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In quasi-two-dimensional conditions, different invasion patterns can be observed when a fluid displaces another fluid with a higher viscosity. The transition from interfacial instability to fracture during gas invasion remained poorly understood, and classification criteria for different invasion patterns demanded further improvement. In this study, single-point compressed gas injection experiments were conducted in magnesium lithium phyllosilicate (MLPS) suspensions in a rectangular Hele-Shaw cell. Interestingly, with the increase of the injection pressure and the decrease of the concentration of MLPS suspension, the gas invasion pattern underwent a transition from viscous elastic fracture, elastic fracture, elastic viscous fingering to viscous fingering, in which viscous elastic fracture was observed for the first times. We detailly discuss the characteristics and occurrence conditions of each invasion pattern. Furthermore, by analyzing the velocity field of each invasion pattern, it is found that the relationship between the velocity direction around the gas and the gas growth direction varies with different invasion patterns. A simple and effective quantitative indicator is constructed to distinguish the different invasion patterns. Following the identification of invasion patterns, a further investigation was conducted into the relationship between invasion patterns and experimental conditions. By utilizing the relationships among injection conditions and material rheological properties, two dimensionless numbers, Bingham number and Weissenberg number, are conducted, which have an impact on the various invasion patterns and invasion process. A unified phase diagram based on the Bingham number and Weissenberg number was also proposed to incorporate the possible gas invasion patterns in the MLPS suspension. [ABSTRACT FROM AUTHOR]

Oscillatory shear tests are frequently used to determine viscoelastic properties of complex fluids. Both the amplitude and frequency of the input signal can be independently varied, allowing rheologists to probe a wide range of material responses. Historically, most oscillatory tests have focused on the measurement and application of the total strain. However, the total strain is a composite parameter consisting of recoverable and unrecoverable components. Use of only the total strain therefore provides an incomplete description of the rheology. In this work, we provide a mathematical derivation for the determination of the recoverable and unrecoverable components in steady-state linear viscoelastic oscillatory flows via a simple experimental procedure. The relationship between the total strain and its components is fully explored and challenged in the context of how rheologists define moduli and common rheological models. These relations are demonstrated via experimental measurements on model viscoelastic and viscoplastic materials: wormlike micelles and Carbopol 980. Additionally, we show how the derived mathematics fully details the conditions where the Cox-Merz rules are valid in terms of recovery rheology. Finally, we demonstrate how a thorough understanding of the strain components can be used to create a simple nonlinear model that reproduces all common amplitude sweep behaviors. [ABSTRACT FROM AUTHOR]

We derive explicit analytical expressions for the recurrence relations using the analytical matrix method for frequency response and the Bautista-Manero-Puig model for complex fluids. The BMP model is derived from the Extended Irreversible Thermodynamics formalism and has been shown to be useful in predicting the complex rheological behavior of self-associative systems. All harmonics of the alternating normal and shear stresses in oscillatory shear with various amplitude oscillatory regimes (AOS) can be calculated analytically, i.e., small amplitude oscillatory shear (SAOS), medium amplitude oscillatory shear (MAOS), and large amplitude oscillatory shear (LAOS). We show that incorporating the effects of the first and second normal stress differences for all AOS regimes leads to the emergence of higher harmonics. We establish the limits between the different AOS regimes based on criteria suggested by the analytical method. For some typical systems, such as CTAB-NaSal, we found a satisfactory quantitative agreement with the measured behavior of AOS. [ABSTRACT FROM AUTHOR]

Developments in the last century, and especially in the last 50 years, have advanced understanding of suspension rheology greatly. Here, a limited review of suspension work over this period is presented, emphasizing advances over the last three decades in understanding of the particle pressure and strong shear thickening, which were motivated by crucial experimental observations, computational advances, and a critical review, all from the 1980s. This review serves as a preview to some outstanding challenges in suspension mechanics. This article considers primarily dispersions of spherical particles, which serve not only as a model material for understanding the rheology of more complex fluids of practical relevance, but also as a basic system for the study of nonequilibrium statistical physics. [ABSTRACT FROM AUTHOR]

An overarching challenge in rheology is to develop constitutive models for complex fluids for which we lack accurate first principles theory. A further challenge is that most experiments probing dynamical structure and rheology do so only in very simple flow fields that are not characteristic of the complex deformation histories experienced by material in a processing application. A recently developed experimental methodology holds potential to overcome this challenge by incorporating a fluidic four-roll mill (FFoRM) into scanning small-angle X-ray scattering instrumentation (sSAXS) (Corona, P. T. et al. Sci. Rep.8, 15559 (2018); Corona, P. T. et al. Phys. Rev. Mater6, 045603 (2022)) to rapidly generate large data sets of scattering intensity for complex fluids along diverse Lagrangian flow histories. To exploit this uniquely rich FFoRM-sSAXS data, we propose a machine learning framework, Scattering-Informed Microstructure Prediction under Lagrangian Evolution (SIMPLE), which uses FFoRM-sSAXS data to learn an evolution equation for the scattering intensity and an associated tensorial differential constitutive equation for the stress. The framework incorporates material frame indifference and invariance to arbitrary rotations by data preprocessing. We use autoencoders to find an efficient reduced order model for the scattering intensity and neural network ordinary differential equations to predict the time evolution of the model coordinates. The framework is validated on a synthetic FFoRM-sSAXS data set for a dilute rigid rod suspension. The model accurately predicts microstructural evolution and rheology for flows that differ significantly from those used in training. SIMPLE is compatible with but does not require material-specific constraints or assumptions. [ABSTRACT FROM AUTHOR]

Developing constitutive models that can describe a complex fluid's response to an applied stimulus has been one of the critical pursuits of rheologists. The complexity of the models typically goes hand-in-hand with that of the observed behaviors and can quickly become prohibitive depending on the choice of materials and/or flow protocols. Therefore, reducing the number of fitting parameters by seeking compact representations of those constitutive models can obviate extra experimentation to confine the parameter space. To this end, fractional derivatives in which the differential response of matter accepts non-integer orders have shown promise. Here, we develop neural networks that are informed by a series of different fractional constitutive models. These fractional rheology-informed neural networks (RhINNs) are then used to recover the relevant parameters (fractional derivative orders) of three fractional viscoelastic constitutive models, i.e., fractional Maxwell, Kelvin-Voigt, and Zener models. We find that for all three studied models, RhINNs recover the observed behavior accurately, although in some cases, the fractional derivative order is recovered with significant deviations from what is known as ground truth. This suggests that extra fractional elements are redundant when the material response is relatively simple. Therefore, choosing a fractional constitutive model for a given material response is contingent upon the response complexity, as fractional elements embody a wide range of transient material behaviors. [ABSTRACT FROM AUTHOR]

This study introduces the Constitutive Neural Network (ConNN) model, a machine learning algorithm that accurately predicts the temporal response of complex fluids under specific deformations. The ConNN model utilizes a recurrent neural network architecture to capture the time dependent stress responses, and the recurrent units are specifically designed to reflect the characteristics of complex fluids (fading memory, finite elastic deformation, and relaxation spectrum), without presuming any equation of motion of the fluid. We demonstrate that the ConNN model can effectively replicate the temporal data generated by the Giesekus model and the Thixotropic-Elasto-Visco-Plastic (TEVP) fluid model under varying shear rates. To test the performance of the trained model, we subject it to an oscillatory shear flow, with periodic reversals in flow direction, which has not been trained on. The ConNN model successfully replicates the shear moduli of the original models, and the trained values of the recurrent parameters match the physical prediction of the original models. However, we do observe a slight deviation in the normal stresses, indicating that further improvements are necessary to achieve more rigorous physical symmetry and improve the model prediction. [ABSTRACT FROM AUTHOR]

In flows with re-entrant corners, polymeric fluids can exhibit a recirculation region along the wall upstream from the corner. In general, the formation of these vortices is controlled by both the extensional and shear rheology of the material. More importantly, these regions can only form for sufficiently elastic fluids and are often called "lip vortices". These elastic lip vortices have been observed in the flows of complex fluids in geometries with sharp bends. In this work, we characterize the roles played by elasticity and shear thinning in the formation of the lip vortices. Simulations of the Newtonian, Bird-Carreau, and Oldroyd-B models reveal that elasticity is a necessary element. A systematic study of the White-Metzner, finitely extensible non-linear elastic (FENE-P), Giesekus and Rolie-Poly models shows that the onset and size of the elastic lip vortex is governed by a combination of both the degree of shear thinning and the critical shear rate at which the thinning begins. (left) Contour plots of velocity magnitude for the FENE-P model showing the decrease in size of the elastic lip vortex as maximum extensibility decreases. (right) Length of the elastic vortex as a function of a modified Carreau number showing a modelindependent effect of the degree and onset of shear thinning. [ABSTRACT FROM AUTHOR]

The current experimental study investigates the rheological characterization of Carbopol gel solutions into a pipe flow using in situ visualization. The shear rate and shear stress profiles for different pressure-driven values are presented and correlated to obtain new steady-state flow curves compared with rheometrical data performed with cross-hatched parallel plate (CHPP) and smooth concentric cylindrical (SCC) geometries at a rotational rheometer. The rheological behavior for the test performed by the in situ visualization was well fitted by the generalized Herschel-Bulkley model, and different values for the coefficient of consistency (K), flow behavior index (n), and yield stress (τ0) were fitted for the three gel solutions due to the presence of wall slip behavior. The discrepancies between the values of the rheological parameters suggest that conventional rheometrical measurements, which avoid the slippage of the fluid, lead to an overestimation of these parameters, and as a consequence, these discrepancies are extended to the dimensionless numbers calculated for the hydrodynamic flow description. Also, the experimental plug core velocity was compared with the analytical value obtained by the rotational rheometer tests to calculate an equivalent slip velocity, and such velocity depicts a quasi-linear trend with the wall shear stress. This is supported by the apparent viscosity profiles along the pipe diameter, suggesting that the slippage is an inherent characteristic of polymer gel solutions, and it is disseminated by the presence of layers near the pipe wall where the Newtonian like-behavior is presented. Finally, applying in situ visualization technique assures a better rheological characterization and accurate description of the flow conditions for fluids with complex behavior. [ABSTRACT FROM AUTHOR]

Because they are sensitive to mechanical properties of materials and can propagate even in opaque systems, acoustic waves provides us with a powerful tool for local rheological characterization of various systems. While most common acoustic techniques rely on time-of-flight measurements, acoustic spectra of speed and attenuation contain rich information on the propagation medium which led to the development of acoustic spectroscopy techniques. They however remain underused in the field of complex fluids because of the difficulty to interpret quantitatively acoustic signals. In this note, we use a simple ultrasound spectroscopy set up to investigate the gelation dynamics by precipitation of a silica colloidal gel. First, we show that a simple analysis of acoustic attenuation allows to define a gelation time, which is proportional to this obtained with rheological measurements. This validates the possibility to use acoustic spectroscopy for monitoring of gelation process, and our setup also has the possibility to perform mappings, showing that the formed gels display some heterogeneity. By studying in more detail acoustic spectra, we finally attempt to relate more precisely acoustic measurements with mechanical parameters of the material. [ABSTRACT FROM AUTHOR]

3D printing is changing the way we conceive, design, and build 3D objects in mechanical, biomedical, aerospace, construction, automotive and maritime industries. In the current work, the nonlinear rheological behaviour of polymer melts is measured through a table-top 3D printer (3D RheoPrinter) that, smartly modified, allows inline investigation of viscosity, extrudate swell and melt fracture. By using a piezoresistive mini-transducer, the innovative system is designed to be applicable to all Fused Deposition Modelling (FDM) 3D printers by a simple and cost-effective modification of a state-of-art nozzle. The measurements of the nonlinear rheological behaviour are compared with traditional, rotational rheology. Two biodegradable polymers, i.e. polylactic acid and polycaprolactone, are investigated as model systems to test the 3D RheoPrinter. The results of the shear viscosity and the first normal stress difference coefficient, as function of shear rate, show a good agreement between the 3D RheoPrinter and rotational rheometer with an error of about 6% for a confidence interval of 96%. Moreover, the 3D RheoPrinter can still be used as 3D printer. In the last part of this work, it is presented a printing test for building 3D structures in which the results show controllable resolution by means of the measured rheological information such as the extrudate swell. The vision of this work is that an inline rheological characterization, possible with the developed 3D RheoPrinter, can enable automatic process optimization and quality assurance to the 3D printing community. The social and scientific impacts of this work are maximized by the cost-efficiency and simplicity of the design that makes it within reach of the general public. The 3D RheoPrinter opens for a rheological experimentation to a broad audience and it offers important insights to bring FDM to the next level of resolution. [ABSTRACT FROM AUTHOR]

A myriad of empirical and phenomenological constitutive models that describe different observed rheologies of complex fluids have been developed over many decades. With each of these constitutive models' strength in recovering different rheological responses, algorithms that allow the data to automatically select the appropriate constitutive relations are of great interest to rheologists. Here, we present a rheology-informed neural network (RhINN) that enables robust model selection based on available experimental data with minimal user intervention. We train our RhINN on a series of experimental data for different complex fluids and show that it is capable of finding the appropriate model with the lowest number of fitting parameters for each data set. Finally, we show that uniform selection of a handful of data over the entire accessible shear rates does not affect the RhINN's accuracy, while providing a specific range of data (and omitting the rest) results in an erroneous model determination. [ABSTRACT FROM AUTHOR]

An innovative experimental apparatus for the direct measurement of yield stress was conceived and realized. It is based on a torsion pendulum equipped with a magnetic dipole and a rotating cylinder immersed in the material to be investigated. The pendulum equilibrium state depends on the mechanical torque applied due to an external magnetic induction field, elastic reaction of the suspension wire, and shear yield stress. Experimental results are reported showing that the behavior of the pendulum rotation angle, in different equilibrium conditions, provides evidence of the yield stress presence and enables its evaluation by equilibrium equations. The dependence on time of the equilibrium approach was also studied, contributing to shed light on the relaxation effect in the transition from a fluid-like to solid-like behavior, as well as on the eventual thixotropic effects in non-Newtonian fluids. The validity of the proposed technique and related experimental apparatus was tested in aqueous Carbopol solutions, with different weight percentages. The linear procedure, combined with the effectiveness and reliability of the proposed experimental method, candidates it to be used for the study of peculiar behaviors of other yield stress complex fluid such as blood, crude waxy oils, ice slurries, and coating layer used in the food industry and also for fault sliding in geodynamics. [ABSTRACT FROM AUTHOR]

In view of the increasing attention to the time responses of complex fluids described by power-laws in association with the need to capture inertia effects that manifest in high-frequency microrheology, we compute the five basic time-response functions of in-series or in-parallel connections of two elementary fractional derivative elements known as the Scott-Blair (springpot) element. The order of fractional differentiation in each Scott-Blair element is allowed to exceed unity reaching values up to 2 and at this limit-case the Scott-Blair element becomes an inerter—a mechanical analogue of the electric capacitor that its output force is proportional only to the relative acceleration of its end-nodes. With this generalization, inertia effects may be captured beyond the traditional viscoelastic behavior. In addition to the relaxation moduli and the creep compliances, we compute closed-form expressions of the memory functions, impulse fluidities (impulse response functions) and impulse strain-rate response functions of the generalized fractional derivative Maxwell fluid, the generalized fractional derivative Kelvin-Voigt element and their special cases that have been implemented in the literature. Central to these calculations is the fractional derivative of the Dirac delta function which makes possible the extraction of singularities embedded in the fractional derivatives of the two-parameter Mittag-Leffler function that emerges invariably in the time-response functions of fractional derivative rheological models. [ABSTRACT FROM AUTHOR]

The objective of this study is to discuss the micro rheology of mucus (Jeffrey nanofluid) which is a complex biological fluid that protects lungs from pollutants, bacteria, and allergens that can be inhaled during the breathing process. To see the insight of pollutants and effect of surrounding temperature in the mucus, momentum, energy, and concentration equations are modeled with the help of metachronal wave formed by cilia beating. The governing system of equations are modeled in the wave and fixed frame and simplified by the lubrication approach. The velocity profile for recovery and effective stroke is compared and it is analyzed that effective stroke possess high magnitude of velocity when compared with the recovery stroke. The flow of mucus with pollutants and surrounding temperature is calculated with homotopy perturbation method and software "MATHEMATICA." The results within the given domain are convergent under the different parameters appearing into the system of equations. The physical interpretation of involved parameters is explained through graphs. [ABSTRACT FROM AUTHOR]

Complex flow fields including high elongational deformation occur in numerous industrial processes such as spraying, coating, fiber spinning, and screen or inkjet printing. Fully exploiting the potential of these technologies suffers from a lack of knowledge regarding how elongational flow properties of the processed fluids affect the results of these operations. Here, we present two strategies that allow for varying the elongational flow behavior independent of shear rheology. First, two acrylic thickener solutions that differ with respect to the fraction of hydrophobic co-monomers and hence with respect to their degree of inter- and intramolecular hydrophobic association were mixed to vary the elongational relaxation time, as determined using capillary breakup elongational rheometry (CaBER), by almost two orders of magnitude without affecting shear viscosity of these solutions in a wide, processing-relevant shear rate range. Second, a substantial increase in the elongational flow resistance was achieved by adding a small amount of plate-like particles without affecting the shear viscosity of these thickener solutions. A fourfold increase of the elongational relaxation time was observed upon the addition of 3.5 vol.% of glass flakes to such a highly shear-thinning system. A similar effect was also observed for an industrial waterborne automotive basecoat due to added aluminum flakes. This work may be useful for product development since the control of extensional viscosity can improve technological applications, and the introduced model systems may therefore be used for systematic, goal-oriented investigations of the relevance of elongational flow properties in the technological processes mentioned above. [ABSTRACT FROM AUTHOR]

Complex fluids are widely used in formulated products to impart rheological properties like stability, performance, and aesthetics. Yield stress fluids are a particularly important example, enabling applications like particle suspension, surface coating, and therapeutic delivery. Recent work has shown that particle suspension in yield stress fluids can be a strong function of yielding and flow heterogeneities, especially in anisotropic fiber dispersions. This work uses a microbubble technique to study the deformation before and during yielding of microfibrous cellulose suspensions. We note significant variations in suspension performance as a result of fiber rearrangement and heterogeneities. Strong strain rate dependencies are found to vary local network yield strength, and confocal microscopy quantifies structural reinforcement and deformation rate effects. The observed behavior indicates a two-fluid interpretation may help interpret sparse network flow and suspension properties. [ABSTRACT FROM AUTHOR]

Nonlinear rheometry of interfaces is very challenging because of the limits of current day instrumentation and the intricate coupling of flows at interfaces and in the bulk. The use of time periodic flows may constitute a first step in addressing this issue. Fourier transform rheology (FTR) measurements with quasi-monolayers at the air-water interface are used in order to assess the suitability of the different devices to investigate nonlinear interfacial shear viscoelasticity. The probe material was a poly (methyl methacrylate) forming a soft glassy interface, whereas complementary measurements were performed with a polystyrene latex suspension forming a predominantly viscous interface at intermediate surface coverages. The obtained data with the magnetic rod rheometer (ISR) were compared against those obtained with the bicone and the double wall ring geometries attached to standard rotational rheometers. In particular, an unexpected appearance of even in addition to odd harmonics is discussed in terms of flow field asymmetry.Graphical abstract [ABSTRACT FROM AUTHOR]

Shear banding occurs in complex fluids that exhibit a non-monotonic constitutive instability, such as wormlike micelles, and potentially also in polymeric fluids with presumably monotonic constitutive behavior. However, velocity profiles for shear thinning fluids in geometries possessing a stress gradient, such as Taylor-Couette flow, could be misidentified as shear banding. To address this, we present a model-free experimental procedure to distinguish shear banding from strong shear thinning using high-resolution velocimetry. The approach is developed and validated using simulations using the d-Giesekus model and is based upon the behavior of the width of the apparent interface between the high and low shear rate regions. It is then tested using experimental data for model wormlike micellar solutions. The method allows shear banding to be distinguished from shear thinning in cases where this difference is otherwise indistinguishable. As a by-product, it also provides an estimate of the stress diffusivities for shear banding fluids. [ABSTRACT FROM AUTHOR]

In this work, the micro-macro approach is used to simulate the flow of dilute polymer solutions by means of a kinetic model coupled with the dynamics of a transient network. The transient network modeling is based on the original formulation, in which the kinetics of microstates describes the complexity of interactions among the macromolecules suspended in a Newtonian solvent (Rincón et al, J Non-Newton Fluid 131:64-77, 2005). The average concentration of microstates, at a given time, defines a variable maximum segment length (variable extensibility) of the molecular FENE model. The non-Newtonian contribution to the extra stress tensor is computed according to the Brownian configuration-fields method. Comparisons with the Oldroyd-B model validates its limiting behavior. Numerical results show the influence of the solvent to total viscosity ratio and shear rate, on the transient and steady rheological phenomena of complex fluids with microstates. [ABSTRACT FROM AUTHOR]

A stability analysis of planar shear flow shear of a homogeneous, complex fluid predicts that shear banding instabilities can grow in fluids with a shear thinning strength above W = 1 and dampen out in fluids with W < 1. The shear thinning strength, $$ W\left(\dot{\gamma}\right)=-\kern0.32em \partial \kern0.5em \log \eta /\partial \kern0.5em \log \dot{\gamma} $$ , arises naturally as the lead material function for the stability analysis of shear thinning fluids. The onset of shear banding is modeled as shear-thinning instability, which is attributed to anomalously strong shear thinning. Not considered here are inertial or elastic instabilities. In lack of suitable viscosity data from experiments, a Carreau powerlaw fluid and a Carreau-Yasuda powerlaw fluid serve as testbeds for the W-criterion. The analysis shows that the limiting high shear viscosity, η , plays an important role in shear banding and that the ratio of the limiting high shear viscosity and zero shear viscosity, η / η , has to be sufficiently small for shear banding to occur. The main purpose of this brief communication is to share this new stability criterion. Extensive testing is still needed and is planned for future study. [ABSTRACT FROM AUTHOR]

Teresa has supervised several PhD and MSc students, disseminating her interest and knowledge on rheology to many others. Graph Professor Maria Teresa Cidade graduated in Chemical Engineering at Instituto Superior Técnico, University of Lisbon (IST/UL, 1983) and obtained a PhD from the School of Science and Technology, NOVA University Lisbon (NOVA, 1994). Teresa disseminated and spread her profound knowledge and good practices to many institutions, in Rheology, in Materials Science and Engineering and in associated areas. [Extracted from the article]

In this work, the rheological behavior of complex fluids is analyzed with a model based on the classical transient network formulation, in which the description of nonlinear viscoelasticity and time-dependent phenomena considers spatial and temporal variations of the entanglement density in the flow region. The entropic law of the segments that join entanglement points of macromolecules (or dispersed phase) is modeled with a Warner spring law with variable extensibility. The structure modification is described with a function that is dependent of a kinetic process that involves the formation of a more entangled microstate on one extreme, and a flow-induced degradation of the transient network with variable entanglement density on the other extreme (disentangled microstate). Predictions of the model under simple shear, inception of shear flow, stress relaxation, interrupted shear, and shear-thickening are compared with those of other models and with experiments. [ABSTRACT FROM AUTHOR]

In recent years, large amplitude oscillatory shear (LAOS) has become a prime tool to investigate the nonlinear rheology of complex fluids. More specifically, most studies use LAOS data as a macroscopic probe for hypothetical microscopic changes, sometimes successfully. Nevertheless, we would like to raise awareness on the potential impact of secondary flows triggered by instabilities in LAOS performed in curved shear flows exemplified by the Taylor-Couette geometry. First, we show that even for Newtonian fluids, where no micro-structural changes are expected, complex flow patterns can emerge in the LAOS data. Second, we stress the potential impact of similar effects in complex fluids by studying LAOS flows of a well-known shear-banding surfactant solution. Our hope is that our thorough study of the Newtonian case together with preliminary experiments on a complex fluid will reinforce the already successful analogy existing between inertial and elastic instabilities, suggest caution for future LAOS studies focused solely on a structural perspective, and open new research avenues combining flow instabilities and unsteady flows in complex fluids. [ABSTRACT FROM AUTHOR]

The use of nonstandard geometries like the vane is essential to measure the rheological characteristics of complex fluids such as non-Newtonian fluids or particle dispersions. For this geometry which is of Couette type, there is no analytical simple model defining the relation between the shear stress and the torque or relating the angular velocity to the shear rate. This study consists on calibrating a nonstandard vane geometry using a finite volume method with the Ansys Fluent software. The influence of geometrical parameters and rheological characteristics of the complex fluids are considered. First, the Newtonian fluid flow in a rotative vane geometry was simulated and a parametric model is derived therefrom. The results show an excellent agreement between the calculated torque and the measured one. They provide the possibility to define equivalent dimensions by reference to a standard geometry with concentric cylinders where the relationships between shear stress (resp. shear rate) and the torque (resp. the angular rotation) are classical. Non-Newtonian fluid flows obeying a power law rheology with different indices were then simulated. The results of these numerical simulations are in very good agreement with the preceding Newtonian-based model in some ranges of indices. The absolute difference still under 5 % provided the index is below 0.45. Finally, this study provides a calibration protocol in order to use nonstandard vane geometries with various heights, gaps, and distance to the cup bottom for measuring the rheology of complex fluids like shear thinning fluids and concentrated suspensions. [ABSTRACT FROM AUTHOR]

We present a comprehensive review of the current state of fracture phenomena in transient networks, a wide class of viscoelastic fluids. We will first define what is a fracture in a complex fluid and recall the main structural and rheological properties of transient networks. Secondly, we review experimental reports on fractures of transient networks in several configurations: shear-induced fractures, fractures in Hele-Shaw cells, and fracture in extensional geometries (filament-stretching rheometry and pendant drop experiments), including fracture propagation. The tentative extension of the concepts of brittleness and ductility to the fracture mechanisms in transient networks is also discussed. Finally, the different and apparently contradictory theoretical approaches developed to interpret fracture nucleation will be addressed and confronted to experimental results. Rationalized criteria to discriminate the relevance of these different models will be proposed. [ABSTRACT FROM AUTHOR]

The Responsive Particle Dynamics model is a very efficient method to account for the transient forces present in complex fluids, such as solutions of entangled polymers. This coarse-grained model considers a solution of particles that are made of a core and a corona. The cores typically interact through conservative interactions, while the coronae transiently penetrate each other to form short-lived temporary interactions, typically of entropic origin. In this study, we reformulate the resulting rheological model within the general framework of nonequilibrium thermodynamics called General Equation for the Nonequilibrium Reversible-Irreversible Coupling. This allows us to determine the consistency of the model, from a mechanistic and thermodynamic point of view, and to isolate the reversible and irreversible contributions to the dynamics of the model system. [ABSTRACT FROM AUTHOR]

We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in a cone-plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results for flow in the cone-plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively deformed into the non-linear regime-this onset closely coincides with the appearance of higher harmonics in the periodic stress signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients e and v, which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically, we observe that at large strain amplitudes ( γ ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding, the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have a measurable effect on the stability boundaries of the shear-banded flow. [ABSTRACT FROM AUTHOR]

Traditional torsional resonators, often obtaining the viscoelastic moduli of complex fluids only at one or several given discrete frequencies, lack the continuously varying frequency capability. This is an obvious disadvantage of the traditional torsional resonator technique. This paper presents an improved strategy, based on our previous discrete-frequency-measuring method (Wang et al., J Rheol 52:999-1011, ), to overcome such restriction and thus accomplish the continuously varying frequency capability of the traditional torsional resonator for measuring the viscoelastic properties of complex fluids. The feasibility of this strategy is demonstrated with the Newtonian fluids (several water-glycerol solutions) of viscosities varying from 10 to 1,400 cp by using our homemade torsion resonator apparatus in the 10 ~ 2,500 rad/s frequency range (continuous frequencies). Some results for typical viscoelastic polymers (two polyethylene oxide (PEO) aqueous solutions) are also given. Additionally, a comparison of the PEO results is made with the common rheometer technique. It is demonstrated that this improved strategy could enable the traditional torsional resonators, with one oscillating resonance mode, to work as the microrheological technique and the common rheometer technique in the continuous frequency range. [ABSTRACT FROM AUTHOR]

The rheological properties of plastic and viscoplastic complex fluids have been investigated using constant velocity squeeze flow rheometry at small gaps in order to examine the influence of the resulting flow confinement. The fluids investigated were aqueous carbopol suspensions and three commercial products (Tesco English mustard, Tesco value lemon curd and Heinz tomato ketchup (HTK)). The bulk rheological properties were measured using parallel plate rheometry. Rough plates were used to eliminate wall slip in both rheometric configurations. The commercial products are Herschel--Bulkley fluids and, for gaps less than a critical value of ~200 µm, the yield stresses tended to large values. There was a corresponding trend to small values for the flow consistencies except for HTK, which has a negligibly small bulk value. A possible explanation is that the micro-structural correlation lengths of these fluids are of the order of that of the critical separation. The yield stresses of the carbopol suspensions also increased sharply at the critical gap but then decreased gradually with decreasing gap separation. The transition may correspond to the formation of a jammed state with the subsequent plastic flow at smaller gap separations arising from localised shearing. [ABSTRACT FROM AUTHOR]

Abstract  We review and compare the phenomenological aspects and physical origin of shear localization and shear banding in various material types, namely emulsions, suspensions, colloids, granular materials, and micellar systems. It appears that shear banding, which must be distinguished from the simple effect of coexisting static-flowing regions in yield stress fluids, occurs in the form of a progressive evolution of the local viscosity toward two significantly different values in two adjoining regions of the fluids in which the stress takes slightly different values. This suggests that from a global point of view, shear banding in these systems has a common physical origin: Two physical phenomena (for example, in colloids, destructuration due to flow and restructuration due to aging) are in competition, and depending on the flow conditions, one of them becomes dominant and makes the system evolve in a specific direction. [ABSTRACT FROM AUTHOR]

Abstract  In this work, the rectilinear Poiseuille flow of a complex liquid flowing in a vibrating pipe is analyzed. The pipe wall performs oscillations of small amplitude that can be adequately represented by a weakly stochastic process, for which a quasi-static perturbation solution scheme is suggested. The flow is analyzed using the Bautista–Manero–Puig constitutive equation, consisting on the upper-convected Maxwell equation coupled to a kinetic equation to account for the breakdown and reformation of the fluid structure. A drastic enhancement of the volumetric flow is predicted in the region where the fluid experiences pronounced shear-thinning. Finally, flow enhancement is predicted using experimental data reported elsewhere for wormlike micellar solutions of cetyl trimethyl ammonium tosilate. [ABSTRACT FROM AUTHOR]

Abstract  In this review, I present an idiosyncratic view of the current state of shear banding in complex fluids. Particular attention is paid to some of the outstanding issues and questions facing the field, including the applicability of models that have “traditionally” been used to model experiments; future directions and challenges for experimentalists; and some of the issues surrounding vorticity banding, which has been discussed theoretically and whose experiments are fewer in number yet, in many ways, more varied in character. [ABSTRACT FROM AUTHOR]

The effect of shear on electrical conductivity (rheo-conduction) is studied to give information about particle behaviour in suspensions. Past work is reviewed, and expressions are derived for the rheo-conduction of a suspension of nonconducting spheroids in a conducting matrix for current flow, parallel and normal to the suspension flow direction. A simple apparatus to study rheo-conduction in pipe flow is described, and measurements of steady and time-dependent effects are reported for various suspensions of colloidal particles. Suspensions of anisometric rod- and platelike particles at low concentrations showed rheo-conductive changes of sign, magnitude and relaxation that were consistent with the particle shape, concentration and interactions. The rheo-conductive response decreased with increasing volume fraction for platelike kaolinite particles, attributed to orientational jamming. Spherical latex particles gave unexpected rheo-conductive changes consistent with shear disruption of a conductive network of particles. It is concluded that rheo-conduction measurements are a useful adjunct to conventional rheometry. [ABSTRACT FROM AUTHOR]

We obtain the linear viscoelastic shear moduli of complex fluids from the time-dependent mean square displacement, <Δ r2( t)>, of thermally-driven colloidal spheres suspended in the fluid using a generalized Stokes–Einstein (GSE) equation. Different representations of the GSE equation can be used to obtain the viscoelastic spectrum, G˜( s), in the Laplace frequency domain, the complex shear modulus, G*(ω), in the Fourier frequency domain, and the stress relaxation modulus, G r( t), in the time domain. Because trapezoid integration ( s domain) or the Fast Fourier Transform (ω domain) of <Δ r2( t)> known only over a finite temporal interval can lead to errors which result in unphysical behavior of the moduli near the frequency extremes, we estimate the transforms algebraically by describing <Δ r2( t)> as a local power law. If the logarithmic slope of <Δ r2( t)> can be accurately determined, these estimates generally perform well at the frequency extremes. [ABSTRACT FROM AUTHOR]

The time-dependent transformation of an ionically charged lamellar phase ( Lα-phase) into a vesicle phase under the influence of shear is investigated using rheological and conductivity measurements. The Lα-phase consists of the zwitterionic surfactant tetradecyldimethylaminoxide (C14DMAO), hexanol, oxalic acid and water. The experiments were carried out on the Lα-phase in a well defined state. It was prepared by a special route from the neighbouring L3-phase that consists of 100 mM C14DMAO, 250 mM hexanol and 5 mM oxalicdiethylester (OEE). The OEE hydrolyses in the L 3-phase to oxalic acid and ethanol. The result is a virgin Lα-phase which consists of stacked bilayers and which has not been exposed to shear. When this low-viscous phase is subjected to shear it is transformed into a highly viscous vesicle phase. The transformation of the Lα-phase into vesicles under constant shear was monitored by recording the viscosity and conductivity with time. It is observed that at least three different time constants can be distinguished in the transformation process. The conductivity passes through a minimum (τ1) in the direction of shear. The viscosity first passes through a minimum (τ2) and then over a maximum (τ3). It is concluded that τ1 belongs to the complete alignment of the bilayer parallel to the wall, τ2 to the beginning of the break-up of the bilayers to the vesicles and τ3 to the complete transformation of the Lα- to the vesicle phase. When the shear rate was varied, it was noted that the product of the time constants and shear is constant. [ABSTRACT FROM AUTHOR]