Evolution Of Viscosity Research Articles

Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain size sensitive creep: 3. Application of grain growth and flow laws on peridotite ultramylonite

Microstructures of a layered peridotite ultramylonite from the Oman ophiolite are compared with that of experimentally deformed samples. Average grain sizes and grain size ratios of olivine and pyroxene from each layer are compared with respect to the fraction of pyroxene (fpx) in the layer. Grain size of the pyroxene is almost constant among different fpx layers, whereas olivine grain size decreases significantly with increasing fpx, both of which were characteristic features found in forsterite + enstatite aggregates after grain growth experiments (Tasaka and Hiraga, 2013). Furthermore, the Zener relationship (log dol/dpx versus log fpx) found in the ultramylonite is remarkably comparable to that observed in our experiments. These observations indicate effective pinning of olivine grain growth due to the presence of pyroxene grains during the deformation of the rocks. Olivine grains in layers with fpx ≥ 0.03 do not exhibit lattice-preferred orientation (LPO), whereas the grains in layers with fpx < 0.03 exhibit LPO, indicating that deformation proceeded via diffusion- and dislocation-accommodated creep in the former and the latter layers, respectively. We simulated the evolution of grain size and viscosity in the shear zone based on our grain growth and flow laws obtained for diffusion creep of forsterite + enstatite (Tasaka and Hiraga, 2013; Tasaka et al., 2013) and successfully reproduced the observed grain sizes in the ultramylonite. We therefore conclude that the relative values of the kinetic parameters, some of which are functions of the fpx, are applicable to nature.

Time-Dependent Shear-Induced Nonlinear Viscosity Effects in Dilute CTAC/NaSal Solutions: Mechanism Analyses

The time-dependent shear-induced nonlinear viscosity effects of dilute surfactant solutions (CTAC/NaSal) at constant shear rate were tested by using the rheometer Couette cell. The apparent viscosity evolution curve can be divided into five stages: weak shear-thickening (Stage I), weak shear-thinning and plateau (Stage II), sharp shear-thickening (Stage III), oscillating adjustment (Stage IV), and rough plateau (Stage V). In Stage I, the stretching effects of shear flow lead to the weak increase in apparent viscosity at the inception of shearing. The apparent viscosity curve firstly decreases in Stage II and then levels off. The apparent viscosity plateau is caused by the forming and slipping of micellar lumps at the inner cylinder wall surface. Once the volume of lump exceeds a certain degree, the nucleation process of forming SIS is triggered, which is the beginning of Stage III and then the apparent viscosity increases sharply. The variations of apparent viscosity in adjusting period are rather complicated in Stage IV, and the variations mainly depend on the situation of SISs network. In Stage V, coupled with obvious oscillations, the apparent viscosity maintains a basically constant plateau value, indicating that the SISs network is fully developed and saturated at the corresponding shear rate.

Stokes–Einstein violation and fragility in calcium aluminosilicate glass formers: a molecular dynamics study

A series of classical molecular dynamics simulations of calcium aluminosilicate (CAS) (CaO–Al2O3)1–x(SiO2)x melts with varying silica content x, along the CaO/Al2O3 concentration ratio R = 1, have been performed for the purpose of studying (i) the evolution of fragility with silica content and (ii) the temperature and composition evolution of self-diffusion coefficients, viscosity and α-relaxation times. Our results indicate a decrease in the fragility of the CAS systems along R = 1 from calcium aluminate to pure silica. It is found that upon cooling, the Stokes–Einstein relation breaks down at a temperature significantly higher than the critical temperature of the mode-coupling theory TC for all compositions and also above the melting point.

Application of the systemic rheology to the in situ follow-up of viscosity evolution with reaction time in the synthesis of urethane prepolymers

For the first time, the in situ follow-up of the viscosity evolution with reaction time during the synthesis of isocyanate-terminated urethane prepolymers has been done. For that, the systemic rheology has been applied with the purpose of mimic the unit operations of adhesive processing, particularly the mixing and chemical reaction. Several behaviors were revealed with the methodology applied. For all the synthesis prepared at 100°C, we observed that the viscosity evolution tended toward a plateau, i.e., the viscosity reached constant values. However, for all the synthesis made at 60°C, the viscosity plateau was not at all observed and the viscosity remarkably increased during the studied reaction time. The viscosity decreased with an increase of: (a) molecular weight of the PPG or PPG blends, and (b) NCO/OH ratio. We also found that the latent pre-reaction time increased and the PPG's reactivity decreased when the molecular weight of PPG was increased. Moreover, at low NCO/OH ratio, the viscosity quickly increased when the amount of PPG-400 related to the PPG-2000 was high. Surprisingly, this behavior was not observed at high NCO/OH ratio.

Chemorheological study of a polyfurfuryl alcohol resin system—Pre-gel curing stage

The rheological and chemorheological behavior of a new biomass-based polyfurfuryl alcohol (FA) resin aimed to be used as a matrix in composite materials is studied in this work. The viscosity dependence on the shear rate and temperature of the resin is studied under steady and oscillatory conditions. The FA resin exhibits a Newtonian flow behavior within the shear rate range tested. The dependence of the viscosity of the uncured resin on temperature is measured and modeled. The flow activation energy, as calculated by the Arrhenius model, is 63.3kJmol−1. The chemorheological study of the curing process showed that the flow activation energy of the resin is increased when the amount of catalyst is increased. Furthermore, the curing parameters of the FA resin using three amounts of catalyst of 2, 4, and 6% (w/w), are obtained by the Arrhenius model. The curing activation energy is found to be about 96kJmol−1 and with no dependence on the amount of catalyst. However, a significant difference of the logarithm of the pre-exponential curing parameter is found. This parameter increases from 28.9 to 30.7 when the amount of catalyst is increased from 2 to 6% (w/w). Based on the established values of the Arrhenius model parameters, predictions are made for the evolution of viscosity of the resin during isothermal curing with different temperatures, and different amounts of catalyst. This is of great importance in the design of the curing processes of the FA resin during development and improvement of composite manufacturing processes.

Short-Term Flow Induced Crystallization in Isotactic Polypropylene: How Short Is Short?

The so-called “short-term flow” protocol is widely applied in experimental flow-induced crystallization studies on polymers in order to separate the nucleation and subsequent growth processes [Liedauer et al. Int. Polym. Proc. 1993, 8, 236–244]. The basis of this protocol is the assumption that structure development during flow can be minimized and the rheological behavior, i.e., the viscosity, does not change noticeably. In this work we explore the validity of this assumption for short but strong flows and reveal the structure formation during the early stages of crystallization. Viscosity and structure evolution of an isotactic polypropylene (iPP, Mw ≈ 365 kg/mol and Mw/Mn = 5.4) melt at 145 °C are measured during the short-flow period (0.2–0.25 s) using the combination of a slit rheometer and fast X-ray scattering measurements. For high enough (apparent) shear rates (≥240 s–1) a viscosity rise during flow is observed; i.e., the condition for “short-term flow” is not satisfied. With a time delay with respect to the viscosity rise, the development of shish is observed at a position halfway the length of slit, along the flow direction, by means of ultrafast time-resolved SAXS measurements. Depending on the shear rate, these shish are detected during (shear rates ≥ 400 s–1) or after flow (240 s–1 ≤ shear rates < 400 s–1). For even lower shear rates of 160 and 80 s–1, the viscosity does not change significantly, and instead of shish, oriented row nuclei (X-ray undetectable) are generated. These two shear conditions qualify as short-term flow. A full understanding of the coupled flow and crystallization phenomena requires that the transient and nonhomogeneous behaviors, both in flow and in flow gradient direction, have to be taken into account. This can only be done by a full numerical model, and therefore, the results presented in this paper also provide a valuable data set for future numerical studies.

Dielectric Analysis on the Microscopic Characteristics of the Cloudy Silwet L-77 Solution

Dielectric relaxation spectra of the cloudy Silwet L-77 solutions were investigated from 40 Hz to 110 MHz. A striking Maxwell-Wagner relaxation was observed at about 106 Hz, which was assigned to the interfacial polarization between the micelle particle (L1) and the continuous water (W). Based on the concentration and the temperature dependence of the dielectric increment and the low-frequency conductivity (σ l ), the critical data and the corresponding micellar structures of the Silwet L-77 solutions under cloudy state were clarified. Furthermore, the viscosity evolution in the W-L1 region was confirmed according to an anomalous lower temperature dependence of σ l .

Short-term and long-term irreversibility in particle suspensions undergoing small and large amplitude oscillatory stress

The short-term and long-term irreversible behaviors of suspensions of rigid particles in oscillatory shear flow are studied by measuring the evolution of complex viscosity in time and applying of nonlinear analysis of the responded strain signal under the controlled-stress mode, and complemented by optical measurements on the particle motion. The short-term transition time for the system to reach a quasisteady state is an approximately bell-shaped function of the amplitude of the strain response, thus showing a critical strain amplitude accounting for the peak transition time. The short-term behavior is caused by the particle self-organization due to collisions between particles. At longer time scales, the complex viscosity of the suspension increases when probed by forces that elicit small strain amplitudes and decreases when stresses that result in large strain amplitudes are applied. It is proposed that the long-term behavior for stresses eliciting small strain amplitude is induced by the shear-induced diffusion of particles which self-organize into a crystal-like microstructure that can be easily annulled in oscillatory flow with large strain amplitude, while for stresses causing large strain amplitude the dominant microstructure is formed immediately via the oscillation.

Photoinduced aging and viscosity evolution in Se-rich Ge-Se glasses

We propose here to investigate the non-equilibrium viscosity of Ge-Se glasses under and after light irradiation. Ge10Se90 and Ge20Se80 fibers have been aged in the dark and under ambient light, over months. During aging, both the relaxation of enthalpy and the viscosity have been investigated. The viscosity was measured by shear relaxation-recovery tests allowing the measurement of non-equilibrium viscosity. When Ge10Se90 glass fibers are aged under irradiation, a relatively fast fictive temperature decrease is observed. Concomitantly, during aging under irradiation, the non-equilibrium viscosity increases and reaches an equilibrium after two months of aging. This viscosity increase is also observed in Ge20Se80 fibers. Nevertheless, this equilibrium viscosity is far below the viscosity expected at the configurational equilibrium. As soon as the irradiation ceases, the viscosity increases almost instantaneously by about one order of magnitude. Then, if the fibers are kept in the dark, their viscosity slowly increases over months. The analysis of the shear relaxation functions shows that the aging is thermorheologically simple. On the other side, there is no simple relaxation between the shear relaxation functions measured under irradiation and those measured in the dark. These results clearly suggest that a very specific photoinduced aging process occurs under irradiation. This aging is due to photorelaxation. Nevertheless, the viscosity changes are not solely correlated to photoaging and photorelaxation. A scenario is proposed to explain all the observed viscosity evolutions under and after irradiation, on the basis of photoinduced transient defects.

Stiffening of the Cement Paste Monitored Using Vibrating Fork Technique

This study explored the stiffening behavior of the cement pastes using a new vibration folk technique. The viscosity evolutions of the cement pastes were evaluated using either a rotational viscometer or a tuning fork vibration viscometer. Results showed that the viscosities were increased with hydration time. The viscosity increases of the cement pastes measured by the vibro-viscometer were more stable than those measured by the rotational viscometer. In addition, the subtle changes in the paste viscosities during the very early hydration were well detected by the vibro-viscometer. The new vibration folk technique has been shown a useful tool to characterize the microstructure changes in the early cement pastes.

Temperature evolution of the shear viscosity in a unitary Fermi gas

We present an ab initio determination of the shear viscosity for the unitary Fermi gas based on finite temperature quantum Monte Carlo (QMC) calculations and the Kubo linear-response formalism. The results are confronted with the bound for the shear viscosity originating from hydrodynamic fluctuations. Assuming smoothness of the frequency dependent shear viscosity $\ensuremath{\eta}(\ensuremath{\omega})$, we show that the bound is violated in the low temperature regime and the violation occurs simultaneously with the onset of the Cooper pairing in the system. In order to preserve the hydrodynamic bound in QMC $\ensuremath{\eta}(\ensuremath{\omega})$ has to possess a sharp structure located in the vicinity of zero frequency which is not resolved by an analytic continuation procedure.

Study on the rheological properties of Portland cement pastes with polycarboxylate superplasticizers

The coupling effects of temperature and time on the fluidity of fresh cement mixtures were investigated. Mini-cone tests on cement mortars and rheological tests on cement pastes under different temperatures (0 to 60 °C) were conducted to characterize the development of the fluidity of fresh cement mixtures over time. In addition, total organic carbon tests were performed to quantify the adsorption amount of superplasticizers on the cement surface. The amount of free water in cement pastes was determined via centrifugation. Isothermal calorimetry was employed to characterize the hydration kinetics of cement under different temperatures. Results show that the spread diameter of mortars decreases in a roughly linear fashion over elapsed time. Higher temperature facilitates a sharper decrease in fluidity with time, although the initial fluidity of fresh mortars is not significantly affected by temperature. Higher temperature results in a greater amount of adsorbed polycarboxylate ester/ether on the cement surface and a lower amount of free water in fresh cement pastes, which is believed to result from the higher hydration rate of cement. The evolution of rheological properties over time can be attributed to the development of hydration degree. Relative hydration degree is introduced to indicate the development of rheological properties with time. Two models to describe the evolution of yield stress and plastic viscosity for fresh cement pastes were developed.

Viscosity of carbon nanotubes water-based nanofluids: Influence of concentration and temperature

Experimental results on the steady-state viscosity of carbon nanotubes water-based nanofluids are presented considering the influence of particle volume fraction and temperature ranging from 0 to 40 °C. The suspensions consist of multi-walled carbon nanotubes dispersed in de-ionized water and they are stabilized by a surfactant. The aspect ratio of nanotubes is close to 160 and the particle volume fraction varies between 0.0055% and 0.55%. It is shown that the nanofluids behave as shear-thinning materials for high particle content. For lower particle content, the nanofluids are quite Newtonian. It is also observed that the relative viscosity of nanofluids at high shear rate does not vary with temperature. Moreover, the evolution of relative viscosity at high shear rate is well predicted by the Maron–Pierce model considering the effect of nanoparticles agglomerates.

Salt effects on thermodynamic and rheological properties of hydrate forming emulsions

Results from an experimental study of the effect of aqueous phase salinity on a density-matched 40% (v/v) aqueous fraction hydrate-forming emulsion are reported. Using micro-differential scanning calorimetry and rheometry, a correlation of the rheological properties to thermodynamic driving forces is obtained. The cyclopentane hydrate-brine system liquidus line is determined from the calorimetric study. Equilibrium temperature and thermodynamically allowed conversion of water to hydrate are computed from the hydrate-brine phase diagram. Good agreement was found between the experiments and calculations by an established thermodynamic simulation tool. Rheological properties, including the viscosity evolution time, final viscosity and its shear rate dependence, and yield stress of the hydrate-forming emulsion are determined; the material viscosity increases markedly as hydrate forms and the mixture develops a yield stress. The viscosity evolution time is smaller at lower temperature, i.e., at higher subcooling relative to the hydrate equilibrium dissociation temperature. The final hydrate slurry exhibits shear thinning effects describable by the Ostwald-de Waele model, and displays very little hysteresis in the shear rate dependence. Maximum viscosity is attained when the thermodynamic water to hydrate conversion is 61–85% while the yield stress was found to be a maximum at about 80% conversion; this non-monotonic rheological property dependence on the thermodynamically allowed water to hydrate conversion demonstrates that both liquid (brine) and solid hydrate content are important to the mechanical properties of the network structure developed when hydrate is present.

Evolution of the Coefficient of Dynamic Viscosity with Growing Damage in Metals

Coefficient of dynamic viscosity (η) for uniaxial rectangular bar of steel is calculated using creep simulation in Ansys. Norton Secondary creep law with coefficients at 600 ᵒC is used. Steel bar is then inflicted with some damage elements in order to investigate the evolution of dynamic viscosity for varying degree of fatigued specimen at constant rate and temperature. Isotropic damage parameter (D) based on the volume fraction of damaged elements is used to formulate a linear relation between the coefficient of viscosity and D. It is found that with 0.001 increase is damage parameter (D), almost 0.6% decrease on the value of Coefficient of dynamic viscosity (η) is observed.

Rheological Study of Mutarotation of Fructose in Anhydrous State

Rheological measurement was employed to study the mutarotation of D-fructose in anhydrous state. By monitoring the evolution of shear viscosity with time, rate constants for mutarotation were estimated, and two different stages of this reaction were identified. One of the mutarotation stages is rapid and has a low activation energy, whereas the other is much slower and has a much higher activation energy. Possible conversions corresponding to these two phases are discussed. This work demonstrates that, in addition to the routine techniques such polarimetry and gas-liquid chromatography, rheological measurement can be used as an alternative method to continuously monitor the mutarotation of sugars.

Relationship Between pH and Carbonation Ratio to Measure CO2 Capture Efficiency by NH3 Solvent

This paper presents an established relationship between the pH of ammonia solution and its carbonation ratio (α). The aim of this correlation is to track the amount of CO2 absorbed or the amount of CO2 released from ammonia solvent. Furthermore, this paper studies the evolution of density and viscosity of ammonia solution with the carbonation ratio because those physical properties can significantly affect liquid side mass transfer coefficient. Experiments have been carried out in a thermoregulated Lewis-type cell reactor and the measures have been done using three concentrations of ammonia (3, 5 and 7 wt %) and temperatures (293, 303 and 313 K). Results show that the estimated function has a high correlation coefficient which indicates a relationship between variables. All results show a linear correlation between viscosity and carbonation ratio, and between density and carbonation ratio.

Rheological study of the SAN modified epoxy–DDM system: relationship between viscosity and viscoelastic phase separation

The rheological behavior and structural transitions in an epoxy–amine system and poly(styrene-acrylonitrile) SAN modified epoxy–amine systems were studied by, optical microscopy and rheometry. The SAN modified epoxy blends undergo viscoelastic phase separation. The concentration of SAN has a profound effect on viscoelastic phase separation and hence the rheological behavior of modified epoxy systems. The evolution of complex viscosity is closely related to viscoelastic phase separation. The increase in complex viscosity shifted to shorter times upon increasing the concentration of SAN due to viscoelastic phase separation. The complex viscosity profiles follow an exponential growth with phase separation at various cure temperatures. The relaxation time of viscosity growth depends on both the concentration of SAN and cure temperature. The relaxation time of viscosity growth can be described by the WLF equation.

Rheological studies of tautomerization kinetics in supercooled glibenclamide drug

Rheological measurements have been applied to study the tautomerization of the pharmaceutically active compound glibenclamide. The rate constant and activation energy of the imidic-acid-amide transformation have been successfully determined by monitoring the evolution of shear viscosity. The kinetic parameters from rheological measurements agree reasonably well with the data previously obtained from dielectric spectroscopy. The present Brief Report demonstrates that rheology can provide a fast and precise way to characterize the reaction kinetics of tautomerization.

Effect of room-temperature out-time on tow impregnation in an out-of-autoclave prepreg

Tow impregnation as a function of material out-time was investigated for an out-of-autoclave carbon fiber–epoxy prepreg. Prepreg was aged at ambient temperature for 56days. Every 7days, laminates were laid up and cured using vacuum bag only processing. Void content was calculated through image analysis of polished sections. Experimental results were used to validate an analytical model for tow impregnation. Model predictions were based on flow kinetics during processing conditions, taking into account increasing degree of cure and evolution of resin viscosity as a function of ambient aging time. The study found that no significant tow porosity occurred within the material’s stated out-life, that tow porosity increased once this out-life was exceeded and eventually stabilized due to the room-temperature vitrification of the resin. The model’s predicted trends were consistent with experimental results, suggesting that an increase in resin viscosity is indeed the main cause of out-time induced tow porosity and providing a means of predicting laminate quality as a function of room temperature aging time.