Equilibrium Viscosity Research Articles

Slot coating of time-dependent structured materials: Effects of thixotropy on the flow dynamics and operating limits

Some of the most common liquid-like formulations rooted in the coating industry consist of rheologically complex structured materials such as inks, paints, and slurries. Yet, the impact of time-dependent structuring effects due to thixotropy on thin film coating applications remains elusive and still unclear. Here, we present a computational study of the effects of thixotropy on slot coating of time-dependent structured materials. By coupling a recent fluidity-based constitutive model for thixotropic materials with a well-established finite element/elliptic mesh generation method for free surface flows, we assess the role of thixotropy by comparing the predictions from the thixotropic model with those from a simple Generalized Newtonian Fluid model that uses the same flow curve for the material steady-state equilibrium viscosity. We find that thixotropy can indeed have a major impact on slot coating applications, potentially bringing strong implications not only to the flow dynamics but also to the operating limits of the process. In conclusion, the results and discussions we present in this study underscore the importance of accounting for thixotropy to genuinely model and fundamentally understand the behavior of time-dependent structured materials with complex rheology in processing flows like those ubiquitous across the broad fields of coating science and engineering.

The rheology of rhyolite magma from the IDDP-1 borehole and Hrafntinnuhryggur (Krafla, Iceland) with implications for geothermal drilling

Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at ∼ 700 K, at a rate of between 7 and 80 K.min−1. We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H2O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H2O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123–1193 K, the viscosity of the stored magma is ∼ 3×105 Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to ∼ 2×109 Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Transferable Anisotropic Mie Potential Force Field for Alkanediols.

The development of force fields for polyfunctional molecules, such as alkanediols, requires a careful account of different average intramolecular conformations for gas states compared to dense liquid states, where intra- and intermolecular hydrogen bonds compete. In the present work, the transferable anisotropic Mie (TAMie) potential is extended to 1,n-alkanediols. Using the convention that intramolecular nonbonded interactions up to and including the third neighbor are excluded, all force field parameters developed previously for 1-alcohols were transferred to 1,5-pentanediol and beyond, with good agreement with experimental phase equilibrium data. To obtain trans-gauche ratios of 1,2-ethanediol and 1,3-propanediol that are consistent with experimental results, the propensities for intra- and intermolecular hydrogen bonds had to be balanced. This was achieved by parameterizing the intramolecular dihedral energy functions governing the O-C-C-O and O-C-C-C angles while intramolecular charge-charge interactions were active. All partial charges belonging to a functional group are collected in a charge group and all interactions among two charge groups are evaluated even if they are separated by less than three bonds. With this approach, it is possible to apply the nonbonded parameters from 1-alcohols to alkanediols without further refinement. The agreement with experimental phase equilibrium and shear viscosity data is of similar quality as for the 1-alcohols and the trans-gauche ratio agrees with literature results from spectroscopic measurements and ab initio calculations.

On the Shear-Thinning of Alkanes

The approximate power law dependence of the apparent viscosity of liquids on shear rate is often argued to arise from a distribution of energy barriers. However, recent work on the Prandtl model, which consists of a point mass being dragged by a damped, harmonic spring past a sinusoidal potential, revealed a similar dependence of the friction on velocity as that of many liquids. Here, we demonstrate that this correlation is not only qualitative but can also be made quantitative over a broad temperature range using merely three dimensionless parameters, at least for alkanes, in particular n-hexadecane, at elevated pressure p. These and other observations made on our all-atom alkane simulations at elevated pressure point to the existence of an elementary instability causing shear-thinning. In addition, the equilibrium viscosity shows power law dependence on p near the cavitation pressure but an exponential dependence at large p, while the additional parameter(s) in the Carreau–Yasuda equation compared to other rheological models turn out justifiable.

Experimental study on the viscosity of trifluoroiodomethane (R13I1) with polyol ester and polyvinylether lubricating oils

Trifluoroiodomethane (R13I1) has attracted extensive attention because of its excellent environmental protection performance, and can be used as a component of environmentally friendly refrigerant blends. To better understand the cycle performance of R13I1 as a refrigerant, it is necessary to study the thermodynamic properties of the R13I1/lubricating oil mixture. In this work, the viscosities of polyol ester (POE) oil (RL 32H) and polyvinylether (PVE) oil (FVC 32D) saturated with R13I1 were measured at temperatures ranging from 293.15 to 353.15 K and pressures up to 0.6 MPa using a custom equilibrium viscosity system. The viscosities of R13I1/RL 32H and R13I1/FVC 32D showed similar trends with temperature, pressure, and solubility. With the increase of the solubility of R13I1 in the oil, the viscosity of the mixture decreased sharply at first and then tended to be flat. The dissolved R13I1 in oil greatly reduced the viscosity of lubricating oil, and the effect of R13I1 on the viscosity of RL 32H was greater than that of FVC 32D. The experimental viscosity data were correlated using the modified Setchenow-type equation, the Eyring-NRTL model, and the Eyring-MTSM model, and the calculated results of the models were in good agreement with the experimental data.

Size-dependent viscosity of silica optical fiber under high temperature

Viscosity of optical fiber plays an important role in high temperature applications in harsh environments. A size-dependent viscosity phenomenon of silica optical fiber under high temperature is observed by the stretching method with in-fiber regenerated fiber Bragg gratings (RFBGs). Higher viscosity is derived from optical fiber with larger diameters. The mechanisms of this size-dependent relationship of high temperature viscosity of silica optical fiber are discussed, and the difference in fictive temperature of silica optical fibers is considered as the mainly contribution. An equivalent fictive temperature of optical fiber at a metastable structure state is used to explain the difference in equilibrium viscosity. Then the relationship between equilibrium viscosity and fictive temperature for silica optical fiber at 1000 °C is derived.

Gel-sol transition of thermoresponsive poly(vinyl alcohol) solution: Validation of the universal critical scaling relations

While undergoing gelation transition, a material passes through a distinctive state called the critical gel state. In the neighborhood of this critical gel state, how viscosity, equilibrium modulus, and relaxation times evolve are correlated by scaling relations, and their universality has been validated for materials undergoing the sol-gel transition. In this work, we extend this approach for the gel-sol transition of a thermoresponsive polymeric system of aqueous poly(vinyl alcohol) (PVOH) gel that passes through the critical state upon increasing temperature. We observe that, in the neighborhood of the critical gel state, the equilibrium modulus and viscosity demonstrate a power law dependence on the relative distance from the critical state in terms of normalized temperature. Furthermore, the relaxation times in the gel and the sol state shows symmetric power law divergence near the critical state. The corresponding critical power law exponents and the dynamic critical exponents computed at the critical gel-sol transition state validate the scaling and hyperscaling relations originally proposed for the critical sol-gel transition very well. Remarkably, the dependence of complex viscosity on frequency at different temperatures shows a comprehensive master curve irrespective of the temperature ramp rate independently in the gel and the sol state. This observation demonstrates how the shape of relaxation time spectrum is independent of both the temperature as well as the ramp rate. Since sol-gel and the gel-sol transitions are opposite to each other, the applicability of the scaling relations validated in this work suggests broader symmetry associated with how the structure evolves around the critical state irrespective of the direction.

Molecular Dynamics Study on Structure, Vibrational Properties, and Transport Coefficients of Liquid Alumina

The structure, vibrational density of states, and transport coefficients of liquid alumina were studied using molecular dynamics simulations. At the temperature of 2500 K, 3000 K, 3500 K, and 4000 K, systems with three different densities were constructed, respectively, including the configurations with densities of 2.81 g/cm3 and 3.17 g/cm3, and the relaxed ones with nearly zero pressure at each temperature. With the changes in temperature or density, the transformations on the structural, vibrational and transport properties were discussed. The Born-Mayer-Huggins type of atomic interactions was used, with newly optimized parameters. The analysis of the interatomic correlations indicated that the short-range order of liquid alumina was mainly constructed by AlO4 tetrahedra, also a certain number of AlO3 and AlO5 was present. Meanwhile, the structural transitions on the elemental units occurred as either the temperature or density increased. Two primary frequency bands were observed in each vibrational density of states spectrum, with the higher frequency bands produced by the O atom vibrations, and the lower frequency ones generated by the Al atom vibrations. Self-diffusion coefficients were estimated using the linear behavior of the mean-squared displacement at long time, while by using the Green-Kubo relation during equilibrium molecular dynamics simulations, thermal conductivities and viscosities were calculated. Significantly, the viscosity at 2500 K with a density of 2.81 g/cm3 was equal to 25.23 mPa s, which was very close to the experimental finding.

Combined organic reagents for co-conditioning of sewage sludge: High performance in deep dewatering and significant contribution to the floc property

In this study, we have examined a new all-organic conditioning method for sewage sludge pretreatment, featuring the combined use of poly dimethyldiallylammonium chloride (PDMD) and tannic acid (TA). The impact of the reagents and the specific dosing order were identified for process optimization. Results showed that TA + PDMD mode (i.e., TA addition followed by PDMD) has better sludge filterability and dewaterability. With the optimal dosage, it decreased specific resistance of filtration by 90.60%, capillary suction time by 65.08%, and reduced moisture content of dewatered sludge cake to 53.46%, giving the highest dewatering performance reported so far. In addition, a significant positive correlation was found between viscosity and dehydration performance (Rp = 0.9898, p < 0.05). The explanation was the enhanced interparticle interaction and structural strength, indicated by higher apparent viscosity and yield stress, thixotropic hysteresis loop, solid-like viscoelastic property, and higher fractal dimension (Df) value as well. The enhanced compressibility (0.69 versus 1.06 of raw sludge) and SEM image further confirmed the compact and permeable structure of the conditioned sludge. And for dosing order, it was proved that TA-first conditioning method led to full reaction of protein precipitation, which produced increasingly compact flocs consequently. In conclusion, the floc property played an important role in condensed sludge formation and in deep dewatering process, which offset the side effect of undesirably higher equilibrium viscosity of the conditioned sludge.

Influence of swelling-degradation degree on rheological properties, thermal pyrolysis kinetics, and emission components of waste crumb rubber modified bitumen

This study investigated the effects of swelling-degradation degree on the rheological properties and thermal pyrolysis behaviors of crumb rubber modified bitumen (CRMB). The equilibrium viscosity of CRMB in 160 °C-swelling process is 10.5 Pa·s at 8 h. During the 200℃-degradation procedure, the viscosity increased to a maximum value of 20.58 Pa·s at 7.5 h and gradually reduced to a stable value of 11.09 Pa·s after 35 h. The swelling and degradation processes exhibit the opposite influence on the rheological properties of CRMB. Moreover, the degradation process accelerated the pyrolysis rate of CRMB but hindered the release of toxic gas CO and hydrocarbons.

A numerical and experimental investigation into the influence of CO2 on the stability of W/O emulsion

The formation of water in oil (W/O) emulsion can result in flow issues in the reservoir and a high cost of demulsification throughout the water treatment process. Without treatment, the W/O emulsion takes a long time to settle, which results in blockage in porous media. CO2 has strong interaction with crude oil and so has the potential to alter the settling process of W/O emulsion. The purpose of this paper is to undertake a numerical and experimental investigation into the influence of CO2 on the emulsion stability. To anticipate the settling process of a W/O emulsion in a high-pressure environment, a mathematical model was created that included a CO2 diffusion model, a viscosity prediction model, and an emulsion settling model. A high-pressure reactor was used to model the emulsion demulsification process at three different pressure conditions and to evaluate the model's prediction capability. The results indicate that under a high-pressure CO2 atmosphere, the demulsification process of a W/O emulsion can be accelerated. The CO2 dissolution and diffusion into the oil phase might enhance the likelihood of droplets colliding, resulting in the demulsification of the W/O emulsion system. Pressure has a significant influence on the stability of the emulsion. High pressure accelerates the CO2 dissolution process and significantly reduces the time required for the emulsion system to reach equilibrium. Additionally, the oil/water ratio can affect the stability of a W/O emulsion. At the same pressure, an emulsion with a higher oil/water ratio required a shorter time to reach equilibrium and had a higher equilibrium viscosity.

In English

The interfacial properties (dynamic and equilibrium surface tension, viscosity and elasticity moduli) of chitosan lactate have been studied at the liquid/air interface by the oscillating drop shape method. Isotherms of dynamic surface tension of chitosan lactate are similar to dependences for other polyelectrolyte solutions, in particular for proteins. Chitosan is a weak cationic polyelectrolyte which can change its conformation from a linear rod to a chaotic and compacted coil. Therefore, the experimental dependence of the equilibrium surface tension on concentration of chitosan lactate was analyzed with the adsorption model proposed earlier for proteins. This model accounts the possibility of polyelectrolyte molecules existence in surface layer in n states with different molar surface varying from the maximum value at very low surface coverage by polyelectrolyte molecules to a minimum value at high surface coverage. Good agreement between the calculated and experimental values of surface tension was observed. The dependences of the elasticity and viscosity moduli of chitosan lactate solutions on the drop oscillations frequency are conditioned by the influence of exchange processes both between the surface layer and the bulk solution and in the surface layer itself. An increase of the solution concentration intensifies the exchange processes, and an increase of the oscillation frequency suppresses them. It is shown that the dependence of the surface viscoelasticity modulus of chitosan lactate is extreme in nature with a pronounced maximum. The reason for such behavior is the possibility of changing the molar surface area of the polyelectrolyte at the interface dependent on the amount of adsorption and its structural properties. Attempt of theoretical description of the viscoelasticity modulus within the framework of model accounting mono- or bilayer adsorption did not lead to a satisfactory result, possibly due to barrier adsorption mechanism of chitosan. But bilayer model provide qualitative description of extreme behavior of surface viscoelasticity on concentration. The values of the surface viscoelasticity modulus of chitosan lactate occupy an intermediate position in comparison with the data available in the literature for globular and flexible-chain proteins, that is consistent with their molecular structure. In addition, the work shows the applicability of the adsorption model, developed earlier for proteins in the framework of a nonideal two-dimensional solution theory, for describing the surface properties of other polyelectrolytes. This makes it possible to obtain qualitative and quantitative information about the processes occurring in the systems under study.

Dehydration characteristics and evolution of physicochemical properties of Platycodon grandiflorum (Jacq. A.DC.) roots (PGR) during pulse-spouted microwave vacuum drying (PSMVD)

Platycodon grandiflorum (Jacq. A.DC.) root (PGR) is a famous edible and medicinal herb which contains lots of bioactive components that were beneficial for people health. Drying process is a necessary post-harvest technology for fresh PGR to lower the moisture content and extend shelf-life of PGR. But it also causes irregular cellular deformation and dynamically changing viscoelastic properties of PGR, which are critical for the drying kinetics and the quality attributes of materials. Herein, it is essential to understand the micro-level cellular changes, viscoelastic properties, and their relationship with the moisture loss during drying. PGR slices at different drying stages were investigated for the evolution of physicochemical properties (moisture changes, micro-level cellular attributes, and viscoelastic properties) and their correlations during pulse-spouted microwave vacuum drying (PSMVD). Low field nuclear magnetic resonance (LF −NMR) results showed that the removal of moisture from the PGR mainly depends on the loss of free water instead of bound water. Furthermore, magnetic resonance imaging (MRI) demonstrated the uniform removal of water from PGR slices during PSMVD process. As the drying proceeding from stage I to V, irregular cellular enlargement and intracellular cavities appeared in the tissues of PGR slices, and the fractal dimension (Df) values of PGR slices increased from 1.62 to 2.08. Through dynamic mechanical analysis (DMA), both the equilibrium modulus (Ee) and viscosity (μ0) exhibited a descending trend first then ascending trend, indicating that the hardness and stiffness of PGR slices were decreased first then increased during PSMVD process. This study further established the correlation of microcellular structure and viscoelastic properties with moisture loss during PSMVD process. Thus, this work identified the evolution of physicochemical properties of PGR during PSMVD process and future would help to build a predicted machanic model for the material deformation during drying.

Rheology of bentonite dispersions: Role of ionic strength and solid content

The rheological behavior of the bentonite dispersion was studied at different ionic strength and solid content. The yield and flow behavior of the dispersion changed with both the ionic strength and the solid content. Curves of the equilibrium viscosity normalized by two characteristic parameters, namely, the yield stress and the characteristic viscosity, formed a master curve. The electrostatic interaction reflected by the yield stress hindered the orientation/alignment of clay platelets estimated by the characteristic viscosity, giving rise to a power law relationship between the two characteristic parameters. Models based on the DLVO (Derjaguin and Landau, Verwey and Overbeek) theory were developed to describe the change of the yield stress and the characteristic viscosity as functions of both ionic strength and bentonite solid content. The master viscosity curve allows viscosity curves at different solid content and ionic strength to be determined once the yield stress and the characteristic viscosity of the dispersion are known. A constitutive model for the master viscosity curve as a function of the normalized shear stress and rate was proposed.

The effect of shear on the liquid–liquid transition and crystallization of the undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (Vit1) bulk metallic glass forming alloy

An entropy driven liquid–liquid transition (LLT) from a fragile (less ordered) to a strong (highly ordered) liquid occurs in the phase during undercooling. In this work, we show that this ordering transition as well as the applied shear rate affect the onset of crystallization. By recording simultaneously melt viscosity and temperature profiles, we quantitatively determine the shift in the upper part of the time-temperature-transformation diagram of Vit1 to shorter times with increasing shear rate. This acceleration in nucleation rate can be explained by the classical nucleation theory of crystals only if we take into consideration the effect of both shear flow and equilibrium viscosity. A critical assessment of the results concludes that shearing must first trigger the nucleation of the strong liquid from the fragile liquid and that the crystallization proceeds in a second step from the strong liquid. The fragile-to-strong transition decreases the configurational entropy of the liquid leading to a smaller interfacial energy between liquid and crystal, thus reducing the activation barrier for crystallization.

Molecular dynamics study on the viscosity of glass‐forming systems near and below the glass transition temperature

AbstractTemperature‐dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (Tg) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear‐induced variation in shear modulus or potential energy of the glass‐forming system. The shear viscosity as a function of steady‐state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero‐strain‐rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near Tg of four glass‐forming systems (Kob–Andersen system, silica, Cu45.5Zr45.5Al9, and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below Tg; the upper‐bound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~1032 and 1025 Pa·s, respectively.

Vapor–Liquid Equilibrium, Surface Tension, and Dynamic Viscosity for the Propan-1-ol + Dibutyl Ether Binary Mixture

Vapor–Liquid Equilibrium, Surface Tension, and Dynamic Viscosity for the Propan-1-ol + Dibutyl Ether Binary Mixture

Further evidence against the alleged failure of the classical nucleation theory below the glass transition range

AbstractWe collected a plethora of new data to test the hypothesis that the failure of the Classical Nucleation Theory (CNT) below the glass transition range is just an experimental artifact. Since reaching the steady‐state nucleation regime takes a significant time for treatments below the glass transition temperature, data collected in this temperature range tend not to have reached a steady state. Because of this potential problem, we examined the CNT using new experimental data for three stoichiometric silicate glasses: Li2Si2O5, BaSi2O5, and Na4CaSi3O9. We also measured the equilibrium viscosity for the studied glass batches and used it as a proxy for the effective diffusion coefficient. The analysis was conducted by applying a steady‐state criterion and evaluating the error propagation throughout all calculations. Using this rigorous procedure, we have not observed the alleged CNT failure. Our comprehensive results support recent studies questioning this possible CNT failure helping solve a longstanding problem in glass science.

Solid-liquid equilibrium and kinematic viscosity of binary mixture of fatty acid alkyl esters

The separation and recovery of fatty acid alkyl esters (FAAE) is of great significance for various industries. Crystallization is a well-suited separation process for fatty acid mixtures which may be evolved by the understanding of solid-liquid equilibrium (SLE). The SLE and kinematic viscosity (v) at (T=298.15–318.15 K) and (P=0.1 mPa) for binary mixture of FAAE: methyl decanoate (1)+ethyl decanoate or methyl dodecanoate or methyl tetradecanoate (2) has been studied. The deviation in kinematic viscosity (Δv) data was derived using experimental v data. The measured SLE exhibit general single eutectic point like other organic mixtures. The SLE data fit well with universal quasi-chemical (UNIQUAC) equation. The v values were interrelated using different equations recommended by Heric-Brewer, Krishnan-Ladda, and Lulian et al. Theoretically estimated values using these empirical equations are in accordance with the experimental values with percentage standard deviation less than 0.35. The v were also analyzed by means of the McAllister equation. The Δv data were also fitted with the Redlich-Kister polynomial equation. All three binary mixtures showed negative Δv values.

Rheological and Microbiological Characteristics of Hops and Hot Trub Particles Formed during Beer Production.

During the production of beer, and especially beer wort, the main wastes are spent grain and hot trub, i.e., the so-called “hot break.” Combined with yeast after fermentation, they represent the most valuable wastes. Hot trub is also one of the most valuable by-products. Studies on the chemical composition of these sediments and their rheological properties as waste products will contribute to their effective disposal and even further use as valuable pharmaceutical and cosmetic raw materials. So far, hot trub has been studied for morphology and particle distribution depending on the raw material composition and beer wort extract. However, there are no preliminary studies on the rheological properties of hot trub and hops. In particular, no attention has yet been paid to the dependence of these properties on the hop variety or different protein sources used. The aim of this study was to examine the effect of different hopping methods on hot trub viscosity and beer wort physicochemical parameters. Additionally, the hop solutions were measured at different temperatures. A microbiological analysis of hop sediments was also performed to determine the post-process survival of selected microorganisms in these wastes. For manufacturers of pumps used in the brewing industry, the most convenient material is that of the lowest viscosity. Low viscosity hot trub can be removed at lower velocities, which reduces costs and simplifies washing and transport. The sediments also had similar equilibrium viscosity values at high shear rates.