Steady Shear Conditions Research Articles

Rheological Response of Polylactic Acid Dispersions in Water with Xanthan Gum.

In this work, the rheological behavior of stable poly(lactic acid) (PLA) dispersions in water, intended for coating applications, was investigated. The newly prepared dispersion consists of PLA particles with an average diameter of 222 ± 2 nm based on dynamic light scattering (DLS) and scanning electron microscopy (SEM) analyses, at concentrations varying in the 5–22 wt % range. Xanthan gum (XG), a bacterial polysaccharide, was used as a thickening agent to modulate the viscosity of the formulations. The rheological properties of the PLA dispersions with different XG and PLA contents were studied in steady shear, amplitude sweep, and frequency sweep experiments. Under steady shear conditions, the viscosity of all the formulations showed a shear-thinning behavior similar to XG solutions in the whole investigated 1–1000 s–1 range, with values dependent on both PLA particles and XG concentrations. Amplitude and frequency sweep data revealed a weak-gel behavior except in the case of the most diluted sample, with moduli dependent on both PLA and XG contents. A unified scaling parameter was identified in the volume fraction (ϕ) of the PLA particles, calculated by considering the dependence of the continuous phase density on the XG concentration. Accordingly, a master curve at different volume fractions was built using the time–concentration–superposition approach. The master curve describes the rheological response of the system over a wider frequency window than the experimentally accessible one and reveals the presence of a superimposed β relaxation process in the high-frequency region.

Shear flow of associative polymers in aqueous solutions

The rheological behavior of two associative polymers in aqueous solutions, poly(vinyl alcohol) (PVA) and polyurethane (PU) was carefully investigated in steady shear conditions at different temperatures, ranging from 4 °C to 80 °C. Also, dynamic light scattering measurements were carried out for rested and sheared samples and distinctive features were observed. The flow curves evidenced an associative tendency occurring in a certain shear rate range and the amplitude of this phenomenon is influenced by temperature and polymer composition. At moderate shear rates, the intermolecular interactions determine the formation of supramolecular structures that increase the viscosity of the system. Higher shear forces orient the macromolecules along the flow direction and improve the interaction of hydrophilic groups with the water molecules. On the other hand, the temperature and concentration increases allow the manifestation of hydrophobic interactions. The gelation as a function of temperature was followed in temperature sweep tests and the influence of PVA/PU composition was evidenced. The overall behavior of the system is dictated by the balance between these interactions at rest or during shear.

Parametric analysis of the transient back extrusion flow to determine instantaneous viscosity

Rotational rheometers are extremely sensitive instruments that are able to provide very accurate measurements of low values of different material functions, such as viscosity and viscoelastic moduli, under certain ideal flow conditions. Nevertheless, they fail in providing reliable data for the viscosity when measured at short time scales due to either instrument or fluid inertia. In 1948, Bikerman [“A penetroviscometer for very viscous liquids,” J. Colloid Sci. 3, 75 (1948)] proposed the penetroviscometer device as an instrument to measure the viscosity of highly viscous (102–104 Pa · s) Newtonian fluids under steady shear conditions. In the penetroviscometer, the fluid sample undergoes a kind of flow, which is currently known as back extrusion flow, and it is nowadays used to characterize the rheological properties of foodstuff under steady flow conditions. In this study, we perform a parametric analysis (blockage ratio, initial position, velocity profile of the inner cylinder, and viscosity of Newtonian fluids) of the back extrusion flow to determine its potential to provide reliable viscosity measurements at short time scales. Three-dimensional numerical simulations including free surface effects and inertial terms allowed us to determine its limits of operation. We additionally provide an analytical expression to calculate the time-resolved viscosity, relating the force and velocity of the inner cylinder by means of a geometric factor.

Rheological properties of thickened barium liquids prepared with xanthan gum‐based thickener and barium powder used in the diagnosis of dysphagia: Effect of thickener concentration and serving temperature

In this study, thickened barium liquids (TBL) were prepared by mixing a xanthan gum (XG)-based thickener at different concentrations (1.0%–3.0%) with a barium powder. The rheological properties of TBL and thickened non-barium liquids (TNL) at different thickener concentrations were examined under steady and dynamic shear conditions. The values of flow and dynamic rheological parameters (K, σoc, G′, and G″) of TBL were considerably higher than those of TNL. The extents of the TBL thixotropy loops decreased with an increase in thickener concentration, showing that at higher concentrations, the degree of shearing-induced structure breakdown was lower than at lower concentrations. Compared with those of TNL, the rheological characteristics of TBL were more strongly dependent on thickener concentration, indicative of a strong molecular interaction between XG and barium at high thickener concentrations. These results could facilitate the development and preparation of videofluoroscopic contrast media with rheological properties suitable for the diagnosis of dysphagia. Practical applications Thickened liquids (TL) prepared with XG-based food thickeners are widely used for the effective management of patients with dysphagia (difficulty of swallowing). In terms of clinical practice, the rheological characteristics of thickened barium liquids (TBL) prepared with both XG-based thickener and barium powder are very important for the preparing the contrast media used for videofluoroscopic swallow study (VFSS) which the swallowing safety and efficiency with a bolus are assessed. Our results showed that compared with those of TL without barium, the rheological characteristics of TBL were more strongly dependent on thickener concentration, indicative of a strong molecular interaction between XG and barium at high thickener concentrations. These results presented in this study will be useful for the development and preparation of videofluoroscopic contrast media with rheological properties suitable for the diagnosis of dysphagia.

Buriti Oil Emulsions as Affected by Soy Protein Isolate/High-Methoxyl Pectin Ratio, Oil Content and Homogenization Pressure.

SUMMARYResearch backgroundEmulsion technology is a suitable way of encapsulating, protecting and releasing hydrophobic bioactive compounds for application in food industries, but they are thermodynamically unstable systems. Good results have been achieved for emulsions stabilized by protein-polysaccharide complexes subjected to high-pressure homogenization. Improved stabilization of oil-in-water emulsions results from electrostatic complexes formed between proteins and polysaccharides at pH lower than the protein isoelectric point, which adsorb at the oil-water interface. In addition, polysaccharides contribute to emulsion stability by increasing viscosity of the continuous phase. The aim of this work is to investigate the production of carotenoid-rich buriti oil emulsions using soy protein isolate and high-methoxyl pectin as stabilizers.Experimental approachUsing a rotatable central composite experimental design, we assessed the effects of oil content, soy protein isolate/high-methoxyl pectin ratio and homogenization pressure on the stability, droplet size, electrical conductivity, electrical charge, microstructure and rheological behaviour of the emulsions.Results and conclusionsAn optimized emulsion was produced with 28% buriti oil, 55% soy protein isolate, and homogenization pressure of 380·105 Pa. This emulsion was stable for at least seven days, presenting reduced average droplet size, low electrical conductivity and high modulus of negative charges. The mechanical spectra showed that the emulsion behaved as a viscoelastic gel under oscillatory, non-destructive shearing, whereas shear-thinning behaviour took place under steady shear conditions.Novelty and scientific contributionThe optimized buriti oil emulsions stabilized by soy protein isolate and high-methoxyl pectin could be suitable for fat substitution, energy reduction and carotenoid enrichment in food products, such as dairy and bakery products, ice cream, salad sauces and vegetable-based cream.

Saturated granular flows: constitutive modelling under steady simple shear conditions

In this paper, the authors analyse numerical and experimental results concerning either dry or saturated granular flows under steady, simple shear conditions. A new constitutive model is introduced, on the base of the mixture theory, according to which granular and liquid phases are considered separately. The constitutive relationship refers to the representative elementary volume and assumes that the mean values of all kinematic variables, of both granular and liquid phases, coincide. For the two phases, a parallel scheme is chosen. As regards the granular contribution, the authors employ an already conceived constitutive model where the critical state concept and the kinetic theory of granular gases are merged, and in which the granular temperature plays the role of state variable for the material. Under saturated conditions, the new model accounts for granular–liquid coupling effects. In fact, the liquid viscosity is a function of granular concentration, whereas the evolution of granular temperature is influenced by the liquid molecular viscosity. The model is validated against numerical results and critically discussed. For sufficiently small values of concentration, the transition from a Newtonian to a Bagnoldian regime, for increasing values of strain rate, is correctly reproduced.

Rheological Characteristics of Waxy Rice Starch Modified by Carboxymethyl Cellulose.

The effects of carboxymethyl cellulose (CMC) at different concentrations (0, 0.2, 0.4, and 0.6% w/w) on the rheological properties of waxy rice starch (WRS) pastes were evaluated under both steady and dynamic shear conditions. The flow properties of WRS-CMC mixtures were determined from the rheological parameters of power law and Casson models. All samples demonstrated a clear trend of shear-thinning behavior (n=0.33~0.34), with a marginal difference shown between n values. The addition of CMC to WRS increased the apparent viscosity (ηa,100), consistency index, and Casson yield stress values. The dynamic moduli [storage modulus (G′), loss modulus (G″), and dynamic viscosity (η*)] and ratio of G″/G′ values of WRS-CMC mixtures also increased with an increase in CMC concentration; the higher dynamic rheological properties observed at higher CMC concentrations may be attributed to an increase in the viscoelasticity of the continuous phase in the starch-gum mixture system. Dependence of ηa,100 on temperature followed the Arrhenius model for all samples. The Cox-Merz rule was not applicable to WRS-CMC pastes with different CMC concentrations, demonstrating that there was a deviation between η* and steady shear viscosities for all samples. Therefore, the synergistic effect of CMC on the rheological properties of WRS pastes appeared to be the result of coacervation.

Individual Molecular Dynamics of an Entangled Polyethylene Melt Undergoing Steady Shear Flow: Steady-State and Transient Dynamics.

The startup and steady shear flow properties of an entangled, monodisperse polyethylene liquid (C1000H2002) were investigated via virtual experimentation using nonequilibrium molecular dynamics. The simulations revealed a multifaceted dynamical response of the liquid to the imposed flow field in which entanglement loss leading to individual molecular rotation plays a dominant role in dictating the bulk rheological response at intermediate and high shear rates. Under steady shear conditions, four regimes of flow behavior were evident. In the linear viscoelastic regime (), orientation of the reptation tube network dictates the rheological response. Within the second regime (), the tube segments begin to stretch mildly and the molecular entanglement network begins to relax as flow strength increases; however, the dominant relaxation mechanism in this region remains the orientation of the tube segments. In the third regime (), molecular disentangling accelerates and tube stretching dominates the response. Additionally, the rotation of molecules become a significant source of the overall dynamic response. In the fourth regime (), the entanglement network deteriorates such that some molecules become almost completely unraveled, and molecular tumbling becomes the dominant relaxation mechanism. The comparison of transient shear viscosity, , with the dynamic responses of key variables of the tube model, including the tube segmental orientation, , and tube stretch, , revealed that the stress overshoot and undershoot in steady shear flow of entangled liquids are essentially originated and dynamically controlled by the component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths.

Motility and cell shape roles in the rheology of growing bacteria cultures.

Cell shape, size and self-motility appear as determinant intrinsic cell factors in the rheological behavior of living bacterial cultures during the growth process. In this work three different species were considered due to their differences on these intrinsic characteristics: two different strains of Staphylococcus aureus - strain COL and its isogenic cell wall autolysis mutant, RUSAL9 - both non-motile and Escherichia coli and Bacillus subtilis - both presenting intrinsic motility. In situ real-time rheology, was used to characterize the activity of growing bacteria, under steady-shear conditions, in particular the viscosity growth curve was measured, for a constant shear flow rate, presenting for all studied cultures, different and rich flow curves. These complex rheological behaviors are a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties, where cell size and shape and intrinsic motility are major players.

Magneto-mechanical response of additive-free Fe-based magnetorheological fluids: role of particle shape and magnetic properties

The flow properties of carbonyl iron (CI) and electrolytic iron (EI) micro-particles based magnetorheological fluids (MRFs) were studied under steady shear conditions at different applied magnetic field strengths (B). The CI particles were spherically-shaped having a wide size distribution ranging from ∼0.2 to ∼3 μm whereas the EI particles were irregularly-shaped with much bigger particles of sizes between ∼20 μm and 80 μm. To prepare the MRFs, 40 wt% each of CI and EI micro-particles were separately dispersed in silicone oil. In the MRFs the particles do not contain any surfactant/coating and the MRFs do not include any stabilizers in them. According to the magnetorheological results, although the yield strength (τY) and viscosity (η) of both MRFs increase with increase in B, there is deviation from the parallelism in increase of τY and η with B. This suggests that τY and η depend strongly on the physical parameters of Fe-particles such as the shape, size and saturation magnetization. The effect of saturation magnetization on steady state shear response of the MRFs dominates over the effect of shape and size of the particles.

Steady-shear magnetorheological response of fluids containing solution-combustion-synthesized Ni-Zn ferrite powder

Magnetically soft nickel-zinc ferrite (Ni0.5Zn0.5Fe2O4) powder with high saturation magnetization was synthesized by solution combustion route using metal nitrates as precursors and glycine as fuel. The particles were found to have irregular morphology. Three different concentrations of magnetorheological fluids (MRFs) were prepared by dispersing 10, 20 and 40 wt% of these particles in thin silicone oil. The behaviours of the MRFs were studied under steady shear conditions at different applied magnetic field strengths (B). The yield strength (τY) and viscosity (η) of all the MRFs were found to increase with B and particle fill fraction ϕ, while the response of the MRFs was strongly influenced by the morphology, microstructure and saturation magnetization of the particles. Owing to the low density of the particles, the observed off-state viscosity is high. However, the excellent thermo-oxidative and chemical stabilities of these magnetic oxide particles than metallic magnetic particles make these MRFs dependable for applications in harsh working environments. In addition, the low cost and feasibility of large scale preparation of these magnetic oxides make these MRFs further attractive for industrial applications.

3D-printed graphene-Al2O3 composites with complex mesoscale architecture

Composites of reduced graphene oxide (rGO) and Al2O3 with complex three-dimensional (3D) mesoscale architecture were prepared by a combination of 3D printing technique and thermal reduction process. For this purpose, stable aqueous graphene oxide (GO) suspensions were prepared using GO sheets synthesized previously from graphite powders according to a modified Hummers method. The printability of the resulted inks was investigated by rheological tests under steady and dynamic shear conditions. GO-Al2O3 structures with a 3D interconnected network were fabricated and thermally reduced in an N2 atmosphere. Finally, the reduction and distribution, as well as the integrity of rGO sheets in the ceramic matrix were evaluated using scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD) analysis. The reported 3D printed rGO-Al2O3 composite offers a strategy for the fabrication of other graphene-ceramic composites with a high geometric surface area for a broad range of technological applications.

Evaluation of Rheological, Textural and Thermal Properties of Quinoa (Chenopodium Quinoa Willd) Based Breakfast Puree

This study was conducted to determine the rheological, thermal, textural and color properties of quinoa based breakfast puree possessing high nutritional value and composed of mainly non-dairy milk (soy milk) which could be an alternative to conventional gluten-containing dairy breakfast meals. The rheological properties were evaluated under steady and dynamic shear conditions. The steady shear test was carried out at 27 oC using a rotational type of viscometer. Quinoa puree showed pseudo plastic non-Newtonian and time-independent flow behavior. Rheological data was modeled by using Power and Herschel Buckley Models (R2>0.99). Dynamic shear properties were determined by applying sweep test using a rheometer. G' (elastic modulus) was reported to be greater than G (viscous modulus) exhibiting elastic behavior. Specific Heat (Cp) of quinoa puree was determined endothermically using Differential Scanning Calorimeter (DSC). The sample thermogram showed no glass transition point. But the melting point appeared at elevated temperature (above 120 oC). Textural properties of the puree in terms of consistency, cohesiveness and firmness confirmed its viscoelastic nature. Water activity was about 0.97 and the color was reasonably light and brownish.

Rheological Behavior of Nanosilica Suspensions in Poly(Ethylene Oxide) Solutions with Sodium Chloride

Suspensions of silica nanoparticles showed shear-thickening profiles under steady shear conditions up on addition of a small amount of poly(ethylene oxide) (PEO). The suspensions turned into gels upon shaking and their fluidity was recovered several minutes after resting. We studied the rheological properties of these shake gels with small amounts of sodium chloride (NaCl). Gelation occurred at lower shear rates upon addition of small amounts of NaCl. In addition, the time taken by the gelated samples to recover their original viscosity increased with the increasing NaCl content. The weakened repulsive interactions between the silica particles upon NaCl addition lead to particles in closer proximity, and three-dimensional networks of PEO chains are easily formed as the electric double layer of the particles becomes thinner.

Steady shearing flow of a moderately entangled polyethylene liquid

The rheological properties and dynamical responses of a monodisperse polyethylene (PE) liquid, C700H1402, were examined using equilibrium molecular dynamics and nonequilibrium molecular dynamics simulations of the atomistically detailed molecules. Equilibrium structural and dynamical properties of the PE liquid, such as the disengagement time ( τd), Rouse time ( τR), entanglement time, ( τe), reptation tube diameter, number of entanglements, and the distribution of the chain end-to-end vector, each followed very closely the predictions of the Doi and Edwards theory. Under steady shear conditions, the rheological and dynamical responses exhibited starkly different behavior as functions of shear rate, which could be categorized within four distinct shear rate regions; namely, γ τe−1. In the first region, the topological properties of the liquid remained relatively unperturbed from quiescent conditions and the rheological characteristic functions remained constant th...

Comparison of rheological behaviors with fumed silica-based shear thickening fluids

Shear thickening fluids (STFs) of differing compositions were fabricated and characterised in order to observe the effect of varying chemical and material properties on the resultant rheological behavior. Steady shear tests showed that for a given carrier fluid and particle size exists an optimum weight fraction which exhibits optimal shear thickening performance. Testing also showed that increasing particle size resulted in increased shear thickening performance and its onset whilst altering the carrier fluid chemistry has a significant effect on the thickening performance. An explanation is provided connecting the effect of varying particle size, carrier fluid chemistry and weight fraction to the resultant rheological behavior of the STFs. Two STFs were chosen for further testing due to their improved but contrasting rheological behaviors. Both STFs displayed a relationship between steady and dynamic shear conditions via the Modified Cox-Merz rule at high strain amplitudes (γ≥500%). Understanding the effects of particle and liquid polymer chemistry on the shear thickening effect will assist in ‘tailoring’ STFs for certain potential or existing applications.

Measurement techniques for steady shear viscosity of Mozzarella-type cheeses at high shear rates and high temperature

While measuring steady shear viscosity of Mozzarella-type cheeses in a rotational rheometer at 70 °C, three main difficulties were encountered; wall slip, structural failure during measurement and viscoelastic time dependent effects. Serrated plates were the most successful surface modification at eliminating wall slip. However, even with serrated plates shear banding occurred at higher shear rates. Because of the viscoelastic nature of the cheeses, a time dependent viscous response occurred at shear rates <1 s−1, requiring longer times to attain steady shear conditions. Prolonged continuous shearing altered the structure of the molten cheeses. The effects of structural change were greatly reduced by minimising the total accumulated strain exerted on the sample during flow curve determination. These techniques enabled successful measurement of steady shear viscosity of molten Mozzarella-type cheeses at 70 °C at shear rates up to 250 s−1.

Comparison of rheological properties of native and pregelatinized potato starches

Rheological properties of native potato starch (NPS) and pregelatinized potato starch (PPS) were investigated under steady and dynamic shear conditions at 20°C and 60°C. The consistency index, apparent viscosity, and yield stress values of PPS were much lower than those of NPS. The flow behavior of PPS is more temperaturedependent. Dynamic moduli (G′ and G″) and complex viscosity of PPS were much lower than those of NPS, while tan δ values of PPS were in the range of 2.30–3.50 (tan δ>1), indicating that PPS is more viscous than elastic. Rheological properties of NPS are strongly influenced by pregelatinization.

Assessing the critical concentration of NH2 terminal groups on the surface of MWNTs towards chain scission of PC in PC/SAN blends: effect on dispersion, electrical conductivity and EMI shielding

The localization and dispersion quality of as received NH2 terminated multiwall carbon nanotubes (MWNT-I) and ethylene diamine (EDA) functionalized MWNTs in melt mixed blends of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) were assessed in this study using rheo-electrical and electromagnetic interference (EMI) shielding measurements. In order to improve the dispersion quality and also to selectively localize MWNTs in the PC phase of the blends, EDA was grafted onto MWNTs by two different strategies like diazonium reaction of the para-substituted benzene ring of MWNTs with EDA (referred to as MWNT-II) and acylation of carboxyl functionalized MWNTs with thionyl chloride (referred to as MWNT-III). By this approach we could systematically vary the concentration of NH2 functional groups on the surface of MWNTs at a fixed concentration (1 wt%) in PC/SAN blends. XPS was carried to evaluate the % concentration of N in different MWNTs and was observed to be highest for MWNT-III manifesting in a large surface coverage of EDA on the surface of MWNTs. Viscoelastic properties and melt electrical conductivities were measured to assess the dispersion quality of MWNTs using a rheo-electrical set-up both in the quiescent as well as under steady shear conditions. Rheological properties revealed chain scission of PC in the presence of MWNT-III which is due to specific interactions between EDA and PC leading to smaller PC grafts on the surface of MWNTs. The observed viscoelastic properties in the blends were further correlated with the phase morphologies under quiescent and annealed conditions. Electromagnetic interference (EMI) shielding effectiveness in X and Ku-band frequencies were measured to explore these composites for EMI shielding applications. Interestingly, MWNT-II showed the highest electrical conductivity and EMI shielding in the blends.

Electrical conductivity of poly(vinylidene fluoride)/polyaniline blends under oscillatory and steady shear conditions

Blends of poly(vinylidene fluoride) (PVDF) and polyaniline (PAni) were prepared through melt blending in a batch mixer. The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy (TEM) and combined electro-rheological measurements. Through TEM analysis, it was possible to observe that all blends showed typical phase separation with the presence of conductive polymer aggregates. Deformations imposed during a strain sweep caused, not only disturbance of the linear viscoelastic behavior, but also changes in electrical conductivity. The oscillatory shear altered the morphology, breaking the PAni domains into smaller ones. This effect increases the distance between them and, consequently, resulted in a decrease of the electrical conductivity. The measurements under quiescent conditions and steady shear proved that the disturbance in morphology for PVDF/PAni system is non-recoverable. Through combined electrical and rheological measurements, it was possible to achieve good correlation between the electrical and flow behavior of PVDF/PAni blends.