Time-dependent Rheological Properties Research Articles

Effect of Citric Acid on the Time-Dependent Rheological Properties of Magnesium Oxychloride Cement

AbstractThis study was intended to evaluate how citric acid influences the time-dependent rheological properties of magnesium oxychloride cement (MOC). For this purpose, rheological parameters, set...

Evolution of the Size Distribution of an Emulsion under a Simple Shear Flow

Understanding the rheology of immiscible liquids mixtures, as well as the role played by its micro-structures are important criteria for the production of new materials and processes in industry. Here, we study changes over time of the droplet size distributions of emulsions induced by slow shearing flows. We observe that the initial heterogeneous microstructure may evolve toward more complex structures (such as bimodal distribution) as a result of coalescence and rupture of droplets. These dynamic structures were produced using a flow cell made up of two parallel disks, separated by a gap of 100 µm. The steady rotation of the lower disk generates a simple shear flow of γ˙=0.75 s−1, during ~400 s. After a brief rest time, this procedure was repeated by applying a step ramp until the maximum shear rate of 4.5 s−1 was reached, using step increments of 0.75 s−1. During the last portion of the flow and during the rest time in between flows, structures of emulsions were characterized. Initially, a broad single-peak distribution of drops was observed, which evolved toward a rather narrower bimodal distribution, at first due to the coalescence of the smaller droplets and subsequently of the larger drops. The rupture of drops at higher shear rates was also observed. The observed evolutions also presented global structures such as “pearl necklaces” or “bands of particles”, the latter characterized by alternating bands of a high density of particles and regions of the continuous phase with only a few droplets. These changes may indicate complex, time-dependent rheological properties of these mixtures.

Quasi-Static Acoustic Tweezing Thromboelastometry

Acutely unwell patients often develop coagulopathy due to depletion of both pro- and anti-coagulants. They are at high risk of both bleeding and thrombotic complications and require monitoring of their coagulation status. Available contact assays for blood plasma and whole blood coagulation have low predictive power in patients with coagulopathy or take a significant amount of time to obtain diagnostic data. We have developed quasi-static acoustic tweezing thromboelastometry (QATT) as a noncontact method for real-time assessment of blood coagulation. In our method, 4-5 microliter human blood drops are levitated in air by the acoustic radiation force. The sample drop location and deformation are controlled by quasi-statically changing the acoustic pressure. Time-dependent rheological properties of the samples (e.g. sample elasticity) are determined from digital images at each pressure level. Results of our study show that increasing elasticity during coagulation causes blood drops to deform less, leading to steeper deformation/location curves. By extracting a linear regime slope, the samples exhibit a unique slope profile (tweezograph) as they begin to clot. By using tweezographs, we were able to measure the reaction time, coagulation rate and maximum clot stiffness. The exposure of whole blood samples to pro- or anti-coagulants led to significant changes in tweezographs within 10 minutes of sample tweezing, thus allowing detection of hyper- or hypo-coagulable states. Similarly, we detected significant changes between low, normal, and high fibrinogen level control plasma samples, which points out to the ability of our method to measure the fibrinogen level. We demonstrated that QATT can rapidly yield information about coagulation properties of human blood and requires a very small sample volume (∼4 microliter). The advantages of small sample size, non-contact and rapid measurement make this technique desirable for real-time monitoring of blood coagulation in acutely unwell patients.

Time-Dependent and Time-Independent Rheological Characterization of Date Syrup

The time-dependent and time-independent rheological properties of Barhi date syrup have been investigated. Rheological measurements were performed with a rotational viscometer with parallel plate geometry. The date syrup showed thixotropic behavior and a first order exponential decay model characterized the time-dependent behavior. The rate constant of the structure breakdown was found to be a function of shear rate. The steady shear flow measurements showed that the date syrup is a non-Newtonian material fit the power law model (p <0.001). The Arrhenius model described the effect of temperature on consistency coefficient; the estimated parameters from the Arrhenius equation were used to develop a prediction rheological model for the apparent viscosity. The model accurately predicts the experimental data even when extrapolating beyond parameter estimation temperature range. The time-independent viscosity model was satisfactory for modeling date syrup despite the presence of thixotropic behavior.

Rheological mechanical properties of altered-rock

In order to investigate the rheological properties of altered-rock in Xiaowan Hydropower Project, triaxial creep tests were carried out on altered-rock specimens using a rock rheology testing machine. The rheological test results show that the time-dependent rheological properties of altered-rock are very significant. Based on the triaxial rheological test results, the relationship between axial strain and time under different confining pressures is investigated. The rheological effect on the stress–strain curve of altered-rock is discussed. Moreover, the relationship between axial strain rates and deviatoric stress under different stress levels is discussed in detail. The rheological failure mechanism under different confining pressures is also analysed. The triaxial rheological law of altered-rock specimen is achieved. The Burgers rheological model is used to fit the creep curves, and its parameters are obtained. The result shows that Burgers model can exactly describe the rheological properties of altered-rocks in Xiaowan Hydropower Project.

Effect of High Pressure Treatment on Rheological Characteristics of Egg Components

The objective of this study was to evaluate the effect of high pressure treatment on rheological properties of various egg components (i.e., egg white, egg yolk, and whole liquid egg). A five-level central composite design with three independent variables (pressure, 281.8–618.2 MPa; temperature, 8.2–41.8°C; pressure holding time, 1.6–18.4 min) was employed. Samples were packed in plastic pouches, heat sealed, and subjected to pressure treatment in an isostatic press. Shear and time-dependent rheological properties were evaluated using a 3-segment (each 5 min) program with the shear rate initially increasing from zero to 100 s−1 (upward curve), then held at 100 s−1 (hold curve), and finally decreasing from 100 to 0 s−1 (downward curve) using a parallel plate rheometer. Power law model was fitted to the upward (virgin) and downward curves while Weltman time dependency model was fitted for the hold curve (R2 > 0.90). Rheological properties were most significantly affected by pressure followed by holding time and temperature for all egg components. In general, they showed thixotropic shear thinning behavior and reduction in time dependency, which was dependent on the level of processing treatment used (combination of pressure, time, and temperature level). The high pressure treatment resulted in a progressive transition of egg components from flowable liquid-like behavior (218–350 MPa) to semi-viscous (350–500 MPa) to highly-viscous (500–618 MPa) behavior. Rheology-based optimal conditions satisfying HP pasteurization needs were identified.

A micro-mechanical study of coarsening and rheology of colloidal gels: Cage building, cage hopping, and Smoluchowski's ratchet

We study via theory and dynamical simulation the evolving structure, particle dynamics, and time-dependent rheological properties of an aging colloidal gel, with a focus on the micro-mechanics that drive coarsening and age-related changes in linear-response behavior. When colloids in suspension attract one another, the attractions can lead to phase separation into particle-rich and particle-poor regions separated by a single interface. But this transition is sometimes interrupted before full separation occurs. With certain particle concentrations and interparticle potentials, the attractions that promote phase separation also inhibit it, frustrating the separation and “freezing in” a nonequilibrium particle configuration, resulting in a space-spanning gel. With attractions on the order of a few kT, gelation can produce nonfractal bicontinuous morphologies. In such “reversible” gels, thermal fluctuations are strong enough to rupture bonds and reform new ones, allowing restructuring of the gel over time. But, because particle diffusion is dramatically slowed by interparticle attractions, the march toward equilibrium is frustrated. Prior studies of colloidal gels have examined evolution of length scales and dynamics such as decorrelation times or heterogeneity. Left open were additional questions such as how the particle-rich regions are structured (liquidlike, glassy, and crystalline), how restructuring takes place (via bulk diffusion, surface migration, and coalescence of large structures), and the impact of the evolution on rheology. In this study, we conduct dynamic simulations to elucidate the post-gelation evolution of a system of 750 000 Brownian spheres interacting via a hard-sphere repulsion and short-range attractions of order kT, as would be generated by a polymer depletant, for example. We find that the network strands comprise a glassy, immobile interior near random-close packing, enclosed by a liquidlike surface along which the diffusive migration of particles drives coarsening. We show that coarsening is a three-step process of cage forming, cage hopping, and cage arrest, where particles migrate to ever-deeper energy wells via “Smoluchowski's ratchet.” Both elastic and viscous high-frequency moduli are found to scale with the square-root of the frequency, similar to the perfectly viscoelastic behavior of nonhydrodynamically interacting, purely repulsive dispersions. But here, the behavior is elastic over all frequencies, with a quantitative offset between elastic and viscous moduli which owes its origin to the hindrance of diffusion by particle attractions. Propagation of this elasticity via the network gives rise to age-stiffening as the gel coarsens. This simple phenomenological model suggests a rescaling of the moduli on network length scale which, when carried out, collapses each modulus for all gel ages onto a single universal curve. A theoretical model inspired by the Rouse model is advanced and, from it, we have obtained an analytical expression that captures the effects of (finite) structural aging on rheology: The moduli are linear in the network size, suggesting that linear mechanical response can be determined at any age by measurement of dominant network length scale—or vice versa.

Effect of high pressure homogenization (HPH) on the rheological properties of tomato juice: Time-dependent and steady-state shear

High pressure homogenization (HPH) is a non-thermal technology that has been widely studied as a partial or total substitute for the thermal processing of food. Although microbial inactivation has been widely studied, there are only a few works in the literature reporting the physicochemical changes caused in fruit products due to HPH, especially those regarding the rheological properties. The present work evaluated the effect of HPH (up to 150MPa) on the time-dependent and steady-state shear rheological properties of tomato juice. HPH reduced the mean particle diameter and particle size distribution (PSD), and increased its consistency and thixotropy. The rheological results were in accordance with the PSD observed. The rheological properties of the juice were evaluated by the Herschel–Bulkley and Falguera–Ibarz models (steady-state shear) and Figoni–Shoemaker and Weltman models (time-dependent). The parameters of these equations were modelled as a function of the homogenization pressure. The models obtained described the experimental values well, and contributed to future studies on product and process development.

Design of a ‘driven cylinder’ viscometer for bone cement rheological characterization

The long-term success of surgical applications requiring bone cement is strongly influenced by the cement’s rheological properties. Previous studies showed a clear non-Newtonian, pseudoplastic, time-dependent rheological behaviour of acrylic bone cement during the working time. However, the particular nature of this material raises issues about its rheological characterization: the aim of this paper is to develop a new device, called ‘driven cylinder’ viscometer, able to accomplish the rheological characterization of acrylic bone cement. Three different fluids were tested by using the ‘driven cylinder’ viscometer: (a) liquid soap, (b) low-viscosity bone cement, and (c) vertebroplasty bone cement. For the sake of comparison, the rheological behaviour of the fluids (a) and (b) was also evaluated using traditional viscometers. The developed device allowed evaluation of the constitutive rheological parameters for non-Newtonian, power-law fluids. Independent evaluation of the viscosity of liquid soap and low-viscosity bone cement carried out by means of parallel-plate rotational rheometers showed a good agreement with the trends obtained by the ‘driven cylinder’ viscometer. The device versatility suggests its application for a complete description of pseudoplastic and time-dependent rheological properties of acrylic bone cements, which is a mandatory step for virtual tools of cement-based surgical procedures.

Effects of Fat Replacers and Sweeteners on the Time-Dependent Rheological Characteristics and Emulsion Stability of Low-Calorie Pistachio Butter: A Response Surface Methodology

The effect of three fat replacers (xanthan gum, Reihan seed gum, and Balangu seed gum) and two sweeteners (sucrose and isomalt) on time-dependent rheological properties of low-calorie pistachio butter were analyzed using response surface methodology. The steady shear behavior of all samples was shear thinning (n = 0.156–0.6175), and power law model was fitted the upward and downward curves properly (R 2 = 0.847–0.998). Balangu seed gum (0.01–0.04 wt.%), Reihan seed gum (0.01–0.023 wt.%), xanthan gum (0.06–0.1 wt.%), isomalt (0–1 wt.%), and sucrose (0.25–1 wt.%) were the levels investigated. A central composite design was used to develop models for the responses. The obtained experimental data were fitted to a second-order polynomial equation and also analyzed by appropriate statistical methods. In most cases, increasing the sweetener level led to a significant decrease in consistency coefficients. However, the effect on the flow behavior index was not significant. The effect of gum concentration on the rheological parameters was not significant (p ≤ 0.1), except for formulas prepared using Balangu seed gum. All formulas studied were stable on shelf.

Rheological Behavior of Tomato Juice: Steady-State Shear and Time-Dependent Modeling

Introduction The present work has evaluated the time-dependent and steady-state shear rheological properties of tomato juice.

Epithelial Coating Mechanisms by Semi-Solid Materials: Application to Microbicide Gels

Many epithelial surfaces have natural coating by polymeric materials, e.g. mucus. Foreign materials may be introduced for coating, e.g. for lubrication or drug delivery: examples are vaginal gels delivering mucosal antigens or topical microbicides. We present here a next generation biophysical vaginal coating model, which supersedes our previous work. The model characterizes the vagina as an elastic tube with a flattened lumen. The walls have porous surfaces through which natural vaginal fluid transudates, contacting and diffusing into a gel coating layer within the lumen. Spreading of the gel layer is driven by gravity and other trans-luminal pressure gradients, and wall elasticity. Gel rheology is characterized by the Carreau constitutive equation, including the presence of a yield stress. The model determines the local dilution of gel as water is transported into it, which is linked to local dilution and time-dependent rheological properties. This association is obtained experimentally. Gel coating flow is computed, accounting for variable properties at each spatial location and time step. A set of current and prototype microbicide gels is being evaluated. Results show the predominance of yield stress at later times during flow; the flow ceases when remaining vaginal wall distension is insufficient to develop shear stresses that exceed the yield stress. Dilution is most important near the vaginal walls and the leading edge of the spreading bolus. It is there that dilution proceeds most quickly, where the local viscosity of the gel drops most, and where spreading accelerates most. For the test gels, there are trade-offs amongst the dilution-dependent yield stress, limiting low shear viscosity, and rate of shear thinning, in rates of epithelial coating. Practically, these provide flexibility in optimizing gel compositions for target rates of epithelial coating. [Supported by NIH AI48103, CHRP ID07-B-135]

Low Shear Rheology of Concentrated Tomato Products. Effect of Particle Size and Time

Time-dependent rheological properties of three tomato paste suspensions in the concentration range of 200-1,000 g paste/kg suspension have been investigated by using the vane geometry at shear rates gamma >sigma y): before homogenization the suspensions exhibited a steady-state viscosity at large deformations, whereas after homogenization, the transient viscosity continuously decreased. That behavior was attributed to flocculation of the particles.

Time Dependent Rheological Characteristics of Pineapple Jam

Jam is an intermediate moisture food containing fruit pulp, pectin, sugar and acid. The effect of sugar and pectin concentration, pH, shear rate and temperature on the time dependent rheological properties of pineapple jam was studied using a rheometer. Pineapple jam exhibited thixotropic behavior. Shear stress of the pineapple jam at a particular time of shearing depended on the shear rate, temperature and composition. Weltman, Hahn, and Figoni and Shoemaker, models were applied to describe the time dependent flow properties of pineapple jam. Hahn model described adequately the rheological characteristics of pineapple jam.

Long-term failure of a layered viscoelastic composite material with a crack under a time-dependent load

The long-term failure of a layered viscoelastic composite caused by precritical propagation of a coin-shaped crack is studied. It is assumed that the crack is located inside a viscoelastic layer (the layer of binder) parallel to the layer orientation. The crack development due to stretching of the composite massive by uniformly distributed external forces increasing with time is described. It is assumed that these forces act perpendicularly to the plane of crack propagation. The investigation is carried out within the framework of Boltzmann-Volterra linear theory for resolving integral operators with difference kernels describing the deformation of a material with time-dependent rheological properties. An irrational function of the viscoelastic integral operator is presented in the form of a proper continued fraction and transformed using the method of operator continued fractions. Numerical solutions are obtained for resolving integral operators with the kernel in the form of Rabotnov exponential-fractional function. The kinetics of crack growth with a prefailure zone commensurable with the crack length is described. A comparison with the results obtained in terms of the concept of thin structure of the crak tip is given.

Development of a crack with a considerable prefracture zone in a viscoelastic orthotropic plate under variable loads

The study is made of the delayed fracture of a viscoelastic orthotropic plate caused by subcritical advancement of a rectilinear microcrack, which is located along one of the orthotropic axes. The crack develops because of stretching of the plate by uniformly distributed increasing and cyclic external forces perpendicular to the crack line. The investigation is carried out within the framework of the Boltzmann-Volterra theory for resolvent integral operators of difference type, which describe the deformation of a material with time-dependent rheological properties. The analytical form of the kernel of an irrational function of a linear combination of the above integral operators is determined by the method of operator continued fractions. Numerical calculations are conducted for resolvent bounded integral operators with a kernel in the form of Rabotnov's fractional-exponential function. The kinetics of growth of a crack with tip regions commensurable with the crack length is studied. A comparison with the results obtained within the framework of the concept of the thin structure of the crack tip is given.

The delayed fracture of an isotropic viscoelastic plate with a microcrack under a varying load

The delayed fracture of an isotropic viscoelastic plate due to subcritical growth of a rectilinear microcrack of normal separation is studied. The paper deals with the development of the crack due to the stretching of the plate by uniformly distributed increasing and cyclic external loads applied perpendicularly to the crack line. The investigation is carried out within the framework of the Boltzmann-Volterra theory for resolvent integral operators of difference type, which describe the deformation of materials with time-dependent rheological properties. Numerical calculations are performed for resolvent integral operators with a kernel in the form of Rabotnov's fractional-exponential function. The kinetics of a crack with a tip zone commensurable with the crack length is studied. The results are compared with those obtained on the basis of the hypothesis on the thin structure of the crack tip.

Rheology of Sodium Caseinate Stabilized Oil-in-Water Emulsions

We report on shear rheological measurements at 30°C of fine oil-in-water emulsions (volume–surface average diameter < 0.5 μm) prepared at pH 6.8 with sodium caseinate as the sole emulsifier (1–6 wt%) andn-tetradecane as the dispersed phase (10, 35, or 45 vol%). Strong sensitivity of rheological behavior to total protein concentration was indicated by both steady-state viscometry and small-deformation oscillatory experiments. The behavior can be classified into three types, depending on the protein/oil ratio. (1) Emulsions containing insufficient protein for (near-) saturation protein surface coverage develop a time-dependent increase in low-stress apparent viscosity and associated shear-thinning behavior; this can be attributed to bridging flocculation. (2) Emulsions having full protein surface coverage but relatively little excess unadsorbed protein in the continuous phase are stable Newtonian liquids. (3) Emulsions containing a substantial excess of unadsorbed sodium caseinate exhibit considerable pseudoplasticity which can be attributed to depletion flocculation. Taken as a whole, the time-dependent rheological properties for this set of emulsions as a function of protein content and oil volume fraction are largely consistent with our previous results on the creaming stability and the particle gel microstructure for these same emulsion systems. In particular, the reversible flocculation of emulsion samples of high protein content is readily explicable in terms of depletion flocculation of droplets by unadsorbed protein existing in the form of approximately spherical caseinate submicelles.

Rheological behaviour of gelled, waxy North Sea crude oils

The design of subsea pipelines carrying waxy crude oil, especially in Northern and arctic areas, is largely controlled by start-up pressure drop requirements. The correct measurement of rheological parameters associated with start-up and line-clearing of gelled crude oil pipelines, is thus important for proper design of pumping capacity and pipeline dimensions. In Part 1, the yielding behaviour of some waxy North Sea crude oils was studied by means of different controlled stress experiments, using a controlled stress rheometer (CSR) with cone-and-plate geometry and a model pipeline. Strong dependence of yield stress on cooling rate and stress loading rate was observed. Reasonably good estimates of yield stress, as compared to model pipeline results, were obtained with the CSR. CSR measurements are thus considered to be a valuable supplement to model pipeline experiments, particularly for comparative purposes, such as flow improver screening. In Part 2, the time-dependent rheological properties of two waxy oils were studied under controlled stress and controlled shear rate conditions, mainly in the cone-and-plate geometry. Two different approaches for modelling of viscosity decay at constant shear stress, based on a first-order rate equation and the Bingham rheological equation of state respectively, reproduced experiments reasonably well. Equilibrium properties obtained in different instruments (controlled stress/ controlled shear) and geometries (cone-and-plate and concentric cylinder) agreed within about 15% for shear rates between 14 and 700 s −1. At constant temperature, the broken-down apparent viscosity recovered moderately at rest, whereas the breakdown of static yield stress was essentially irreversible. Slight temperature cycling (±1° C) induced rebuilding of a weak structure, but did not restore the original gel strength completely.

Flow birefringence of polymer melts: Application to the investigation of time dependent rheological properties

Transient stresses including normal stresses, which are developed in a polymer melt by a suddenly imposed constant rate of shear, are investigated by mechanical measurement and, indirectly, with the aid of the flow birefringence technique. For the latter purpose use is made of the so-called stress-optical law, which is carefully checked.