Showing papers on "Rheometer published in 2018"

Distinguishing dynamic and static yield stress of fresh cement mortars through thixotropy

Abstract: The dynamic and static yield stress of fresh cement mortar were measured in a rotational rheometer with a vane geometry using shear rate and shear stress-controlled protocols, respectively. Through a shear rate-controlled steady-state protocol, the equilibrium flow curve is measured and fitted with the Bingham model to obtain dynamic yield stress. A negative slope in the equilibrium flow curve, shear banding and stick-slip phenomena are observed and discussed. Through a stress-controlled creep-recovery protocol, viscosity bifurcation behavior is captured and static yield stress is marked as the creep stress when the bifurcation occurs. Finally, the discrepancy between dynamic and static yield stress is tied to thixotropy.

Effect of Particle Size and Cohesion on Powder Yielding and Flow

Abstract: The bulk properties of powders depend on material characteristics and size of the primary particles. During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The measurement of powder yield and flow properties is still a challenge and will be addressed in this study. In the framework of the collaborative project T-MAPPP, a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals: i) to test the reproducibility/consistency among different shear devices and testing protocols; ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion. The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 μm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 μm, the bulk cohesion increases slightly, while the bulk friction increases considerably—presumably due to particle interlocking effects—up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations.

How viscoelastic is human blood plasma

Abstract: Blood plasma has been considered a Newtonian fluid for decades. Recent experiments (Brust et al., Phys. Rev. Lett., 2013, 110) revealed that blood plasma has a pronounced viscoelastic behavior. This claim was based on purely elastic effects observed in the collapse of a thin plasma filament and the fast flow of plasma inside a contraction-expansion microchannel. However, due to the fact that plasma is a solution with very low viscosity, conventional rotational rheometers are not able to stretch the proteins effectively and thus, provide information about the viscoelastic properties of plasma. Using computational rheology and a molecular-based constitutive model, we predict accurately the rheological response of human blood plasma in strong extensional and constriction complex flows. The complete rheological characterization of plasma yields the first quantitative estimation of its viscoelastic properties in shear and extensional flows. We find that although plasma is characterized by a spectrum of ultra-short relaxation times (on the order of 10-3-10-5 s), its elastic nature dominates in flows that feature high shear and extensional rates, such as blood flow in microvessels. We show that plasma exhibits intense strain hardening when exposed to extensional deformations due to the stretch of the proteins in its bulk. In addition, using simple theoretical considerations we propose fibrinogen as the main candidate that attributes elasticity to plasma. These findings confirm that human blood plasma features bulk viscoelasticity and indicate that this non-Newtonian response should be seriously taken into consideration when examining whole blood flow.

Investigating the performance, chemical, and microstructure properties of carbon nanotube-modified asphalt binder

Abstract: Carbon nanotubes (CNT) hold the potential to enhance the performance of construction materials such as asphalt binders and mixtures. With the goal of designing long-life asphalt pavements, this research report investigates the laboratory performance of CNT-modified asphalt. Mixing combination conditions including shearing time and shearing rate are studied in the first step. Then modified asphalts with different CNT contents are evaluated by penetration, softening point, and ductility indexes. Storage stability of modified asphalt is accessed using the separation test. After that, aged asphalts are obtained by using the thin film oven test (TFOT). Samples of base and CNT-modified asphalts before and after ageing are characterised by running the rotational viscosity (RV) test, dynamic shear rheometer (DSR) test, and bending beam rheometer (BBR) test. Furthermore, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) are employed to characterise the functional groups and microstru...

Characterization of Cell Damage and Proliferative Ability during and after Bioprinting.

Abstract: When a biomaterial solution containing living cells is subject to bioprinting, the cells experience process-induced stresses, including shear and extensional stresses. These process-induced stresses breach cell membranes and can lead to cell damage, thus reducing cell viability and functioning within the printed constructs. Studies have been conducted to determine the influence of shear stress on cell damage; however, the effect of extensional stress has been typically ignored in the literature until the recently collected evidence of its importance. This paper presents a novel method to characterize and quantify the cell damage caused by both shear and extensional stresses in bioprinting. In this method, cell damage law is first established to relate cell damage to shear stress based on the experiments with a rheometer; the process-induced shear stress experienced by cells in bioprinting is represented, and the established cell damage model is applied to calculate the degree of cell damage caused by shear stress in bioprinting; then cell damage caused by extensional stress is inferred from the difference between the total cell damage and the amount of cell damage attributed to shear stress. With the obtained magnitude of extensional stress from fluidic simulation, the model that relates extensional stress to cell damage is established; the bioprinting process-induced cell damage attributed to both shear and extensional stresses is therefore presented. Schwann cells and myoblasts were used as examples to validate the models. Comparison between experimental and simulation results shows the effectiveness of the models presented in this paper. Moreover, the viability and proliferative ability of cells in the first 72 h after bioprinting is investigated, with the results illustrating that the process-induced forces affect not only cell viability but also their proliferative ability after bioprinting.

Out-of-Plane 3D-Printed Microfibers Improve the Shear Properties of Hydrogel Composites.

Abstract: One challenge in biofabrication is to fabricate a matrix that is soft enough to elicit optimal cell behavior while possessing the strength required to withstand the mechanical load that the matrix is subjected to once implanted in the body. Here, melt electrowriting (MEW) is used to direct-write poly(e-caprolactone) fibers "out-of-plane" by design. These out-of-plane fibers are specifically intended to stabilize an existing structure and subsequently improve the shear modulus of hydrogel-fiber composites. The stabilizing fibers (diameter = 13.3 ± 0.3 µm) are sinusoidally direct-written over an existing MEW wall-like structure (330 µm height). The printed constructs are embedded in different hydrogels (5, 10, and 15 wt% polyacrylamide; 65% poly(2-hydroxyethyl methacrylate) (pHEMA)) and a frequency sweep test (0.05-500 rad s-1 , 0.01% strain, n = 5) is performed to measure the complex shear modulus. For the rheological measurements, stabilizing fibers are deposited with a radial-architecture prior to embedding to correspond to the direction of the stabilizing fibers with the loading of the rheometer. Stabilizing fibers increase the complex shear modulus irrespective of the percentage of gel or crosslinking density. The capacity of MEW to produce well-defined out-of-plane fibers and the ability to increase the shear properties of fiber-reinforced hydrogel composites are highlighted.

Introducing a soybean oil-derived material as a potential rejuvenator of asphalt through rheology, mix characterisation and Fourier Transform Infrared analysis

Abstract: The interest in rejuvenators has been growing rapidly in the past few years due to their ability to restore aged binders to their unaged state and the availability of aged recycled materials. The introduction of rejuvenators has made it possible to produce asphalt mixes with high recycled asphalt pavement (RAP) content. Typically rejuvenators need to be added to the RAP bitumen in high dosages, more than 10%, to achieve the desired effect. In this work a new soybean-derived additive is introduced as a rejuvenator, at 0.75% by weight of the bitumen. The effect of adding the soybean-derived rejuvenator on the rheological properties of both a performance grade (PG) 64-28 and a PG 58-28 bitumen is assessed using a dynamic shear rheometer, a bending beam rheometer and a rotational viscometer. Dynamic modulus specimens for both the control and modified blends were prepared using a mixing and compaction temperature of 120°C, as well as a temperature of 140°C, and master curves were constructed using test results...

Combined interfacial shear rheology and microstructure visualization of asphaltenes at air-water and oil-water interfaces

Abstract: Asphaltenes are surface-active polyaromatic molecules in crude oil that are known to deposit in pipelines or stabilize water droplets by flocculating at interfaces resulting highly viscous emulsions, leading to significant flow assurance problems. Commercial dispersants have been developed to disturb asphaltene aggregation to mitigate deposition, but their role on the interfacial properties of asphaltene films is unclear. In this study, we elucidate asphaltene interfacial rheology at air-water and oil-water interfaces at high and low asphaltene surface coverage and in the presence of dispersants. A modified Langmuir trough with double-wall ring rheometer is used to simultaneously visualize the microstructure of asphaltene interface and measure the rheological responses. Two surface coverages, 0.5 and 4 μg cm−2, show widely different rheological responses at air-water interfaces. Strong yielding behavior was observed for higher coverage while a less yielding behavior and wider linear viscoelastic regime we...

Fabrication and Characterization of Isotropic and Anisotropic Magnetorheological Elastomers, Based on Silicone Rubber and Carbonyl Iron Microparticles.

Abstract: This article focuses on studying the rheological behavior of isotropic and anisotropic magnetorheological elastomers (MREs), made of carbonyl iron microparticles dispersed into a silicone⁻rubber matrix by considering 20 and 30 wt % of microparticles. Sample sets were prepared for each composition, with and without the application of an external magnetic field. Experimental measurements of the material rheology behavior were carried out by a shear oscillatory rheometer at constant temperature, to determine both the shear storage modulus (G') and shear loss modulus (G'') for all characterized samples. Then, experimental data collected from the isotropic and the anisotropic material samples were used to plot the Cole-Cole diagrams to quantify the interfacial adhesion between carbonyl iron microparticles and the silicone-rubber matrix. Furthermore, the Fractional Zener Model (FZM) with two spring-pots in series is used for quantitative analysis of collected experimental data.

Interfacial rheology of low interfacial tension systems using a new oscillating spinning drop method.

Abstract: When surfactants adsorb at liquid interfaces, they not only decrease the surface tension, they confer rheological properties to the interfaces. There are two types of rheological parameters associated to interfacial layers: compression and shear. The elastic response is described by a storage modulus and the dissipation by a loss modulus or equivalently a surface viscosity. Various types of instruments are available for the measurements of these coefficients, the most common being oscillating pendent drops instruments and rheometers equipped with bicones. These instruments are applicable to systems with large enough interfacial tensions, typically above a few mN/m. We use a new type of instrument based on spinning drop oscillations, allowing to extend the interfacial rheology studies to low and ultralow interfacial tension systems. We present examples of measurements with systems of high and low tension, discuss the possible artifacts and demonstrate the capability of this new technique. We emphasize that the data shown for low interfacial tensions are the first reported in the literature. The instrument is potentially interesting for instance in enhanced oil recovery or demulsification studies.

Rheology of fumed silica nanoparticles/partially hydrolyzed polyacrylamide aqueous solutions under small and large amplitude oscillatory shear deformations

Abstract: Partially hydrolyzed polyacrylamide (HPAM) is one of the most widely used polymers for enhanced oil recovery operations. However, high temperature and high salinity in oil reservoirs restrict its functionality and performance. To alleviate this, incorporating fumed silica nanoparticles (NPs) in HPAM solutions was found to be very effective in harsh oil reservoir conditions to improve the efficiency of polymer flooding. Studying the flow behavior of hybrid polymer and fumed silica NP solutions under real reservoir conditions can be very challenging and hard to achieve due to continuously converging and diverging flow through porous structures. In this regard, rheological analysis of such systems under well-controlled flow histories within the capability of rotational rheometers can be of great importance to fully understand the mechanical response of these hybrid solution systems. In this study, two types of fumed silica NPs with different surface chemistries and two types of HPAM polymers with different molecular weights were dispersed/dissolved in deionized water. Linear viscoelastic properties of the hybrid solution systems were studied based on their step-stress (creep) and small amplitude oscillatory shear responses. As deformation in porous media can be rapid and large, consideration of nonlinear viscoelastic properties can be very crucial. The stress decomposition method and Lissajous–Bowditch curves were used to describe the intercycle and intracycle shear-thickening and strain-stiffening ratios quantitatively and qualitatively. In brief, linear and nonlinear rheology conjugated with thermogravimetric analysis and cryo-scanning electron microscopy imaging enabled us to characterize viscoelastic properties of the hybrid systems and link our observations to microstructural features. Through polymer bridging, the slightly hydrophobic fumed silica NPs (AEROSIL R816) had a unique ability to form interconnected, predominately elastic network structures in contrast to large agglomerated structures formed by highly hydrophilic AEROSIL 300. This has led to observing very different rheological behaviors, regardless of the HPAM polymer molecular weight, below and above a critical fumed silica NPs concentration.

Influence of Viscosity and Magnetoviscous Effect on the Performance of a Magnetic Fluid Seal in a Water Environment

Abstract: The magnetic fluid seal is one of the most successful applications of magnetic fluids. The theory of magnetic fluid seals in liquid environments has not been developed. This work mainly presents an experimental study of the influence of viscosity and magnetoviscous effects on the performance of a magnetic fluid seal in a water environment. Three engine oil–based magnetic fluids of different viscosities and similar saturation magnetization values were prepared and a multistage magnetic seal structure was designed. The magnetoviscous effect of the magnetic fluids under different working conditions was measured with an advanced rotational rheometer. An experimental platform and a multistage magnetic seal structure were designed for the critical pressure value and durability tests. The experimental results show that the viscosity of a magnetic fluid is a decisive factor in its sealing performance under a water environment and a discussion is presented that can explain the experimental results qualitat...

Polymer Injectivity: Investigation of Mechanical Degradation of Enhanced Oil Recovery Polymers Using In-Situ Rheology

Abstract: Water soluble polymers have attracted increasing interest in enhanced oil recovery (EOR) processes, especially polymer flooding. Despite the fact that the flow of polymer in porous medium has been a research subject for many decades with numerous publications, there are still some research areas that need progress. The prediction of polymer injectivity remains elusive. Polymers with similar shear viscosity might have different in-situ rheological behaviors and may be exposed to different degrees of mechanical degradation. Hence, determining polymer in-situ rheological behavior is of great significance for defining its utility. In this study, an investigation of rheological properties and mechanical degradation of different partially hydrolyzed polyacrylamide (HPAM) polymers was performed using Bentheimer sandstone outcrop cores. The results show that HPAM in-situ rheology is different from bulk rheology measured by a rheometer. Specifically, shear thickening behavior occurs at high rates, and near-Newtonian behavior is measured at low rates in porous media. This deviates strongly from the rheometer measurements. Polymer molecular weight and concentration influence its viscoelasticity and subsequently its flow characteristics in porous media. Exposure to mechanical degradation by flow at high rate through porous media leads to significant reduction in shear thickening and thereby improved injectivity. More importantly, the degraded polymer maintained in-situ viscosity at low flow rates indicating that improved injectivity can be achieved without compromising viscosity at reservoir flow rates. This is explained by a reduction in viscoelasticity. Mechanical degradation also leads to reduced residual resistance factor (RRF), especially for high polymer concentrations. For some of the polymer injections, successive degradation (increased degradation with transport length in porous media) was observed. The results presented here may be used to optimize polymer injectivity.

Effect of Carbon Black on the High and Low Temperature Properties of Bitumen

Abstract: In the present study, the effect of carbon black (CB) as an additive on resistance of bitumen to delay or prevent rutting and low temperature cracking was investigated. For this purpose, different amounts of CB (0, 5, 10, and 15 wt%) were added into bitumen with PG 58-28. Conventional tests (penetration, softening point, Fraass breaking point, ductility, and kinematic viscosity) and Superpave binder tests (rotational viscosity, dynamic shear rheometer, and bending beam rheometer test) were carried out to determine the physical and rheological properties of pure and modified bitumens. In addition, high and low temperature performance grades of pure and modified bitumens were identified according to Superpave binder specification. The results indicated that the addition of CB increased the stiffness and resistance of bitumen to rutting at high temperatures and resistance to thermal cracking at low temperatures of bitumen. Finally, the bitumen becomes more elastic and less susceptible to temperature changes.

High-shear rate rheometry of micro-nanofibrillated cellulose (CMF/CNF) suspensions using rotational rheometer

Abstract: Suspensions of cellulose micro- and nanofibrils are widely used in coatings, fibre spinning, 3D printing and as rheology modifiers where they are frequently exposed to shear rates > 104 s−1, often within small confinements. High-shear rate rheological characterisation for these systems is therefore vital. Rheological data at high-shear rates are normally obtained using capillary and microfluidic rheometers, which are found in relative scarcity within research facilities compared to rotational rheometers. Also, secondary flows and wall depletion prevalent at such high-shear rates often go unnoticed or unquantified, rendering the measurement data unreliable. Reliable high shear rate rheometry using rotational rheometers is therefore desirable. Suspension of TEMPO-oxidised CMF/CNF was tested for its high-shear rate rheological properties using parallel plate geometry at measurement gaps 150–40 µm and concentric cylinder at 1 mm gap. The errors from gap setting, radial dependence of shear stress and wall depletion were quantified and accounted for. Viscosity data from 0.1 to 30,000 s−1 shear rates was constructed using both geometries in agreement. Possibilities of secondary flows, radial migration of fluid and viscous heating were ruled out. Steady shear flow data of CMF/CNF suspension from 0.1 to 30,000 s−1 obtained using rotational rheometer

Empirical modeling of the viscosity of supercritical carbon dioxide foam fracturing fluid under different downhole conditions

Abstract: High-quality supercritical CO2 (sCO2) foam as a fracturing fluid is considered ideal for fracturing shale gas reservoirs. The apparent viscosity of the fracturing fluid holds an important role and governs the efficiency of the fracturing process. In this study, the viscosity of sCO2 foam and its empirical correlations are presented as a function of temperature, pressure, and shear rate. A series of experiments were performed to investigate the effect of temperature, pressure, and shear rate on the apparent viscosity of sCO2 foam generated by a widely used mixed surfactant system. An advanced high pressure, high temperature (HPHT) foam rheometer was used to measure the apparent viscosity of the foam over a wide range of reservoir temperatures (40–120 °C), pressures (1000–2500 psi), and shear rates (10–500 s−1). A well-known power law model was modified to accommodate the individual and combined effect of temperature, pressure, and shear rate on the apparent viscosity of the foam. Flow indices of the power law were found to be a function of temperature, pressure, and shear rate. Nonlinear regression was also performed on the foam apparent viscosity data to develop these correlations. The newly developed correlations provide an accurate prediction of the foam’s apparent viscosity under different fracturing conditions. These correlations can be helpful for evaluating foam-fracturing efficiency by incorporating them into a fracturing simulator.

Nonlinear viscoelastic characterization of human vocal fold tissues under large-amplitude oscillatory shear (LAOS).

Abstract: Viscoelastic shear properties of human vocal fold tissues were previously quantified by the shear moduli (G' and G″). Yet these small-strain linear measures were unable to describe any nonlinear tissue behavior. This study attempted to characterize the nonlinear viscoelastic response of the vocal fold lamina propria under large-amplitude oscillatory shear (LAOS) with a stress decomposition approach. Human vocal fold cover and vocal ligament specimens from eight subjects were subjected to LAOS rheometric testing with a simple-shear rheometer. The empirical total stress response was decomposed into elastic and viscous stress components, based on odd-integer harmonic decomposition approach with Fourier transform. Nonlinear viscoelastic measures derived from the decomposition were plotted in Pipkin space and as rheological fingerprints to observe the onset of nonlinearity and the type of nonlinear behavior. Results showed that both the vocal fold cover and the vocal ligament experienced intercycle strain softening, intracycle strain stiffening, as well as shear thinning both intercycle and intracycle. The vocal ligament appeared to demonstrate an earlier onset of nonlinearity at phonatory frequencies, and higher sensitivity to changes in frequency and strain. In summary, the stress decomposition approach provided much better insights into the nonlinear viscoelastic behavior of the vocal fold lamina propria than the traditional linear measures.

Two-dimensional Fe3O4/MoS2 nanocomposites for a magnetorheological fluid with enhanced sedimentation stability

Abstract: Superparamagnetic Fe3O4 nanoparticles were successfully deposited on the surface of MoS2 nanosheets (Fe3O4/MoS2) by a sonochemical method and the obtained Fe3O4/MoS2 nanocomposites were used as a promising candidate for a magnetorheological (MR) fluid. This MR fluid was prepared from the Fe3O4/MoS2 nanocomposites and its corresponding MR performances were examined using a rotational rheometer. The MR fluid based on Fe3O4/MoS2 showed typical MR effects with increasing viscosity, shear stress, yield stress and dynamic shear modulus depending on the applied magnetic fields. Compared with commercial carbonyl iron (CI) particles, the sedimentation stability of the Fe3O4/MoS2–MR fluid was greatly improved because of its unique two-dimensional structure and the reduced fluid–particle density mismatch. Therefore, the prepared Fe3O4/MoS2-based MR fluid with typical MR effects and good sedimentation stability would have great potential in practical applications.

Measuring and assessing first and second normal stress differences of polymeric fluids with a modular cone-partitioned plate geometry

Abstract: We propose a simple, robust method to measure both the first and second normal stress differences of polymers, hence obtaining the full set of viscometric material functions in nonlinear shear flow. The method is based on the use of a modular cone-partitioned plate (CPP) setup with two different diameters of the inner plate, mounted on a rotational strain-controlled rheometer. The use of CPP allows extending the measured range of shear rates without edge fracture problems. The main advantage of such a protocol is that it overcomes limitations of previous approaches based on CPP (moderate temperatures not exceeding 120 °C, multiple measurements of samples with different volume) and yields data over a wide temperature range by performing a two-step measurement on two different samples with the same volume. The method was tested with two entangled polystyrene solutions at elevated temperatures, and the results were favorably compared with both the limited literature data on the second normal stress difference and the predictions obtained with a recent tube-based model of entangled polymers accounting for shear flow-induced molecular tumbling. Limitations and possible improvements of the proposed simple experimental protocol are also discussed.

Rheological measurements for prediction of pumping and squeezing pressures of toothpaste

Abstract: Methods of non-Newtonian fluid mechanics are applied to predict processability and use of toothpaste, specifically its pumpability through long pipes and squeezability from tubes. The goal is to define procedures which would allow estimating pumping pressure and squeezing force from the data obtained on conventional rotational rheometers which are critical for product development and quality control. 11 commercially available and 28 lab-made model toothpastes are examined. The latter are engineered so as to cover wide range of toothpaste properties ranging from high yield stress products to weakly shear thinning ones by varying the concentration of polymers (xanthan gum and carboxymethyl cellulose) and particulate thickener (silica). Rotational rheometry is represented by narrow-gap Couette concentric cylinders and wide-gap vane-cup geometries. Equilibrium flow curves measured with both geometries are shown to agree, provided proper data processing is used. To assess pumping pressure in long pipes, a capillary rheometer equipped with pipes of different lengths is used. Thus measured pressure drop is compared to the pressures calculated from the equilibrium flow curves. It turns out that while the measurements agree quite well for pastes with higher concentration of silica, there is a systematic overestimation of pressure for pastes with lower concentration of silica and at lower flow rate. A possible explanation for this observation is the presence of wall slippage presumably caused by migration of silica from the pipe wall. This is found to be consistent for the calculations with slip boundary conditions. Vane-cup geometry is used to measure start-up curves from the state of rest which are more representative of tube squeezing during usage. A tube extrusion apparatus is used to quantify squeezability in terms of the force required to pull the tube through two rollers while squeezing toothpaste out of their tubes. Thus measured force correlates with the squeezing pressure calculated using the flow curves measured from the rest.

Stress bifurcation in large amplitude oscillatory shear of yield stress fluids

Abstract: Large amplitude oscillation shear has been an important method to investigate the yielding and flow behavior of yield stress materials. However, there are great uncertainties in determination of the yield stress from the shear stress (or shear strain) dependence of the apparent dynamic moduli or the relative harmonic intensity using Fourier transform rheology. The yield stress from these dynamic methods is also inconsistent with the steady shear and transient shear measurements. We propose a new method, namely, stress bifurcation, based on the geometric average of elastic and viscous Lissajous curves to study the yielding transition of different yield stress fluids in large amplitude oscillatory shear flow. The results prove that typical yield stress fluids such as concentrated emulsions, polymer nanocomposites, microgels, and particulate gels all exhibit stress bifurcations, both inter and intra cyclically, in large amplitude oscillatory shear experiments. Such stress bifurcation phenomena between the average stress-strain (or strain rate) curves are independent of the type of input signal, i.e., stress-controlled versus strain-controlled. A start yield stress (strain) (related to strain) and an end yield stress (strain rate) (related to strain rate), instead of a single critical variable, were suggested to characterize yielding transitions. The frequency dependences of critical stresses, critical strain, and critical strain rate determined by the new method were also investigated systematically for the different kinds of yield stress fluids. A visco-elastic-plastic model, the Kelvin-Voigt-Herschel-Bulkley model, was also adopted to understand the stress bifurcation and frequency dependencies of critical variables in large oscillatory shear flow.Large amplitude oscillation shear has been an important method to investigate the yielding and flow behavior of yield stress materials. However, there are great uncertainties in determination of the yield stress from the shear stress (or shear strain) dependence of the apparent dynamic moduli or the relative harmonic intensity using Fourier transform rheology. The yield stress from these dynamic methods is also inconsistent with the steady shear and transient shear measurements. We propose a new method, namely, stress bifurcation, based on the geometric average of elastic and viscous Lissajous curves to study the yielding transition of different yield stress fluids in large amplitude oscillatory shear flow. The results prove that typical yield stress fluids such as concentrated emulsions, polymer nanocomposites, microgels, and particulate gels all exhibit stress bifurcations, both inter and intra cyclically, in large amplitude oscillatory shear experiments. Such stress bifurcation phenomena between the av...