Steady Shear Measurements Research Articles

Effect of temperature on dynamic and steady-state shear rheological properties of Jambolan (Syzygium cumini) pulp

Jambolan is an exotic fruit rich in phenolic compounds with many health benefits. The study of fruit's rheological properties plays key roles in its industrial processing and obtainment of products. The present work evaluated the time-dependent and steady-state shear rheological properties of Jambolan pulp at several temperatures. The Jambolan pulp presented thixotropy as a time-dependent property and also showed shear thinning behaviour with yield stress in the steady shear measurements. The Figoni-Shoemaker and Hershel-Buckley models adequately described both the product rheological phenomena. During the oscillatory analysis, the pulp showed some viscoelastic properties, increasing when the oscillatory frequency (ω) increased. All previous parameters also verified the impact of the temperature. The increment in temperature resulted in a reduction in consistency levels. In the temperature range studied, the viscoelastic solid component presented higher values than the viscous component. The thixotropic phenomenon and the presence of the yield stress can be related to this. Viscoelastic moduli decreased with the increase in temperature. When relevant, the Arrhenius’ Activation Energy of some model parameters were calculated. The data obtained are applicable for future studies, to improve industrial process design, and to evaluate food properties.

Extrapolation of classical rheometry of plant protein pastes to extrusion conditions

This study explores alternative methods to measure the rheology of pastes based on soy and pea proteins, which are commonly used as starting material in extrusion for the preparation of meat analogs. Currently, it is still a challenge to obtain consistent rheological data on such materials for steady shear at conditions that are relevant for the extrusion of such systems. Hence, possibilities are explored for the extrapolation of results obtained with classical rheometers operating at temperatures below boiling point, to conditions that are relevant for the extrusion of such systems. The above was combined, with findings from off-line and in-line capillary rheometers, which can provide steady shear measurements above boiling point and at hydration levels as used in extrusion.We have analysed the consistency of our new results with previous oscillatory shear measurements, performed at conditions similar to those of meat analog extrusion. For validation of the extrapolation we have analysed whether the new rheology data follow similar scaling rules. As classical rheometers can handle only moderate torques, the measurements are limited to hydration levels higher than those used in extrusion (75–85%). Despite this limitation, strain sweeps with classical rheometers show that in this regime the elastic modulus also scales with Tg/T, the ratio of glass transition, and actual temperature. Yet, its dependency on Tg/T appears differently from measurements performed at extrusion-like conditions. Moreover, our current results indicate a strong thixotropic nature of plant protein pastes at temperatures below boiling point. Consequently, flow curves are only stable if performed from high to low shear rates. Nevertheless, our data can be mapped to a single master curve following the Herschel-Bulkley model, with parameters also following a scaling rule dependent on Tg/T.This Tg/T scaling is used to construct master curves with data from the off-line and in-line capillary rheometers. The obtained shift factors for the flow curves are in agreement with old literature data obtained for SPI, but they deviate significantly from our steady shear measurements with classical rheometers. Based on the results of our current and previous study we have a critical, general discussion on the possibility of extrapolation of classical rheometry of plant protein pastes to process conditions used for production of meat analogs.

Graphene and Nanoclay as Processing Aid Agents: A Study on Rheological Behavior in Polystyrene

The effectiveness of layered particles as processing aid agents in molten polystyrene was studied. Three graphene grades and two clays of different lateral size were selected for this purpose. The morphologies of the composites were observed using scanning electron microscopy. Steady shear measurements were carried out and the Carreau–Yasuda model with yield stress was applied to the experimental results. A decrease in viscosity was observed at 2 wt.% of particle content for almost all composites. The most efficient particle for reducing viscosity was found to be graphene in a loose agglomerated configuration. Graphene and clay particles with similar dispersion states had a similar effect on the viscosity, inducing a decrease by 29% and 22%, respectively, suggesting comparable efficiency as processing aid agents. The observed decrease in viscosity is attributed to the phenomenon of superlubricity, which is a lubricating mechanism that is closely linked to the atomic structure of the particles.

Flow induced attrition of cellulose nanocrystals

To study the potential impacts of shear stress on cellulose nanocrystals (CNCs), a microcapillary rheometer was employed to repeatedly shear approximately 10 mL of 6 wt% aqueous CNC suspension at 25 °C and rates ranging from 1,000 s−1 to 501,000 s−1. A 9 wt% CNC suspension was also tested at 316,000 s−1 for comparison of concentration effects on the behavior of the suspensions. After monitoring viscosity for 25 steady shear measurements, the suspensions processed at 1,000 s−1 decreased in viscosity by approximately 20 %. Higher shear rates produced smaller changes in viscosity, while increasing the concentration produced higher general viscosities. Atomic force microscopy (AFM) and X-ray diffraction (XRD) probed physical changes between the neat and sheared CNC samples. AFM images showed up to a 24 % reduction in length after shearing, but an insignificant reduction in cross-section. XRD showed a slight increase in the ratio of amorphous to crystalline fractions of the CNCs. Additionally, conductometric titration showed insignificant differences between neat and sheared samples. These findings suggest that viscosity changes in CNC suspensions during steady shear flow arise from physical fracturing of the CNCs perpendicular to their length, and not significantly from chemical degradation or reduction in residual amorphous content.

Miscibility studies on carboxymethyl chitosan and poly(N-vinylpyrrolidone) mixtures

The production of polymer mixtures is a widely used method to improve polymer performance, as such mixtures can combine advantageous properties from each component. In this study, mixtures based on carboxymethyl chitosan (CMCh) and poly(N-vinylpyrrolidone) (PVP) were characterized using steady shear measurements, viscometry, infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, and atomic force microscopy. Viscometry and steady shear studies were performed on solutions of the native polymers and their mixtures with various weight proportions (80/20, 50/50, and 20/80%w/w). The rheological tests revealed that the apparent viscosity of solutions of CMCh/PVP mixtures was higher than that of the native polymer solutions. The rheological data showed that CMCh solutions and their mixtures were typical pseudoplastic liquids, which could be accurately described by the Cross and power law models. Viscometric parameters were determined using the method proposed by Garcia et al., which indicated good miscibility between CMCh and PVP in aqueous solution. Furthermore, the morphology, structure, and thermal properties of CMCh films changed when PVP was added. The obtained analytical data showed the formation of stable mixtures of CMCh and PVP, with a high miscibility ratio between these polymers, through intermolecular interactions between the polymer chains.

Rod-climbing rheometry revisited.

The rod-climbing or "Weissenberg" effect in which the free surface of a complex fluid climbs a thin rotating rod is a popular and convincing experiment demonstrating the existence of elasticity in polymeric fluids. The interface shape and steady-state climbing height depend on the rotation rate, fluid elasticity (through the presence of normal stresses), surface tension, and inertia. By solving the equations of motion in the low rotation rate limit for a second-order fluid, a mathematical relationship between the interface deflection and the fluid material functions, specifically the first and second normal stress differences, emerges. This relationship has been used in the past to measure the climbing constant, a combination of the first (Ψ1,0) and second (Ψ2,0) normal stress difference coefficients from experimental observations of rod-climbing in the low shear rate limit. However, a quantitative reconciliation of such observations with the capabilities of modern-day torsional rheometers is lacking. To this end, we combine rod-climbing experiments with both small amplitude oscillatory shear (SAOS) flow measurements and steady shear measurements of the first normal stress difference from commercial rheometers to quantify the values of both Ψ1,0 and Ψ2,0 for a series of polymer solutions. Furthermore, by retaining the oft-neglected inertial terms, we show that the "climbing constant"  = 0.5Ψ1,0 + 2Ψ2,0 can be measured even when the fluids, in fact, experience rod descending. A climbing condition derived by considering the competition between elasticity and inertial effects accurately predicts whether a fluid will undergo rod-climbing or rod-descending. Our results suggest a more general description, "rotating rod rheometry" instead of "rod-climbing rheometry", to be more apt and less restrictive. The analysis and observations presented in this study establish rotating rod rheometry combined with SAOS measurements as a prime candidate for measuring normal stress differences in complex fluids at low shear rates that are often below commercial rheometers' sensitivity limits.

Thickening effects of Ca2+ on apple high-methoxyl pectin: Dependences on Ca2+ concentration and the degree of esterification

To explore the application potential of high-methoxyl pectin (HMP), the effects of Ca2+ (0.00%–0.50%, w/w) on the behavior of three apple HMPs with degrees of esterification (DE) of 60.6% (AP1), 66.1% (AP1) and 72.4% (AP3) were investigated. The results in zeta potential, turbidity, and fluorescence characteristics revealed that Ca2+ promoted the aggregation of HMP molecules. In steady shear measurements, the incorporation of Ca2+ substantially increased the viscosity and thixotropy of HMP systems and changed their shear-thinning behavior in a case-specific manner. In dynamic shear measurements, the frequency sweep revealed strong viscoelasticity of AP1 at Ca2+ concentrations of 0.35% and 0.50% (w/w) and defined other HMP-Ca2+ systems as weak gels or concentrated solutions, while the temperature sweep uncovered that the enthalpy-dominated elasticity of AP1-Ca2+ system, while the facultative enthalpic and entropic nature of the network formation in AP2-Ca2+ system. For the AP3-Ca2+ system, the viscous modulus was entropic, but the elastic modulus was first entropic and then enthalpic upon cooling. Fundamentally, the incorporation of Ca2+ substantially affected the micelles formation and network construction as well as the interactions involved in these systems. All of these changes highly depended both on the DE value of pectin and the concentration of Ca2+. The results concluded herein are expected to encourage the application of HMP in acid- and sugar-restricted food products.

A detailed and systematic study on rheological and physicochemical properties of rhamnolipid biosurfactant solutions

Biosurfactants are widely used in many industrial settings ranging from cosmetic to petroleum industries. Among various biosurfactants available in the market, rhamnolipid is a well-known bacterial biosurfactant produced by the Pseudomonas aeruginosa bacteria. However, despite its wide applications, no detailed and systematic study is available on the rheological characterization of this biosurfactant solution, which is an essential property to investigate for many practical applications. Therefore, this study aims to present a thorough and complete investigation of this biosurfactant's shear and extensional rheological behaviours. While steady shear and small amplitude oscillatory shear (SAOS) measurements were conducted to investigate the shear rheological behaviour, the dripping-onto-substrate (DoS) extensional rheometry technique was used to understand its extensional rheological behaviour. A chemically derived surfactant (cetyltrimethyl ammonium bromide (CTAB)) was also used in our analysis to show and discuss the qualitative and quantitative differences in their rheological behaviours. Along with the detailed rheological study, some studies on the physicochemical properties, such as surface tension, contact angle, particle size analysis, thermal stability, etc., were also conducted to make an overall comparison between the two surfactants. Both surfactants show strong shear-thinning and extensional hardening behaviors in shear and extensional rheological flows, respectively. However, the zero-shear rate viscosity and extensional viscosity are found to be larger for rhamnolipid surfactant solutions than for CTAB. The corresponding shear and extensional relaxation times also follow the same trend. Furthermore, the surface tension is found to be less, and the contact angle is found to be more for rhamnolipid biosurfactant than that for CTAB. Rhamnolipid shows more excellent thermal stability, particularly at high temperatures than CTAB. Therefore, the results and discussion presented in this study will help to choose the present rhamnolipid biosurfactant for any particular application, particularly where the knowledge of the rheological responses of a surfactant solution is essential.

Rheological Behavior of Concentrated GAP/ AP Propellant Suspensions Plasticized by Diglycerol Tetranitrate

AbstractConcentrated GAP/AP propellant suspensions with different solid contents and molecular masses of GAP were prepared, in which diglycerol tetranitrate (DGTN) was the energetic plasticizer. The steady shear measurements were conducted using a rheometer to examine the influences of component characteristics, particle gradation and temperature on rheological behavior. Results indicate that the GAP/DGTN mixture is Newtonian viscous and concentrated GAP/AP suspensions are pseudoplastic fluids. The Ostwald ‐ De Waele model and the Arrhenius ‐ de Guzman equation are utilized to compare the pseudoplastic behavior and the viscosity‐temperature dependence of concentrated suspensions, respectively. The apparent viscosity is exponentially related to solid content and the pseudoplastic behavior gradually increases with solid content revealed by the decreasing power law index (n). Additionally, larger particle size of AP, smaller molecular mass of GAP, or lower temperature make possible the reduced apparent viscosity. Suitable particle gradation reduces the apparent viscosity to the desired value.

Rheological properties of polysaccharides from Pholiota nameko with different temperature extraction: Concentration, pH, temperature, and saltion.

Cold and hot water extracted polysaccharides (CW-PNPs and HW-PNPs) were isolated from Pholiota nameko. The rheological properties of PNPs were investigated by steady shear and oscillatory rheological measurements. The PNPs exhibited typical non-Newtonian and shear-thinning behavior, which are affected by PNP concentration, temperature, pH value, salt ion, and concentration. Specifically, the apparent viscosity of the two PNPs solutions at concentration of 1% (w/w) was shown as HW-PNPs>CW-PNPs. The apparent viscosity of PNPs decreases under acidic and alkaline conditions and when the temperature rises; K+ and Na+ cause the apparent viscosity of CW-PNPs to decrease, while Ca2+ and Al3+ are opposite. The addition of four different salt ions all caused the apparent viscosity of the HW-PNPs to decrease. The results of dynamic rheological experiments show that G' and G″ showed slightly frequency dependency with G' exceeding G″ throughout the accessible range of frequency for CW-PNPs and HW-PNPs.

On the assessment of shear and extensional rheology of thickened liquids from commercial gum-based thickeners used in dysphagia management

In order to promote safe swallowing in dysphagia patients, it is of interest to study the rheological properties of thickened liquids from various commercial gum-based thickening powders using small amplitude oscillatory shear, steady shear and extensional measurements. The active hydrocolloid of the studied samples included xanthan gum, guar gum or tara gum. Shear rheology measurements showed that all samples exhibited shear thinning behaviour that can be described by the Cross model. With comparable viscosity at shear rate 50 s −1 , the xanthan gum-based samples exhibited higher zero shear viscosity and longer relaxation time than the guar gum and tara gum-based samples. Furthermore, they behaved as weak gels while the other samples showed viscoelastic liquid behaviour at IDDSI (International Dysphagia Diet Standardisation Initiative) level 1 to 3. Extensional rheology revealed that the breakup time of these samples were xanthan gum > guar gum > tara gum, suggesting more superior cohesiveness of xanthan gum-based compared to the others. • Water thickened with different types of thickeners can have similar shear viscosity at shear rate 50 s -1 but very different extensional viscosity. • Xanthan gum-based thickened water behaves as concentrated solutions with higher extensional viscosity than guar gum and tara gum-based samples. • Xanthan gum-based thickened water is less likely to fracture during swallowing compared to the guar gum and tara gum-based samples. • The characteristic fluid relaxation time (λ E ) may be used to describe the cohesiveness of thickened fluid/bolus. • Extensional rheology may be an essential tool to define a safe-swallow bolus.

Nonlinear rheological behavior of telechelic ionomers with the distribution of ionic stickers at the ends

This study examines the nonlinear rheology properties of telechelic ionomers with connecting ions at the chain ends. Our previous studies revealed that these telechelic ionomers relaxed in a hierarchical way depending on the average number of connecting ionic groups per chain end m. The terminal relaxation time increased with m because the dissociation was slower for the chain end with more connecting ions. In this study, the stress growth and damping functions are examined for these telechelic ionomer samples and a reference conventional telechelic sample having strictly one ion per chain end, both based on poly(isobutyl acrylate). During both the steady shear and elongation measurements, the stress overshoots are observed, and the corresponding strains at the stress overshoots, γpeak and εH,peak, are insensitive to the Weissenberg number Wi for Wi≫1. This feature suggests that γpeak or εH,peak are critical strains for the induction of dissociation, which is rationalized by the phenomenological analysis of the yielding stress based on the von Mises criterion. The approximate time-strain separability was surprisingly observed for the telechelic ionomer networks with a moderate m, which may be due to the fact that, although the high strain leads to a rupture of strand, the network structure has been maintained and its terminal relaxation is governed by the ionic dissociation (at least in the strain range of our measurements). These arguments are supported by the proper normalization of damping functions obtained for different samples using γ/γpeak as the horizontal axis.

Transient and steady shear rheology of particle-laden viscoelastic suspensions

We study the transient and steady shear rheology of rigid particle suspensions in Boger fluids via complete 3D numerical simulations and experiments. We calculate the transient per-particle extra viscosity and primary stress coefficients for suspensions at different particle volume fractions ϕ for a range of Weissenberg numbers (Wi). The per-particle viscosity ( ηp) and the primary normal coefficient ( ψ1p) increase monotonically to steady state in body-fitted (BF) simulations (for dilute suspensions) and immersed boundary (IB) simulations (for nondilute suspensions). We also present experimental measurements including small amplitude oscillatory shear, steady shear, and transient shear measurements at different particle volume fraction suspensions in a Boger fluid. The simulations and experiments suggest that longer strains are needed to achieve steady state at higher ϕ and Wi. We also show the comparison of the BF and the IB simulations with experimental data for the per-particle viscosity and find excellent quantitative agreement between simulations and experiments at Wi=3 but the IB simulations underpredict the steady values at higher Wi=6. Nevertheless, the IB simulations show an increase in the per-particle viscosity with ϕ as witnessed in the experiments. To understand this behavior, we examine the particle-induced fluid stress (PIFS) and the stresslet contributions using a novel method developed for the IB simulations in this work. We find that the PIFS is independent of ϕ but the stresslet values increase with ϕ. Thus, the particle-particle hydrodynamic interactions in nondilute suspensions affect the stresslet and, in turn, the per-particle viscosity at a given Wi.

The effect of sodium hexametaphosphate on the efficiency of pectin in stabilizing acidified milk drinks

The combined effect of sodium hexametaphosphate (SHMP) (0.0125%, 0.025% and 0.05% w/w) and High methoxyl (HM)-pectin (0.06–0.42% w/w) on the stability and physico-chemical properties of acidified milk drinks (AMDs) was investigated. Analytical centrifugation, particle size distributions, zeta potential, steady shear, and oscillatory rheological measurements were used to characterize the AMD samples. Results indicated that the addition of 0.025% w/w was able to improve stability of the samples by a factor of 1.7 or more, depending on the pectin concentration. In the presence of SHMP, pectin-stabilized AMD samples had smaller particle size and polydispersity. Steady shear rheological data showed that the increase in viscosity due to bridging flocculation at the smallest pectin dosages was reduced by SHMP, whereas a synergistic effect was observed when SHMP was added to samples with a higher concentration of pectin. This study revealed that SHMP could numerically increase the zeta potential of AMD to more negative values. Thus higher stability was achieved by boosting the electrostatic interactions and the formation of soluble protein-pectin complexes. The presence of SHMP augmented the viscosity of the AMD samples with the highest pectin dosages, perhaps owing to an extended structure of pectin in the presence of less available calcium ions. Results of this study contribute to the general understanding of protein-polysaccharide interactions in acidified milk drinks and inspire to finding ways to improve the efficiency of stabilizers.

Effects of deproteinization on rheological properties of polysaccharides from Rosa acicularis ‘Lu He’ and Rosa acicularis Lindl fruits

Polysaccharides from the fruits of Rosa acicularis are one of the main functional components with superior biological activities and physicochemical properties. Rosa acicularis 'Lu He' is a new variety derived from the wild type Rosa acicularis Lindl. A comparative study was performed on the chemical composition and rheological properties of polysaccharides of rosehip from Rosa acicularis 'Lu He' (RLHP) and Rosa acicularis Lindl (RLDP) before and after deproteinization. The polysaccharide extracts, regardless of deproteinization, were acidic heteropolysaccharides composed of 48.3–51.6% carbohydrate and 29.2–43.2% uronic acid on dry weight basis and small amounts of protein (1.8–6.2%) and polyphenol (2.1–6.3%). Steady shear and frequency scanning measurements were used to evaluate the changes in concentration, temperature, pH and salts on the apparent viscosity and dynamic viscoelasticity of polysaccharide of rosehip solutions. The results showed that the apparent viscosity of RLHP was higher than that of RLDP at the same concentrations (2–6%), while apparent viscosity of RLHP and RLDP increased by 7.2% and 20.5% after deproteinization, respectively. Solutions of polysaccharide of rosehip exhibited shear-thinning behaviors, and higher concentrations caused more significant pseudoplastic effects. The solution properties changed from pseudoplasticity to plasticity when the initial apparent viscosity was below 7.5 mPa s. Thixotropy of polysaccharides of rosehips was stronger after protein removal; moreover, the polysaccharides before deproteinization (RLHP-1, RLDP-1) exhibited better gel properties than the deproteinized polysaccharides (RLHP-2, RLDP-2). This rheological property information will help proper product design to ensure the stability of food products enhanced with polysaccharides from rosehips.

Pasting and Rheological Properties of Different Starches Using Starch Cell of Discovery Hybrid Rheometer

The study was conducted to evaluate the pasting and rheological measurements of four different starches (wheat, rice, corn and chickpea) using starch pasting cell of the hybrid rheometer. The experiment was planned to initially record the pasting behavior of starches in the starch cell using heating, holding and cooling program. The cooked gels were then subjected to steady shear and dynamic rheological measurements in the same cell. The highest peak and the final and setback viscosities were recorded for chickpea starch while the lowest values were obtained for the rice starch. The pasting curves of different starches represented the typical curve as observed using the traditional visco-analyzer or visco-amylograph. All the studied starches represented shear-thinning non-Newtonian behavior (n was less than 1) when studied for their steady shear properties. The starches could be classified from less to more pseudoplastic as rice>corn>wheat>chickpea. The activation energy of rice starch was highest (142719 J/mol) while the activation energy of chickpea starch was lowest among all the starches (13291 J/mol). The storage moduli, loss moduli and tan ? were increased with increasing the frequency, and the crossover was also observed in corn, rice and wheat starch at the higher frequency. It is well evident from all the studied parameters that starch cell can be conveniently used to study the pasting and rheological properties of starches.

Structural, rheological, pasting and textural properties of granular cold water swelling maize starch: Effect of NaCl and CaCl2

In this study granular cold water swelling (GCWS) starches were subjected to 0, 50, 100, 150 and 200 mM NaCl and CaCl2 and their effects on microstructure, rheological, pasting, and textural properties were determined. SEM images revealed that NaCl decreased the thickness of GCWS starch paste cell walls while, CaCl2 resulted in formation of denser and more homogenous pastes. The atomic force microscopy (AFM) results showed that NaCl increased the surface wrinkles of starch granules but CaCl2 reduced the roughness of starch granule surface. Zeta potential measurements showed both salts reduced the negative values and CaCl2 was more effective than NaCl. The steady shear measurements showed the shear thinning behavior of the samples. Starch pastes incorporated with NaCl had lower consistency coefficient (K) and apparent viscosity. However, the CaCl2 containing samples were more viscous. The pasting and textural parameters were increased by the increase of CaCl2 but decreased with NaCl.

The impact of dissymmetry on the aggregation properties of heterogemini surfactants

Herein, a series of heterogemini surfactants (YS-12, YS-14, YS-16, and YS-18) have been investigated in aqueous solution. Surface tension, steady shear, and rheological measurements were employed to study the impact of their molecular structure on the aggregation properties. The concentration, counter-ion, and temperature were varied in order to establish a relationship among them. The surface tension measurement results showed that YS-12 and YS-14 possessed superior surface activities among all surfactants. Viscosity and SEM analyses indicated that YS-16 and YS-18 could form complex network structures in the absence of counter-ions, while YS-12 and YS-14 required counter-ion additives. Furthermore, molecular dynamics simulations demonstrated that the length of the shorter hydrophobic chain could affect the hydrophobicity of the whole surfactant.

Pasting, rheological, thermal and gel textural properties of pearl millet starch as modified by guar gum and its acid hydrolysate

The influence of guar gum and its hydrolysate at different concentrations (1 and 2% w/w) on physicochemical, pasting, thermal, rheological and textural properties of pearl millet starch was assessed. The addition of guar gum and its hydrolysate significantly improved the hydration properties of pearl millet starch. NGG significantly (p < 0.05) increased the peak, breakdown, and final viscosities while GGH did not modify the peak and final viscosities but decreased breakdown viscosity. The molecular weight of guar gum had a predominating role in modifying the functional properties of pearl millet starch. Steady shear measurements showed that both NGG and GGH decreased the flow behavior index (n) increased and consistency index (K). Higher G′ and G″ values were obtained after the addition of NGG and GGH, suggesting the formation of more solid like pastes.

Yield stress measurement of a thixotropic colloid

Yield stress measurements have long been considered to be inconsistent and difficult, especially for a thixotropic colloid, since the thixotropic behavior gives rise to time-dependent rheological properties. In this paper, we attempted measuring the yield stress of aqueous xanthan gum (XG) solutions in different XG concentration. Our rheological measurements showed that XG in aqueous solutions, which can be characterized as weak gels, were thixotropic for 0.4, 0.6, 0.8, and 1 wt% XG concentrations. To ensure the accuracy of our rheological measurement data, detailed wall slip and stress relaxation time were first investigated prior to the actual yield stress measurement. A standard sample loading procedure was used to ensure the consistency of the loaded sample. Various methods were employed such as steady shear, oscillatory shear, rheological model fitting, and creep test to determine the yield stress of aqueous XG solutions. A sigmoidal model was also proposed to obtain a yield stress from our steady shear measurement unambiguously. In our steady shear measurements, a resistance to an avalanche of flow is observed and it is likely due to a conformational change of XG polymers under shear. As compared to other measurement methods, it was found that the most reliable way to obtain a yield stress is via small amplitude oscillatory shear measurements, in which a yield stress can be obtained conveniently at the maximum storage modulus, and it is well matched with our steady shear and creep test results.