Weak Strain Overshoot Research Articles

Modulation of acid-induced pea protein gels by gellan gum and glucono-δ-lactone: Rheological and microstructural insights

This study investigated the effect of gellan gum (GG) and glucono-δ-lactone (GDL) on the acid-induced gel properties of pea protein isolate (PPI) pretreated with media milling. The inclusion of GG substantially enhanced the gel hardness of PPI gel from 18.69 g to 792.47 g though slightly reduced its water holding capacity (WHC). Rheological analysis showed that GG increased storage modulus (G’) and decreased damping factor of gels in the small amplitude oscillatory shear region and transformed its strain thinning behavior into weak strain overshoot behavior in the large amplitude oscillatory shear region. SEM revealed that GG transformed the microstructure of gel from a uniform particle aggregate structure to a chain-like architecture composed of filaments with small protein particles attached. Turbidity and zeta potential analysis showed that GG promoted the transformation of PPI from a soluble polymer system to an insoluble coagulant during acidification. When GG content was relatively high (0.2 %-0.3 %), high GDL content increased the electrostatic interaction between PPI and GG molecules, causing their rapid aggregation into a dense irregular aggregate structure, further enhancing gel strength and WHC. Overall, GG and GDL can offer the opportunity to modulate the microstructure and gel properties of acid-induced PPI gels, presenting potential for diversifying food gel design strategies through PPI-GG hybrid systems.

Structure-property relations in rheology of cellulose nanofibrils-based hydrogels

Hydrogels prepared from self-assembled cellulose nanofibrils (CNFs) are widely used in biomedicine, electronics and environmental technology. Their ability to serve as inks for extrusion-based 3D printing is conventionally evaluated by means of rheological tests. A model is developed that describes the response of CNF gels in small- and large-amplitude oscillatory tests in a unified manner. The model involves a reasonably small number of material parameters, ensures good agreement between results of simulation and observations in oscillatory tests and correctly predicts the stress–strain Lissajous curves, experimental data in hysteresis loop tests, and measurements of the steady-state viscosity. The model is applied to analyze how composition and preparation conditions for CNF gels affect transition from shear thinning to weak strain overshoot in large-amplitude shear oscillatory tests. Based on the model, simple relations are derived for the fractal dimension of CNF clusters and the storage modulus of gels prepared in aqueous solutions of multivalent salts. The validity of these equations is confirmed by comparison of their predictions with observations in independent tests.

Impact of an oligomer deep eutectic solvent on structure and properties of natural rubber vulcanizates and their nanocomposites

AbstractRubber nanocomposites exhibit strain softening under large‐amplitude oscillation shear and large‐strain cyclic tension while the softening behaviors are hard to regulate in industry. Herein, an oligomer deep eutectic solvent (DES) synthesized from polyethyleneglycol and tetrabutylammonium bromide is used to mediate the vulcanization and softening behaviors of natural rubber (NR) gums and their nanocomposites. The results show that DES could accelerate NR vulcanization and improve crosslinking density. For the hydrophilic silica nanocomposites, DES could greatly reduce tanδ and weaken weak strain overshoot accompanying Payne and lower percentages damping accompanying Mullins effects. The effects in nanocomposites filled with hydrophilic/hydrophobic silica and carbon black are investigated and compared. The investigation provides a new way for tailoring elasticity and dissipation of vulcanizate nanocomposites filled with hydrophilic silica.

Radio frequency drying on functional diversity of tiger nut flour: Effects on physicochemical, structural, and rheological properties

Tiger nut (TN) is a valuable nutrient and gluten-free tuber. To achieve high-quality TN flour as functional ingredients in food, it is essential to develop effective drying technologies for TN. Five drying methods including natural drying (Control), hot-air drying (HD), radio frequency single drying (RFSD), RF assisted hot-air drying (RFHD), and RF- vacuum drying (RFVD) were selected and compared to determine their effects on physiochemical, structural, and rheological properties of TN flour. Results showed that RF drying (RFD) significantly improved the hydration, oil-absorbing, and antioxidant activity capacity, especially for RFVD. RFHD exhibited greater color (BI = 13.80 ± 0.05 and C = 10.26 ± 0.05) and reducing sugar content (253.50 ± 2.27 mg d.b.) than RFSD and RFVD. The gelatinization temperature, enthalpy value, and particle size (57.30–269.33 μm) of TN flour were reduced. The structural property results indicated that RFD reduced the relative crystallinity and short-range ordering of the flour, altered protein secondary structure, and caused the damaged microstructure in comparison with Control and HD groups. All sample gels exhibited a weak strain overshoot behavior (type III) under large amplitude oscillations, and RFD resulted in a reduced viscoelastic behavior. RFD could be an effective method to produce functional TN flour.

Rheology of plant protein–polysaccharide gel inks for 3D food printing: Modeling and structure–property relations

Plant protein–polysaccharide composite hydrogels and emulsion gels are attractive materials for production of edible inks for 3D food printing. Their ability to satisfy the printability requirements is conventionally evaluated in small- and large-amplitude oscillatory shear tests. A model is derived that describes experimental data in linear and nonlinear rheological tests on protein gels in a unified manner. The model is based on the population-balance approach and involves a reasonably small number of adjustable parameters. Fitting observations on soy protein isolate–xanthan gum, peanut protein–gellan gum, peanut protein–carrageenan, pea protein–carrageenan and pea protein isolate–flaxseed gum gels demonstrates that material parameters evolve consistently with composition of the gels. The governing equations describe adequately the effects of pH and ionic strength of aqueous solutions on the mechanical properties of the gels and predict the stress–strain Lissajous curves and the crossover amplitudes for transition from the solid-like to fluid-like response. Based on the analysis of observations in large-amplitude oscillatory tests with various frequencies, a mechanism is proposed for transition from the type-I (strain thinning) to type-III (weak strain overshoot) response of protein–polysaccharide gels.

Tailoring microstructures of oleogel-based HIPEs to modulate oral processing properties: Rheology, tribology and mastication effects

High internal phase emulsions (HIPEs, 80 wt%) with different structures were developed by using different oleogelators (beeswax and ethyl cellulose) and emulsifiers (whey protein isolate and OSA starch) via a facile shearing approach. The effects of the oleogelators and emulsifiers on the stability and oral processing properties were investigated. The addition of oleogelator resulted in smaller particle sizes and higher accelerated centrifugal stability. Microstructural observations indicated the droplets were tightly packed to form oil-in-water HIPEs. Rheology tests showed that the addition of the oleogelators resulted in strain-thinning behavior and weak strain overshoot for OSA and WPI stabilized HIPEs, respectively. Tribology tests suggested that emulsifiers dominated the lubrication properties of the mixing regime, whereas oleogelators were crucial to the lubricating effects of the hydrodynamic regime. Instrumental mastication assays revealed that the WPI-HIPEs required stronger chewing force, and the addition of oleogelator weakened the normal force. After being masticated by the bionic chewing robot, droplet coalescence and phase separation occurred in the HIPEs, which was attenuated by the addition of oleogelators. These findings will assist in designing the structures of desirable textural attributes and support the creation of functional foods.

Effect of xanthan gum on physicochemical properties and 3D printability of emulsion-filled starch gels

The effects of xanthan gum (XG) concentration (0, 0.32%, 0.64%, 0.96%, 1.28%) on the rheological properties, microstructure, and 3D printing characteristics of emulsion-filled starch gels containing curcumin (XG-EFGs) were investigated. The results showed that the interaction between starch and XG led to an increase in mechanical strength with increasing XG concentration. The gels exhibited weak strain overshoot in large amplitude oscillatory shear tests, and the Lissajous curve shifted from elastic dominance to viscous dominance in the transition from linear to nonlinear behavior. The nonlinear parameters also demonstrated that the addition of XG led to strain stiffening and shear thinning behavior. Combining the rheological properties and 3D printing results, it was concluded that the addition of XG effectively improved the dimensional accuracy and stability of the printed products. Also, the addition of XG decreased the hardness, springiness, and gumminess of the XG-EFGs, changed the color brightness,and resulted in a superior delivery system for curcumin. This research contributes to the development of 3D printed gel foods fortified with lipophilic nutrients.

Rheology of edible soft glassy materials

In this paper we investigate the rheology of three different classes of edible yield stress fluids: a) cookie doughs, b) vegetable/fruit purees, and c) protein-rich doughs as used in meat analogs and 3D food printing. Strains sweeps of different formulations within one class can be mapped to a (partial) master curve, with at least the same strain softening index, and a similar crossover point. For classes showing only a partial master curve, the formulation differs in the response of the loss modulus in the linear viscoelastic (LVE) regime, and the transition regime between LVE and strain softening regimes, where several formulations also show a weak strain overshoot. Amongst different classes, different behaviour in strain softening has been observed. Meat analogs and fruit purees show a ratio of 2 between strain softening indices of elastic and loss modulus, while for cookie doughs and protein-rich inks, the ratio is near unity. A constitutive model using a configurational tensor with shear rate-dependent viscosity and strain-dependent elastic modulus can reproduce all features observed for the different classes and for the various formulations within a class. We conclude that our constitutive model is a good basis to design edible yield stress fluids for fiber-enrichment, starch and/or sugar replacement.

Tunable rheological properties of high internal phase emulsions stabilized by phosphorylated walnut protein/pectin complexes: The effects of pH conditions, mass ratios, and concentrations

The current study reported high internal phase emulsions (HIPEs) stabilized by phosphorylated walnut protein/pectin complexes (PWPI/Pec) and elucidated how their rheological properties were modulated by pH conditions, mass ratios, and concentrations of the complexes. At pH 3.0, the HIPEs stabilized by PWPI/Pec exhibited smaller oil droplet sizes, as well as higher storage modulus (G') and flow stress, in comparison to those stabilized by the complexes formed at pH 4.0–6.0. These observations can be directly linked to pH-dependent changes in particle size, surface hydrophobicity, and wettability of the PWPI/Pec complexes. Rheological analysis revealed that all generated HIPEs displayed weak strain overshoot behavior, irrespective of pH conditions. Notably, HIPEs stabilized by PWPI/Pec at mass ratios of 2:1 and 4:1 showed enlarged oil droplet sizes, lower G' and flow stress but higher flow strain with unaffected loss factor compared to those stabilized by PWPI/Pec 1:1. However, reducing the concentration of PWPI/Pec led to a simultaneous decrease in G', flow stress, and flow strain, along with a significant increase in the loss factor of the HIPEs. Furthermore, the HIPEs formed with 1% PWPI/Pec 1:1 at pH 3.0 demonstrated excellent stability against heat treatment and long-term storage. These results provide valuable insights into the modulation of rheological characteristics of HIPEs and offer guidance for the application of walnut protein-based stabilizers in HIPE systems.

Effect of surface acetylation of chitin nanocrystals on the preparation and viscoelasticity of sunflower seed oil-in-water Pickering emulsions

Acetylated chitin nanocrystals (ChNCs) were used as stabilizer in this work to prepare sunflower seed oil-in-water emulsions for the morphological and rheological studies. The results revealed that the acetylation with moderate degree of substitution (0.38) reduced hydrophilicity and increased surface charge level of rod-like ChNCs, and as a result, significantly improved the emulsifying ability of ChNCs. At the same oil/water ratio and particle loading, the emulsions stabilized with the acetylated ChNCs had far smaller droplet size (∼3 μm) as compared to the emulsions stabilized with the pristine ChNCs (5–7 μm). The increased droplets numbers and improved surface coating level resulted in the enhanced viscous resistance and yield stress level, which improved the physical stability of the acetylated ChNC-stabilized emulsions as a result. In addition, the droplet clusters easily formed in this system, contributing to weak strain overshoot and decreased large-deformation sensitivity during dynamic shear flow. Therefore, the acetylated ChNC-stabilized system showed enhanced transient stress overshoot during startup flow and weakened thixotropy during cyclic ramp shear flow as compared to the pristine ChNC-stabilized system. The relationships between surface acetylation of ChNCs and flow behavior of emulsions were then established, which provide valuable information on the modulation of the ChNC-stabilized Pickering emulsions.

High internal phase emulsions stabilized by walnut protein amyloid-like aggregates and their application in food 3D printing

This study comparatively investigated the rheological properties of high internal phase emulsions (HIPEs) stabilized by walnut protein isolate (WPI) and its amyloid-like aggregate form (WPIA) in detail. WPIA was generated through combined acidic and thermal treatment. Transmission electron microscopy revealed a long rod-like fibrillar structure in WPIA. At pH 3.0, WPI effectively stabilized HIPEs by functioning as a soluble protein emulsifier, while WPIA stabilized Pickering-type HIPEs (HIPPEs) by functioning as solid particles. HIPEs stabilized by WPI at pH 3.0 displayed a slightly higher storage modulus but lower yield strain/stress and a higher loss factor compared to those stabilized by WPIA. Notably, WPIA-stabilized HIPEs showed more pronounced weak strain overshoot behavior. At pH 7.0, WPIA-stabilized HIPEs exhibited a lower storage modulus but higher yield strain compared to pH 3.0 preparations. WPI failed to stabilize HIPEs at pH 7.0 due to its poor solubility. HIPEs stabilized by WPIA at pH 3.0 exhibited good long-term storage stability and had relative resistance to heat treatment and high ionic strength. Furthermore, both WPI- and WPIA-stabilized HIPEs at pH 3.0 exhibited good 3D printability. These findings provide valuable insights into the effect of different folding states of the same protein (WPI and WPIA) on the properties of the resulting HIPEs and offer a novel strategy for expanding the use of walnut proteins in the food industry.

Carboxymethyl chitosan/polyvinyl alcohol double network hydrogels prepared by freeze-thawing and calcium chloride cross-linking for efficient dye adsorption

The discharge of dye wastewater resulting from rapid industrial development has become a serious environmental concern. Therefore, there is a pressing need to develop efficient methods and technologies for the removal of dye pollutants. This study introduced a double network hydrogel, with varying carboxymethyl chitosan (CMCS) contents and polyvinyl alcohol (PVA), employing a combination of freeze- thawing and calcium chloride cross-linking. The investigation focused on the rheological properties of the hydrogels and their removal ability of acidic blue 93 (AB). The results showed that the strength and viscoelastic modulus of composite hydrogels were positively correlated with the CMCS content, and all composite hydrogels exhibited the typical weak strain overshoot behavior. The pore size of the gel initially decreased and then increased, with the densest pores observed at 4 wt% CMCS, showing the optimal removal ability for AB. The adsorption process followed pseudo second-order kinetic model, dominated by external diffusion, and exhibited inhomogeneous multilayer adsorption. This study unveils the potential of CMCS/PVA gels as adsorbents, offering inspirations for the design and development of polyvinyl alcohol-based gels for applications in the food industry.

Pickering emulsions synergistic stabilized with konjac glucomannan and xanthan gum/lysozyme nanoparticles: Structure, protection and gastrointestinal digestion

The effect of konjac glucomannan (KGM) on the stability and digestion characteristics of xanthan gum/lysozyme nanoparticles (XG/Ly NPs) stabilized Pickering emulsions was investigated. Results indicated that the high viscosity of KGM prompted the particles to be adsorbed toward the interface, which decreased the particle size and increased the stability of emulsions. As the concentration of KGM increased, the G′ and G″ of emulsions became larger and approached a “solid-like” state. When the KGM concentration was ≥0.2 %, the large amplitude sweeps of the emulsion exhibited a “weak strain overshoot”. The network structure formed by KGM molecular chain and particles was intertwined around the droplets to form a polysaccharide layer and fibrous network structure. Emulsions containing KGM showed a “spider web” epidermal network pattern. It was found by illumination for 4 h that samples with 0.2 % KGM concentration increased the retention of β-carotene by 18.74 %. KGM decreased the release rate of fatty acids and bioaccessibility by hindering bile salt and lipase adsorption.

The micromorphology and large amplitude oscillatory shear behaviors of hydrocarbon gel fuels filled with fumed silica and aluminium sub-microparticles

The gel network structure filled with attractive nanoparticles (NPs) significantly affects the viscoelasticity of the nanocomposites, which is poorly understood and troubles industrial processes. In this work, the nanoparticle gel matrices were prepared with hydrophilic fumed silica (FS), and filled with aluminium sub-microparticles (Al SMPs) loading in the range of 10%− 50%. The effects of particle content and composition on the large amplitude oscillatory shear (LAOS) behaviors are investigated by stress decomposition methods. Meanwhile, the higher harmonics and the Chebyshev harmonics coefficients were obtained. We found that the gels exhibit consistent progressive stiffening with particles filling, companying with plastic transformation and weak strain overshoot (WSO). With aluminum increasing, nonlinear viscoelastic responses gradually weaken in LAOS, severely decreasing the linear viscoelastic (LVE) region, and were validated by strain stiffening index S, T and high-order harmonics. In confocal microscopy (CFM) images, the aluminum particles were fluorescently labeled and found to be adhered together with FS clusters. This article could provide a perspective on understanding the flow mechanism of particulate network structure by combining morphological details and LAOS behaviors.

MECHANICAL PROPERTIES OF NATURAL RUBBER AND STYRENE–BUTADIENE RUBBER NANOCOMPOSITES WITH NANOFILLERS HAVING DIFFERENT DIMENSIONS AND SHAPES AT LOW FILLER LOADING

ABSTRACT Reinforcement of rubber by nanofillers has been a topic of great interest in recent years. This work compares the reinforcing efficiency of nanofillers with different topologies such as spherical (carbon black and silica), fibrous (silicon carbide nanofibers and carbon nanotubes), and sheetlike (nanoclays, expanded graphite, and graphene) in two different diene rubbers (natural rubber [NR] and styrene–butadiene rubber [SBR]) at low loadings. Tensile strength improved by 88% in the case of NR and 57% in the case of SBR by the addition of just 3 phr of graphene nanoplatelets with high aspect ratio and surface area. An increase in the Mooney–Rivlin constant (C1) with filler loading variation was also observed for these filler systems in NR and SBR. The analysis of the composites using a tube model showed that the confinement of rubber chains due to the presence of fillers with a high aspect ratio gave rise to a lower tube diameter. The addition of nanofillers resulted in higher hysteresis losses, confirming their ability for higher energy dissipation. A higher Payne effect was observed in the composites due to the formation of a percolating filler network, which was accompanied by a weak strain overshoot in the loss modulus. Dynamical mechanical analysis of the composites showed a significant increase in the storage modulus of the composites at both low and room temperatures. The reduction observed in the tan δ was correlated with the crosslink density of the composites.

Phase transitions of cellulose nanocrystal suspensions from nonlinear oscillatory shear

Cellulose nanocrystals (CNCs) self-assemble in water suspensions into liquid crystalline assemblies. Here, we elucidate the microstructural changes associated with nonlinear deformations in (2–9 wt%) CNC suspensions through nonlinear rheological analysis, that was performed in parallel with coupled rheology—polarized light imaging. We show that nonlinear material parameters from Fourier-transform rheology and stress decomposition are sensitive to all CNC phases investigated, i.e. isotropic, biphasic and liquid crystalline. This is in contrast to steady shear and linear viscoelastic dynamic moduli where the three-region behavior and weak strain overshoot cannot distinguish between biphasic and liquid crystalline phases. Thus, the inter-cycle and intra-cycle nonlinear parameters investigated are a more sensitive approach to relate rheological measurements to CNC phase behavior.

Novel high internal phase emulsions with gelled oil phase: Preparation, characterization and stability evaluation

Novel high internal phase emulsions (HIPEs, O/W) with gelled oil phase were fabricated via a facile approach using all-natural ingredients. Oil phase of HIPEs was oleogelated with beeswax (BW), and the effects of BW concentration (0–6%) on structural properties of the emulsions and stability of β-carotene were highlighted. Microstructural observation indicated that the oil droplets (3–4 μm) of the emulsions were tightly packed with protein bridges at the interfaces and BW crystals inside the droplets. BW crystals were also confirmed by XRD analysis, and the increase in BW concentration led to higher crystallinity in HIPEs. Oleogelation of the oil phase offered the emulsions with improved stability against centrifugation, heating and freeze-thawing, and the HIPE with 3% BW presented the best stability. Small amplitude oscillatory shearing tests revealed that storage modulus of the samples was dependent on BW concentration, and creep-recovery evaluation indicated that the HIPEs with BW had higher resistance over deformation. Large amplitude oscillatory shearing tests showed that the HIPEs with higher content of BW (4.5, 6%) had stain thinning behavior, while those with lower content of BW (0, 1.5, 3%) presented weak strain overshoot characteristics. When β-carotene (up to 2.25%) was incorporated in the oil phase of the emulsions, its stability against light and thermal treatment was significantly improved with the presence of BW, and the emulsion with 3% offered the highest protection. These information suggested that oleogelation of the oil phase facilitated the formation of stable HIPEs and can better behave as carriers for liable bioactives.

The study of rheological properties and microstructure of carboxylated nanocellulose as influenced by level of carboxylation

In this work, carboxylated nanocellulose had been prepared by using TEMPO-periodate oxidized reaction, in which the level of carboxylation was controlled by adjusting sodium hypochlorite (NaClO) concentration. The effects of the level of carboxylation on the rheological behavior and microstructure for the carboxylated nanocellulose dispersion had been investigated. It indicated that the nanocellulose dispersion exhibited sol state with low storage (G′) and loss (G″) modulus when NaClO concentration controlled to 2 mmol/g. But nanocellulose dispersion presented a form of gel at high NaClO concentration (6–12 mmol/g) and a critical state (similar values of G′ and G″) was obtained with the NaClO concentration of 4 mmol/g. In addition, both the G′ and G″ of nanocellulose dispersion were increased and then decreased with the increase of NaClO concentration. The highest modulus obtained when NaClO concentration controlled to 8 mmol/g, and a high fractal dimension (Df) was also obtained at this concentration, indicating a compact network structure. The large amplitude oscillation shear (LAOS) suggested that nanocellulose dispersion exhibited Ⅰ behavior (strain thinning) but had a trend of III behavior (weak strain overshoot). The Fourier transform rheology, Chebyshev coefficient, and Lissajous curves were used to analyze the nonlinear behavior, implying a correlation between the nonlinear behavior and the network structure of the nanocellulose dispersion. This work would provide some guidance for regulating the properties of nanocellulose dispersion.

Linear and non-linear rheology of heat-set soy protein gels: Effects of selective proteolysis of β-conglycinin and glycinin

Soy protein is widely used in the food industry as a gelling agent in many food products. Selectively hydrolyzing one of the two main components (β-conglycinin and glycinin) of soy protein is expected to modify its gelling ability in a novel way. To explore these modifications, selective proteolysis was applied on native soy protein isolate (SPI). Degraded β-conglycinin hydrolysate (DβH) and degraded glycinin hydrolysate (DGH) were obtained with a similar degree of hydrolysis (DH), as verified by SDS-PAGE and pH-stat. Heat-set gels were formed by SPI, hydrolysates, and dialyzed hydrolysates at different protein concentrations (6%, 10%, 14%). Rheological properties of these gels in the linear regime were determined by small amplitude oscillatory shear (SAOS) tests, while their non-linear rheology was studied by large amplitude oscillatory shear (LAOS) tests. Scaling theory, CLSM, and SEM images were used to link the rheological behavior to differences in the gel microstructure.Our results showed that hydrolysates had a shorter gelling time and lower gelling temperature, but the non-network peptides made hydrolysate gels less elastic and resulted in a lower critical linear strain than SPI gels. At low concentration and DH, DβH not only had a shorter gelling time but also formed stiffer gels than SPI, which could be due to increased hydrophobic interactions and disulphide bonds, as well as the more homogeneous gel network. On the other hand, DGH formed weaker gels than SPI at every concentration, and their gel structure was coarse, consisting of small and branched flocs. With increased strain amplitude, all gels showed a transition from predominantly elastic to plastic behavior. DβH displayed intercycle strain softening, while DGH and SPI displayed a weak and strong strain overshoot, respectively. In the medium strain range, SPI showed stronger intracycle shear stiffening and DβH gels showed stronger intracycle shear thinning. At the large strain regime, SPI and DβH gels showed abrupt yielding behavior while DGH gels yielded more gradually. Overall, the “rheological fingerprint” of soy protein heat-set gels was established, and these rheological properties can be tailored by selectively hydrolyzing different components of soy protein, and varying the DH.

Viscoelastic behavior of covalently crosslinked hydrogels under large shear deformations: An approach to eliminate wall slip

Linear and nonlinear viscoelastic properties of hydrogels significantly contribute to functionality, long-term performance, and stability of the hydrogels. With respect to the nonlinear viscoelastic response of chemically crosslinked hydrogels, the vast majority of publications have reported the type III response (weak strain overshoot). Herein, to measure the true mechanical response of hydrogels subjected to large shear deformations, we developed a technique by chemically bonding and sandwiching two surfaces of a hydrogel to treated glass slides attached to the oscillating rheometer's metal plates. Employing this method, for the first time, we were able to completely alleviate errors attributed to the wall slip in the rheological measurements of soft materials, enabling the accurate evaluation of nonlinear behavior of hydrogels. The results show that these hydrogels follow a type II (strain hardening) response. It is argued that the observed type III response of hydrogels, widely reported in the literature, originates from the wall-slip condition, rather than the inherent viscoelasticity of the hydrogels. This insight has important implications for the future development of hydrogel-based or other soft materials.