G″), and elastic modulus (G′) over a range of physiological shear rates. UK commercially available dysphagia products at “custard” consistency were examined. Commercially available starch-based dysphagia products were prepared according to manufacturers’ instructions; the viscosity and rheological parameters were tested on a CVOR Rheometer. At a measured shear rate of 50 s−1, all products fell within the viscosity limits defined according to the National Dysphagia Diet Task Force guidelines. However, at lower shear rates, large variations in viscosity were observed. Rheological parameters G*, G′, and G″ also demonstrated considerable differences in both overall strength and rheological behavior between different batches of the same product and different product types. The large range in consistency and changes in the overall structure of the starch-based products over a range of physiological shear rates show that patients could be receiving fluids with very different characteristics from that advised. This could have detrimental effects on their ability to swallow.

On the rheology of a dilute emulsion in a uniform electric field

Abstract: A small-deformation perturbation analysis is developed to describe the effect of a uniform electric field on drop deformation and orientation in linear flows and emulsion shear rheology. All media are treated as leaky dielectrics, and fluid motion is described by the Stokes equations. The one-particle contribution to the effective stress of a dilute emulsion is obtained from the drop stresslet. Analytical solutions are derived as regular perturbations in the limits of small capillary number and large viscosity ratio. The results show that both shape distortion and charge convection modify emulsion rheology. Drop deformation due to application of an electric field in a direction perpendicular to the shear flow gives rise to normal stresses and may lead to shear thickening or shear thinning, depending on the electric properties of the fluids. Charge convection due to the imposed shear affects both the shear viscosity and normal stresses.

Local transient rheological behavior of concentrated suspensions

Abstract: This paper reports experiments on the shear transient response of concentrated non-Brownian suspensions. The shear viscosity of the suspensions is measured using a wide-gap Couette rheometer equipped with a particle image velocimetry device that allows measuring the velocity field. The suspensions made of PMMA particles (31 μm in diameter) suspended in a Newtonian index- and density-matched liquid are transparent enough to allow an accurate measurement of the local velocity for particle concentrations as high as 50%. In the wide-gap Couette cell, the shear induced particle migration is evidenced by the measurement of the time evolution of the flow profile. A peculiar radial zone in the gap is identified where the viscosity remains constant. At this special location, the local particle volume fraction is taken to be the mean particle concentration. The local shear transient response of the suspensions when the shear flow is reversed is measured at this point where the particle volume fraction is well defin...

Study of the flow field in the magnetic rod interfacial stress rheometer.

Abstract: Several technological applications, consumer products, and biological systems derive their functioning from the presence of a complex fluid interface with viscoelastic interfacial rheological properties. Measurements of the "excess" rheological properties of such an interface are complicated by the intimate coupling of the bulk and interfacial flows. In the present work, analytical, numerical, and experimental results of the interfacial flow fields in a magnetic rod interfacial stress rheometer (ISR) are presented. Mathematical solutions are required to correct the experimentally determined apparent interfacial shear moduli and phase angles for the drag exerted by the surrounding phases, especially at low Boussinesq numbers. Starting from the Navier-Stokes equations and using the generalized Boussinesq-Scriven equation as a suitable boundary condition, the problem is solved both analytically and numerically. In addition, experimental data of the interfacial flow field are reported, obtained by following the trajectories of tracer particles at the interface with time. Good agreement is found between the three methods, indicating that both the analytical solution and the numerical simulations give an adequate description of the flow field and the resulting local interfacial shear rate at the rod. Based on these results, an algorithm to correct the experimental data of the ISR is proposed and evaluated, which can be extended to different types of interfacial shear rheometers and geometries. An increased accuracy is obtained and the measurement range of the ISR is expanded toward viscosities and elastic moduli of smaller magnitude.

Soft magnetic carbonyl iron microsphere dispersed in grease and its rheological characteristics under magnetic field

Abstract: Magnetorheological (MR) grease, comprised of a suspension of soft magnetic carbonyl iron (CI) microspherical particles dispersed in a grease medium, was fabricated by a mechanical stirring method. As potential medium oil for MR system, shear viscosity of the pure grease was measured as a function of temperature. Its MR characteristics were investigated using a rotational rheometer under an external magnetic field. Flow curve responses (shear stress and shear viscosity), yield stress, and elasticity were investigated using various magnetic field strengths ranging from 0 to 342 kA/m. It was confirmed that MR grease has a yield stress under no external magnetic field due to the inherent property of grease. In addition, CI based MR grease exhibited a characteristic of a Bingham fluid.

Shear banding in thixotropic and normal emulsions.

Abstract: When made to flow, yield stress materials rarely flow homogeneously. This is mostly attributed to the fact that such materials show a transition from a solid- to a liquid-like state when the stress exceeds some critical value: the yield stress. Thus, if the stress is heterogeneous, so is the flow. Here we consider emulsion flows in a cone-plate geometry that, for Newtonian fluids, correspond to a homogeneous stress situation and show that shear banding can also be observed either due to wall slip or to the existence of a critical shear rate. By means of velocity profiles obtained using a confocal laser scanning microscope combined with a rheometer we conclude that the last type of shear banding occurs only in thixotropic yield stress materials.

Surface rheology: macro- and microrheology of poly(tert-butyl acrylate) monolayers

Abstract: We have studied the surface shear viscoelasticity of poly(tert-butyl-acrylate) Langmuir monolayers spread at the air/water interface, by tracking the Brownian motion of tracer particles with different sizes and surface chemical nature, trapped at the same interface. Surface shear moduli have been extracted from the particles mean square displacements (MSD), using different approaches: hydrodynamic calculations of drag coefficients and direct inversion of the MSD by means of the generalized Stokes–Einstein equation. It has been found that these different theoretical approaches lead to comparable values of the shear interfacial viscosity independent of the polymer concentration and molecular weight. In addition, no effect of the size or chemical nature of the probe has been detected. The results have demonstrated the consistency of the microrheological techniques used, and confirm the existence of entanglements in PtBA monolayers, as recently deduced from dilational elasticity and viscosity measurements, [Maestro et al., Soft Matter, 2010, 6, 4407]. An unexpected result was that the interfacial viscosity values obtained from microrheology have been found to be several orders of magnitude lower than the ones obtained with macroscopic interfacial shear rheometers. At the moment there is no clear explanation for this disagreement, although it is not related to the probe size or their chemical nature. Furthermore, this discrepancy is not related to the analysis methodology used, including the calculation of the two-point correlation function used in 3D microrheology when there are heterogeneities present within the range of the probe size.

A weakly compressible flow model for the restart of thixotropic drilling fluids

Abstract: This study presents a mathematical model to simulate the start-up flow of drilling fluids in drill pipes. The compressible transient flow approach is based on one-dimensional mass and momentum conservation equations, an equation of state and a thixotropic fluid model. In contrast with prior studies that view gel breaking as a visco-plastic phenomenon, thixotropy is approached as an elasto-visco-plastic problem. Such thixotropy model is fit to rheometer data with reasonable agreement. The start-up flow is then simulated, and the results are compared to Newtonian, Bingham and visco-plastic fluid flow models. It can be anticipated that the pressure overshoots observed in start-up flows depend not only on the thixotropy properties of the fluid but also on the flow compressibility and Reynolds number. In addition, the main differences between the visco-plastic and elasto-visco-plastic thixotropy approaches take place at low shear rates; the higher the shear rates, the more similar are the results. Finally, as expected, the pressure overshoots of the thixotropic fluids are not always larger than those of the Bingham and Newtonian counterparts.

Local transient rheological behavior of concentrated suspensions

Abstract: This paper reports experiments on the shear transient response of concentrated non-Brownian suspensions. The shear viscosity of the suspensions is measured using a wide-gap Couette rheometer equipped with a Particle Image Velocimetry (PIV) device that allows measuring the velocity field. The suspensions made of PMMA particles (31$\mu$m in diameter) suspended in a Newtonian index- and density-matched liquid are transparent enough to allow an accurate measurement of the local velocity for particle concentrations as high as 50%. In the wide-gap Couette cell, the shear induced particle migration is evidenced by the measurement of the time evolution of the flow profile. A peculiar radial zone in the gap is identified where the viscosity remains constant. At this special location, the local particle volume fraction is taken to be the mean particle concentration. The local shear transient response of the suspensions when the shear flow is reversed is measured at this point where the particle volume fraction is well defined. The local rheological measurements presented here confirm the macroscopic measurements of Gadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes a step-like reduction, decreases slower and passes through a minimum before increasing again to reach a plateau. Upon varying the particle concentration, we have been able to show that the minimum and the plateau viscosities do not obey the same scaling law with respect to the particle volume fraction. These experimental results are consistent with the scaling predicted by Mills and Snabre (2009) and with the results of numerical simulation performed on random suspensions [Sierou and Brady (2001)]. The minimum seems to be associated with the viscosity of an isotropic suspension, or at least of a suspension whose particles do not interact through non-hydrodynamic forces, while the plateau value would correspond to the viscosity of a suspension structured by the shear where the non-hydrodynamic forces play a crucial role.

Rheology of a suspension of elastic particles in a viscous shear flow

Abstract: In this paper we consider a suspension of elastic solid particles in a viscous liquid. The particles are assumed to be neo-Hookean and can undergo finite elastic deformations. A polarization technique, originally developed for analogous problems in linear elasticity, is used to establish a theory for describing the finite-strain, time-dependent response of an ellipsoidal elastic particle in a viscous fluid flow under Stokes flow conditions. A set of coupled, nonlinear, first-order ODEs is obtained for the evolution of the uniform stress fields in the particle, as well as for the shape and orientation of the particle, which can in turn be used to characterize the rheology of a dilute suspension of elastic particles in a shear flow. When applied to a suspension of cylindrical particles with initially circular cross-section, the theory confirms the existence of steady-state solutions, which can be given simple analytical expressions. The two-dimensional, steady-state solutions for the particle shape and orientation, as well as for the effective viscosity and normal stress differences in the suspension, are in excellent agreement with direct numerical simulations of multiple-particle dispersions in a shear flow obtained by using an arbitrary Lagrangian–Eulerian (ALE) finite element method (FEM) solver. The corresponding solutions for the evolution of the microstructure and the rheological properties of suspensions of initially spherical (three-dimensional) particles in a simple shear flow are also obtained, and compared with the results of Roscoe (J. Fluid Mech., vol. 28, 1967, pp. 273–293) in the steady-state regime. Interestingly, the results show that sufficiently soft elastic particles can be used to reduce the effective viscosity of the suspension (relative to that of the pure fluid).