Yield Stress Behavior Research Articles

Scratch behavior of model polyurethane elastomers containing different soft segment types

The scratch behavior of a series of model cast polyurethane elastomers (CPU) is investigated according to the ASTM D7027/ISO 19252 scratch test methodology. Four model CPU systems considered in this study were synthesized by the same hard segment, but with four different types of soft segment polyols to systematically alter their phase morphology, coefficient of friction (COF), yield stress, tensile strength and damping behavior. Attenuated total reflectance Fourier transform infrared spectroscopy, atomic force microscopy, COF measurement, uniaxial tensile and compressive true stress-strain curves generation, and dynamic mechanical analysis (DMA) were carried out to link intrinsic material properties to the observed scratch-induced deformation mechanisms of the model CPU systems. The observed scratch damage mechanisms of the model CPU systems are found to correlate well with COF, tensile strength, compressive yield stress and DMA damping behavior. The present study demonstrates that the scratch performance of polymers can be predicted by their intrinsic material properties. Implication of the present study for designing scratch resistant polyurethane elastomers is discussed.

Anomalous Behavior of Ultra-Low-Amplitude Capillary Waves. A Glimpse of the Viscoelastic Properties of Interfacial Water?

We investigate, both theoretically and by a differential interferometric technique, the behavior of large-wavelength capillary waves (of the order of 10-4 m) selectively excited at the surface of drops and bubbles with typical eigenfrequencies of the order of 102 Hz. The resonance peaks of gas bubbles or hydrocarbon drops in water (radius less than 1 mm) highlight anomalously small dissipation in the region of ultralow (sub-nanometric) oscillation amplitudes, reaching a plateau at higher amplitudes. This is in sharp contrast to the usual oscillating systems, where an anomalous behavior holds at large amplitudes alone. Dissipation is strongly dependent on the excited vibrational modes and, in spite of remarkable numerical differences, water-vapor and water-hydrocarbon interfaces exhibit the same overall trend. A phenomenological model was developed, based on the assumption that water possesses a threshold viscoelasticity, above which it behaves like a regular viscous fluid. The well-known Deborah number was then estimated within the anomalous region and found to lie in the range of viscoelastic fluids. In agreement with previous studies of nanohydrodynamics (e.g., atomic force microscopy measurements with sub-nanometric tip motions), the present one lends support to the idea that every self-aggregating fluid exhibits yield stress behavior, including classical Newtonian fluids like water. The essential requirement is that the applied perturbation lie below a critical threshold, above which viscous behavior is recovered. Our differential interferometric technique seems particularly suitable for this type of studies, as it allows measurement of long-wavelength capillary waves with sub-nanometric resolution on the oscillation amplitudes.

Influence of synthesis parameters on sol–gel transition and physical properties of Nb2O5 mesoporous ambigels

In this study, Nb2O5 alcogels and mesoporous ambigels were formed via a new acid-catalyzed polymeric synthesis pathway using NbCl5 as a precursor. The effect of hydrolysis, catalyst, and propylene oxide molar ratios on sol–gel transition, rheological properties, and physical properties of Nb2O5 ambigels were investigated. A common behavior for the sol–gel transition of Nb2O5 on the viscosity vs. time curves was identified. The molar ratios of H2O:Nb+5 and HNO3:H2O significantly influenced the gelation time, the rheological properties of alcogels, as well as the physical properties of the respective ambigels. The gels presented yield stress and shear-thinning behavior with different degrees of thixotropy. Microstructure analysis of the ambigels revealed a mesopore network formed by interconnected Nb2O5 nanoparticles. An increase in the molar ratios of HNO3:H2O (lower pH) increased the surface area and pore volume of Nb2O5, and yielded a finer nanostructure of Nb2O5 mesoporous ambigels.

Insights into the rheological behaviors evolution of alginate dialdehyde crosslinked collagen solutions evaluated by numerical models

The elaboration of the rheological behaviors of alginate dialdehyde (ADA) crosslinked collagen solutions, along with the quantitative analysis via numerical models contribute to the controllable design of ADA crosslinked solution system's fluid mechanics performance during manufacturing process for collagen biomaterials. In the present work, steady shear flow, dynamical viscoelasticity, creep-recovery, thixotropy tests were performed to characterize the rheological behaviors of the collagen solutions incorporating of ADA from the different aspects and fitted with corresponding numerical models. It was found that pseudoplastic properties of all samples enhanced with increasing amounts of ADA, which was confirmed by the parameters calculated from the Ostwald-de Waele model, Carreau and Cross model, for instance, the non-Newtonian index (n) decreased from 0.786 to 0.201 and a great increase by 280 times in value of viscosity index (K) was obtained from Ostwald-de Waele model. The forth-mode Leonov model was selected to fit all dynamic modulus-frequency curves due to its higher fitting precision (R2>0.99). It could be found that the values of correlation shear viscosity (ηk) increased and the values of relaxation time (θk) decreased with increasing ADA at the fixed k value, suggesting that the incorporation of ADA accelerated the transformation of the collagen solutions from liquid-like to gel-like state due to more formation of CN linkages between aldehyde groups and lysine residues. Also, the curves of creep and recovery phase of the native and crosslinked collagen solutions were simulated well using Burger model and a semi-empirical model, respectively. The ability to resist to deformation and elasticity strengthened for the samples with higher amounts of ADA, accompanied with the important fact that compliance value (J50) decreased from 56.317Pa−1 to 2.135Pa−1 and the recovery percentage (Rcreep) increased from 2.651% to 28.217%. Finally, it was found that the area of thixotropic loop increased from 8.942Pa/s to 17.823Pa/s with increasing introduction of ADA, suggesting that stronger thixotropic behavior was associated with higher amount of ADA. Furthermore, Herschel–Bulkley model was employed to describe the up and down curves of all samples and it was confirmed that all collagen solutions belonged to pseudoplastic fluid (the flow index<1) without apparent yield stress and shear-thinning behaviors were more obvious with increasing additions of ADA according to the increasing consistency coefficient K values. Overall, this work contributed a new insight into the interactions between collagen and ADA based on quantitative rheological methods reflecting the different rheological properties and the results obtained should be of great utility in the extensive application of ADA crosslinked collagen solutions into diverse collagen-based materials.

Stability of shear banded flow for a viscoelastic constitutive model with thixotropic yield stress behavior

A viscoelastic constitutive model which combines the partially extending strand convection model and a Newtonian solvent is used in the regime of large relaxation time. Prior work on one dimensional time-dependent solutions at prescribed shear stress predicts some of the features expected of thixotropic yield stress fluids, such as delayed yielding. In this paper, we present the linear stability analysis of two-dimensional plane Couette flow, for parameter regimes that support a two-layer arrangement consisting of an unyielded layer and a yielded layer. Asymptotic analysis and computational techniques are applied. We find that the one layer yielded flow can have bulk instabilities which also emerge in the two-layer flow. Bulk instabilities in the yielded phase appear not to have been observed in prior literature. For some parameters, an interfacial mode is unstable and is driven by the normal stress difference across the interface. The yielded zone has the higher first normal stress difference, as for the well-studied Johnson–Segalman model. In order to assess the importance of the sign of the first normal stress difference at the interface, we specifically design a modification to the model to reverse the sign. It is found that instabilities still occur.

Quasi-static bubble in a yield stress fluid: elasto-plastic model

The influence of the yield stress of Carbopol® gel dispersions on the behaviour of quasi-static bubbles was investigated. Many fluids, from many different industrial fields, have yield stress behaviour. Most of them contain gas bubbles. To study bubble behaviour in such suspensions, a transparent model fluid (dispersion of Carbopol® in water) was used. The experimental device allowed to quasi-statically increase bubble internal pressure with small pressure step to reach a maximum target internal pressure and the pressure setpoint was inverted to return to the initial pressure. Hysterical behaviour of the bubbles was highlighted as they did not regain their initial shape because of yield stress. We show that the rheological behaviour is related to the internal pressure, bubble geometry and yield stress in quasi-static conditions. A modification of the Laplace law depending on the yield stress of the fluid and bubble sphericity was proposed and validated.

Rheological properties of reduced fat ice cream mix containing octenyl succinylated pearl millet starch

The octenyl succinyl anhydride (OSA) esterified pearl millet (Pennisetum typhoides) starch was evaluated as fat replacer in soft serve ice cream in comparison to other fat replacers viz. inulin, whey protein concentrate-70 and commercial starch. During temperature sweep test, the yield stress and flow behaviour index of un-pasteurized ice cream mixes increased as the temperature increased from 40 to 80°C, while the consistency index decreased. Consistency index of aged ice cream mixes containing 2% fat replacer was higher as compared to mixes with 1% level. The aged ice cream mixes exhibited non-Newtonian behaviour as flow behaviour index values were less than one. Apparent viscosity (at 50s-1 shear rate) of control as well as ice cream mix containing 1% OSA-esterified pearl millet starch samples was 417 and 415mPas, respectively and did not differ significantly. The overrun of the ice cream (with 5 and 7.5% fat) containing 1 and 2% of above fat replacers ranged between 29.7 and 34.3% and was significantly lower than control (40.3%). The percent melted ice cream was also low for the ice creams containing 2% of above fat replacers at 5% fat content as compared to control. However, sensory acceptability and rheological characteristics of reduced fat ice creams containing 1.0 and 2.0% OSA-esterified pearl millet starch were at par with other fat replacers under the study. Thus, OSA-esterified pearl millet starch has potential to be used as fat replacer in reduced fat ice cream.

Non-Newtonian behavior observed via dynamic rheology for various particle types in energetic materials and simulant composites

The rheological properties of polymer composites highly filled with different filler materials were examined using a stress-controlled rheometer with a parallel-plate configuration, for particle characterization of the filler materials in plastic (polymer) bonded explosive (PBX). Ethylene vinyl acetate (EVA) with dioctyl adipate (DOA) was used as the matrix phase, which was shown to exhibit Newtonian-like behavior. The dispersed phase consisted of one of two energetic materials, i.e., explosive cyclotrimethylene trinitramine (RDX) or cyclotetramethylene tetranitramine (HMX), or a simulant (Dechlorane) in a bimodal size distribution. Before the test, preshearing was conducted to identify the initial condition of each sample. All examined filled polymer specimens exhibited yield stress and shear-thinning behavior over the investigated frequency range. The complex viscosity dependence on the dynamic oscillation frequency was also fitted using an appropriate rheological model, suggesting the model parameters. Furthermore, the temperature dependency of the different filler particle types was determined for different filler volume fractions. These comparative studies revealed the influence of the particle characteristics on the rheological properties of the filled polymer.

A dissipative particle dynamics model for thixotropic materials exhibiting pseudo-yield stress behaviour

Many materials (e.g., gels, colloids, concentrated cohesive sediments, etc.) exhibit a stable solid form at rest, and liquify once subjected to an applied stress exceeding a critical value – a yield-stress behaviour. This can be qualitatively explained by the forming and destruction of the fluid microstructure [1], and it may be modelled as a thixotropic and yield stress material. In this paper, we propose a mesoscopic model which is able to mimic a thixotropic and yield stress behaviour using a particle-based technique known as dissipative particle dynamics (DPD). The DPD technique satisfies conservation of mass and momentum and it has been applied successfully for a number of problems involving complex-structure fluids, such as polymer solutions, suspensions of rigid particles, droplets, biological fluids, etc. In this work, an indirect linkage dissipative particle model (ILDP) is proposed based on qualitative microstructural physics, which results in a non-Newtonian fluid with observed yield stress and thixotropic properties. The model comprises of two types, or species, of DPD particles – with only repulsive conservative force between the same species, and with repulsive force at short range and attractive force at long range between different species. Numerical results show that the proposed DPD fluid can represent some observed complex behaviours, such as yield stress and thixotropic effects.

Yield stress and strength of saline cemented tailings in sub-zero environments: Portland cement paste backfill

The objective of this study is to investigate the time-dependent evolution of the yield stress, strength and deformation behaviour of cemented paste backfill (CPB) that contains Portland cement as the binder with different saline concentrations in a sub-zero environment (−6°C). Yield stress measurement with the vane method, uniaxial compressive testing and microstructure analysis have been conducted on various CPB samples with different sodium chloride (salt; NaCl) concentrations and cured in a sub-zero temperature for different times (0, 0.25, 1, 2 and 4h for yield stress measurement; 7, 28 and 90days for compressive strength testing). The results show that the yield stress decreases with an increase in salinity. The strength of CPB under freezing conditions decreases as the concentration of NaCl is increased. A shift from the strain-softening behaviour of CPB that has no salinity to the strain-hardening behaviour of CPB with high salinity is observed. Additionally, it is also noticed that with curing time, the deformation behaviour of the studied CPB becomes gradually less ductile. The microstructural analysis results show that the binder hydration products are affected by the salinity. There is strong indication that the absorption of the Na+ ions by calcium–silicate–hydrate leads to the decrease of the strength of CPB with different saline concentrations. The results of this investigation provide technical information for the designing of cost-effective, safe and durable CPB structures in sub-zero mining environments.

Comparison of flow behavior and physicochemical characteristics of low-cholesterol mayonnaises produced with cholesterol-reduced egg yolk

Egg yolk is common functional ingredient in food production, mainly in mayonnaise as an emulsifier. Egg yolk cholesterol was removed using β-cyclodextrine, treated with α-amylase, and low-cholesterol mayonnaise produced. The flow behavior of cholesterol-reduced mayonnaise was investigated at different cholesterol-reduced egg yolk and xanthan gum content. The aim of this study was to evaluate the effects of different formulations at various percentages of egg yolk (EY; 4 and 6 percent by weight) and xanthan gum (XG; 0.5 and 0.7 percent by weight) on flow behavior and physicochemical properties of reduced-fat and low-cholesterol mayonnaise prepared with cholesterol-reduced liquid egg yolk and to obtain suitable model for mayonnaise formulations. The aim of the study was to develop a reduced-fat and low-cholesterol mayonnaise with properties similar to those of typical mayonnaise in the market. For this purpose, four reduced-fat and low-cholesterol mayonnaise formulations were prepared in the laboratory using cholesterol-reduced EY. They were compared with a commercial mayonnaise from a local market as a control sample. According to results, color parameters and flow behaviors of the samples compared with the control sample indicated that the samples had higher values of lightness and yellowness, and presented lower values of yield stress and flow behavior index and higher values of consistency coefficient with respect to the commercial product, except recipe that included 6% of EY and 0.7% of XG, which exhibited similar values to the typical commercial one in the market. The Herschel Bulkley model was employed in order to evaluate the effect of EY and XG ratios on mayonnaise rheological behavior. In general, the results obtained suggest that the recipe with EY (6 wt%) and XG (0.7 wt%) had better rheological characteristics and physicochemical properties those prepared with various formulations, maintaining the reduced-fat and low-cholesterol mayonnaise prepared with β-cyclodextrine-treated cholesterol-reduced EY. It can be interpreted that the developed product may have potential use in industrial application.

High Speed Dispensing with Novel 6” Print Head

In this study, a new dispensing print head is introduced covering an operational width of 16 cm and therefore allowing for solar cell processing at industrial throughput rates. Like in previous ten nozzle versions, the interior paste flow was designed by computational fluid dynamics (CFD) using rheological parameters of applied Ag-pastes (commercially available pastes), including yield stress, shear thinning and wall slip behaviour. The novel print head features a homogeneous distribution of Ag-paste from an inlet supply with a diameter of Din = 2mm to an outlet slot with a width of ws = 160 mm. A first printing test followed by geometrical characterization of the dispensed contacts revealed a distribution of the contact finger width of wf = 35±1 μm across the whole wafer (156x156mm2) which demonstrates highest precision of the approach. The print head is directly integrated in an inline feasible dispensing platform, developed by ASYS GmbH. Respective nozzle plates can be customized to any desired front side grid regarding nozzle diameter, nozzle pitch and the total number of nozzles. In the actual version, the print head uses two strokes to print 100 contact fingers. Each of the 50 nozzles prints two adjacent contact fingers while moving up and down at a speed of up to vy = 700mm/s. In a first solar cell test sequence with the new 6” print head, a maximum cell efficiency of η = 21.2% on industrially preprocessed Cz-PERC samples was reached which demonstrates a successful launching of the print head.

Erratum: “Stretch and hold: The dynamics of a filament governed by a viscoelastic constitutive model with thixotropic yield stress behavior” [Phys. Fluids 28(5) 053104 (2016)

Erratum: “Stretch and hold: The dynamics of a filament governed by a viscoelastic constitutive model with thixotropic yield stress behavior” [Phys. Fluids 28(5) 053104 (2016)

Surface chemistry and rheological properties of API bentonite drilling fluid: pH effect, yield stress, zeta potential and ageing behaviour

API bentonite, a key ingredient of drilling muds, displayed a negative zeta potential across the whole pH range of 2.5–12.3. It contained two very rheological active ingredients, sodium montmorillonite or Na+Mt (40.4%) and needle-shape sepiolite (16%). Due to the interplay of these two ingredients, the rheological behaviour displayed by the API bentonite suspensions is quite different to other bentonite suspensions. The gel displayed a very low or no yield stress at low acidic pH and a maximum yield stress at pH 8–12. Magnesium leaching from sepiolite at low pH and its hydrolysis plays a role in affecting the rheological properties at high pH. Yield stress and ageing behaviour was observed in suspensions with at least 3wt% solids. The Herschel-Bulkley (HB) model described the flow behaviour of these gels. Both the HB yield stress and viscosity at a fixed shear rate of the 7wt% gel peaked at pH 8.5 and were smallest at pH 5.2. The ageing or structural recovery behaviour is most pronounced at high pH for the most concentrated gels. Both the Leong and Nguyen-Boger ageing models described the structural recovery behaviour well. The recovery of the recently discovered very open honeycomb structure formed by Na+MT gel, after breakdown by shear was invoked to explain the ageing behaviour.

Steady shear flow behavior of sage seed gum affected by various salts and sugars: Time-independent properties

The rheological properties of food hydrocolloids are remarkably influenced by the quality of solvent/cosolutes in a food system. In this work, the steady shear flow behavior of sage seed gum (SSG, 0.5% w/w) at the presence of different levels of salts (KCl & MgCl2, 0–100mM) and sugars (sucrose, lactose & glucose, 0–6% w/w) was studied. It was found that the rheological properties of SSG were affected by the type of sugars and salts and their concentrations as well. Synergistic interaction was observed between SSG and sugars which enhanced the viscosity of gum solutions, while salts addition diminished the viscosity. SSG solutions exhibited a shear thinning behavior at all conditions tested. Various time-independent rheological models were used to fit the shear stress-shear rate data, although the Herschel-Bulkley (R2=0.994–0.999) and Sisko (R2=0.995–0.999) models showed the best results to describe the flow behavior of SSG. In the presence of salts, the yield stress (τ0), consistency coefficient (k), and flow behavior index (n) values decreased. The k and τ0 values enhanced and the n value lowered in the presence of sugars. Divalent cations of Mg2+ and sucrose roughly showed more effect on rheological parameters than others.

Predicting the apparent viscosity and yield stress of mixtures of primary, secondary and anaerobically digested sewage sludge: Simulating anaerobic digesters

Predicting the flow behaviour, most notably, the apparent viscosity and yield stress of sludge mixtures inside the anaerobic digester is essential because it helps optimize the mixing system in digesters. This paper investigates the rheology of sludge mixtures as a function of digested sludge volume fraction.Sludge mixtures exhibited non-Newtonian, shear thinning, yield stress behaviour. The apparent viscosity and yield stress of sludge mixtures prepared at the same total solids concentration was influenced by the interactions within the digested sludge and increased with the volume fraction of digested sludge – highlighted using shear compliance and shear modulus of sludge mixtures. However, when a thickened primary – secondary sludge mixture was mixed with dilute digested sludge, the apparent viscosity and yield stress decreased with increasing the volume fraction of digested sludge. This was caused by the dilution effect leading to a reduction in the hydrodynamic and non-hydrodynamic interactions when dilute digested sludge was added.Correlations were developed to predict the apparent viscosity and yield stress of the mixtures as a function of the digested sludge volume fraction and total solids concentration of the mixtures. The parameters of correlations can be estimated using pH of sludge. The shear and complex modulus were also modelled and they followed an exponential relationship with increasing digested sludge volume fraction.

Mathematical model for three equal collinear straight cracks: A modified Dugdale approach

The present paper discuss a multiple site damage (MSD) problem of three collinear straight cracks exist in an infinite isotropic elastic perfectly plastic plate. Uniform stress distribution is applied at the infinite boundary of the plate in a direction perpendicular to the faces of the cracks. As a result, yield zones are developed ahead each crack tip. These yield zones are sensitive about the load applied at the boundary of the plate. Since the regions near the crack tips are very week as compared to other parts of the plate, therefore, it is assumed that the behaviour of yield stress of the plate near the yield zones is in linear fashion. Hence, rims of the yield zones are subjected to linearly varying stress distribution to stop further opening of cracks along the real axis. The problem is solved using widely used complex variable technique and closed form expressions are derived for stress intensity factor (SIF), crack tip opening displacement (CTOD) and length of developed yield zones at each crack tip. Under small-scale yielding, numerical results for yield zone length and crack-tip opening displacement as a function of remotely applied stress are obtained and presented graphically.

Stretch and hold: The dynamics of a filament governed by a viscoelastic constitutive model with thixotropic yield stress behavior

The transient behavior of filament stretching is studied for a viscoelastic constitutive model that combines a partially extending strand convection model with a Newtonian solvent. The vertical filament is fixed at the bottom and the top is pulled up and held. Gravity and surface tension are also included in the model though they are not the primary mechanisms in this study. An axisymmetric circular slender jet approximation is applied. An asymptotic analysis for the initial stages of evolution is performed for large relaxation time, so that an interplay of fast and slow time scales emerges, and gives a criterion for whether the fluid yields immediately or whether slow dynamics ensues, depending on elastic stresses, gravity, and capillary stress. The analysis guides the choice of parameters to exemplify thixotropy and yield stress behavior through numerical simulations of the full governing equations from start to finish of the filament evolution. Elastic effects promote a spring back of the filament toward its initial shape, while pulling at the top stretches the filament locally to promote yielding, with the lower portion of the filament remaining unyielded. In addition, a parameter regime that models extensional experiments in the literature for yield stress fluids sheds light on the differences in filament shapes.

Recent Developments and Limitations of the SFG Model

The SFG model was proposed in attempt to provide a consistent description of the stress-strain behaviour of unsaturated soils, including compacted soils and soils dried from slurry. It differs from existing models mainly through two aspects: a consistent description of volume change, yield stress and shear strength behaviour of unsaturated soils and a smooth and natural transition between saturated and unsaturated states. The model has attracted significant attention since it was first proposed and has been extended to cover coupled hydro-mechanical behaviour, hysteretic water retention behaviour and density-dependency. This paper presents a summary of the latest developments of the model, as well as the aspects that require further refinements.

Preparation and rheological behaviors of a nano-yttria anti-bonding paste

Anti-bonding pastes are used on areas that should not be welded during superplastic forming/diffusion bonding processes in multi-layer, complex metal structures. We prepared a nano-Y2O3 anti-bonding paste to prevent the coated anti-bonding paste from flowing to unwanted areas and to improve the thickness uniformity of the coated paste. Nano-spherical Y2O3 powders with a purity of more than 99.99 wt% and a narrow size distribution of 50 to 100 nm were prepared by liquid precipitation followed by sintering. The relationship between the rheological behaviors and dispersion structure of both pastes with and without 1.5 wt% rheology control additive was analyzed in detail. The results indicated that a strong three-dimensional network flocculation structure had formed in the pastes, especially in those with a rheology control additive. The resulting large yield stress, high thixotropy, and solid-like elastic behavior of the paste are favorable for maintaining the original coated shape and offer long-term stability, but are unfavorable for the screen printing (coating) process and thickness uniformity. To solve these problems, a high printing pressure and the use of a screen with high tension and large mesh count (325 mesh) could be adopted. The thermal analysis results show that the paste after SPF/DB processing was easy to clear with no negative influence on the process.