Dependence Of Viscosity Research Articles

Memory Effects Explain the Fractional Viscosity Dependence of Rates Associated with Internal Friction: Simple Models and Applications to Butane Dihedral Rotation.

Barrier-crossing rates of biophysical processes, ranging from simple conformational changes to protein folding, often deviate from the Kramers prediction of an inverse viscosity dependence. In many recent studies, this has been attributed to the presence of internal friction within the system. Previously, we showed that memory-dependent friction arising from the nonequilibrium solvation of a single particle crossing a smooth one-dimensional barrier can also cause such a deviation and be misinterpreted as internal friction. Here we introduce a simple diatom model and show that even in the absence of explicit solvent, internal memory effects arise due to coupling of the reaction coordinate motion with frictionally orthogonal degrees of freedom. This results in a fractional viscosity dependence and a deviation from Kramers' theory, typically attributed to the presence of internal friction. This model therefore mimics several biological processes where a local conformational change of a biomolecule is often influenced by its surroundings. This gives rise to an apparent "internal friction" commonly measured in terms of empirical fitting parameters α and σ. We propose a microscopic measure of this internal friction using Grote-Hynes theory which employs memory-dependent friction. We use butane to demonstrate the effect of coupling strength on the internal friction in realistic systems. This model therefore can serve the purpose of understanding internal friction in biological systems in terms of such coupling.

Structure and dynamic viscosity of CaO–SiO2 and CaO–SiO2–B2O3 model slag systems

Correlation dependencies between the dynamic viscosity of slag and its structural parameters were studied to determine an optimal basicity of silicon smelting slag under the addition of boron oxide to eliminate slagging of the bottom of ore-smelting furnaces. Experimental studies were conducted on CaO–SiO2 and CaO– SiO2–B2O3 model slags obtained at 1600°С. Raman spectroscopic analysis was carried out using a Horiba JobinYvon HR800UV analyzer (France). Theoretical calculations of slag viscosity were performed using Urbain and Mills models. During the experiments, the key structural parameters of slag systems varied within the following limits: the experimental Raman spectrum deconvolution function from 1.41 to 2.45 and optical basicity from 0.58 to 0.68. The obtained experimental and theoretical data were related by mathematical dependencies. It was found that the dynamic viscosity of slag can be promptly determined by Raman spectroscopy on the basis of mathematical models. The dependence obtained shows that slag viscosity decreases upon an increase in the number of bridging oxygen atoms in the silicate anion structure. Notably, this decrease in slag viscosity is observed up to the value of the experimental Raman spectrum deconvolution function of ~1.55-1.60 or slag optical basicity of 0.60–0.62. When B2O3 is added, the viscosity undergoes a further decrease. In practice, for CaO–SiO2 slag systems, the use of boroncontaining flux as a liquefying agent is reasonable at CaO/SiO2 = 0.61–0.63 while maintaining the content of B2O3 in the slag at a level of 1%. The two models (classical and modified) proposed by Urbain were established to be more suitable for theoretical calculation of viscosity in CaO–SiO2 and CaO–SiO2–B2O3 systems. Mills’ model is not suitable for these purposes, since the correlation coefficients in the corresponding mathematical model are not sufficiently large. Further research in this direction is required in order to establish appropriate dependencies of slag viscosity on its structural parameters at different temperatures.

Effect of concentration and electrical charge of maltodextrin, and packing factor on electrospinning processing

The power law ηrel ∼ Ca of maltodextrins with higher entanglement concentrations (Ce) of dextrose equivalent (DE) 4, 10, and 18 exhibits an unusually high exponent a, ranging from 10.34 to 14.38 in congested solutions where particles occupy significant space. This contrasts with the dilute exponents (a ∼2) typically observed in polymer–solvent systems. To address this issue, several viscosity factors were evaluated using the Einstein–Roscoe and Mooney equations, which demonstrated viscosity dependence due to volume fraction (ϕ) > 0.35, shape, crowding factor (k) packing factor (1/k∼0.5) and centered cubic cell (CC), particularly for larger chain sizes (>20 glucoses) in DE-4. Chain electrical charges significantly stiffen the chain and increase its length beyond the theoretically predicted Kuhn persistent length (Lp), which seems to be responsible for the exponential increase in viscosity. Normalizing the viscosity/charge data revealed a log–log plot consistent with the exponent a of the general power law theory. In addition to concentration, electrospinning processing is influenced by strong particle–particle charge interactions, (ϕ), packing factor (1/k), CC, and chain sizes. The Maxwell model demonstrated poor interaction for DE-4 (modulus ∼286 Pa at maximum ϕ due to loose packing of CC, in contrast to DE-18, which exhibited a modulus of 994 Pa for a solution with maximum ϕ> 0.5 and a congested solution of 52% w/v, packing factor of 0.68, and high particle–particle interaction of the body-centered cubic (BCC) cell, favoring electrospun fiber formation. The packing factor, ϕ, and cell type, are highly sensitive and have potential applications in food systems, 3D bioprinting, among many other scientific domains.

Scanning glass microelectrode technique for investigating temperature-dependent electrical properties

Abstract Recent progresses in ionic current analyses related to micro- and nano-object sensing, electrochemical sensors, and liquid pollution monitoring have attracted significant attention. Micro- and nanoscale sensors with high spatial resolution and high signal-to-noise ratios are also effective for obtaining detailed understanding of ion transport phenomena. We have developed a glass microelectrode technique for measuring the electrical potential distribution by scanning through liquids. It enables us to directly evaluate electrical properties with a spatial resolution equal to the glass tip diameter, which is less than 1 μm. Herein, we optimize the channel and cell structures for the analysis of temperature-dependent properties, which allows us to measure the temperature dependence of conductivity and viscosity in the range of 303--333 K based on the Stokes--Einstein relation. The proposed method, which directly measures the spatial distribution of electrical potential, is suitable for analyzing conductivity, viscosity, and concentration without preprocessing calibration.

Composition-dependent diffusion and viscosity behavior in liquid Ti–Al–Ni ternary alloys

We conducted ab initio molecular dynamics simulations to investigate the dynamic properties of the liquid Ti–Al–Ni ternary alloy system at 2033 K. Through simulations across the entire composition range, we revealed the complex compositional dependence of self-diffusion coefficients and viscosity. The self-diffusion coefficients of Ti, Al, and Ni exhibit nonlinear distribution characteristics, while viscosity shows non-monotonic compositional dependence, with lower viscosity in Al-rich regions and higher viscosity in Ni-rich regions. To understand the structural origins of these behaviors, we analyzed chemical short-range order and local fivefold symmetry structures using Warren-Cowley parameters and fivefold symmetry parameters, uncovering a close correlation between atomic-scale structural features and dynamic properties. Evaluation of the Stokes-Einstein relation revealed varying degrees of deviation from its predicted viscosity values across composition regions. We observed a negative correlation between free volume fraction and viscosity, with varying strengths in different regions. These findings provide new perspectives for understanding the complex dynamic properties of Ti–Al–Ni liquid alloys.

Global strong solutions to the radially symmetric compressible MHD equations in 2D solid balls with arbitrary large initial data

In this paper, we prove the global existence of strong solutions to the two-dimensional compressible MHD equations with density dependent viscosity coefficients (known as Kazhikhov-Vaigant model) on 2D solid balls with arbitrary large initial smooth data where shear viscosity μ being constant and the bulk viscosity λ be a polynomial of density up to power β. The global existence of the radially symmetric strong solutions was established under Dirichlet boundary conditions for β > 1. Moreover, as long as β>max{1,γ+24}, the density is shown to be uniformly bounded with respect to time. This generalizes the previous result [Fan et al., Arch. Ration. Mech. Anal. 245, 239–278 (2022)] of the compressible Navier-Stokes equations on 2D bounded domains where they require β > 4/3 and also improves the result [Chen et al., SIAM J. Math. Anal. 54(3), 3817–3847 (2022)] of of compressible MHD equations on 2D solid balls.

Influence of chickpea protein on the pH and temperature dependent viscosity of carboxymethylated starch

Proteins can significantly improve the elasticity and microstructure of starch gels in food. In this work, the influence of chickpea protein flour on the viscoelastic behaviour of carboxymethylated starch (CMS, 92.6 mmol COOH kg−1) gels was studied as function of pH and temperature. A weight ratio CMS:protein flour of 1:0.45 was investigated in the pH range of pH 2.5–8. Above pH 7 presence of 7.5 %w/w chickpea flour lead to an increase in complex viscosity of a 16.5 %w/w CMS solution by a factor of 10. The interaction between CMS and protein above pH 4 accelerates gelation at 37 °C, resulting in an increase in viscosity by a factor of 5, 10 and 120 at pH 5, pH 7 and pH 8 respectively. Model calculations for species dissociation of ammonium groups in basic amino acids and carboxylate groups in CMS indicate that electrostatic interactions led to the observed increase in viscosity. The results form a general model to explain the pH-dependent viscoelastic behaviour of polysaccharide–protein mixtures. The understanding of the mechanism of action between protein and polysaccharides is a condition for targeted analysis and explanation of many phenomena of texture, stability and coacervate formation in food processing.

Density, thermal expansion, dynamic viscosity of halogenated arenes

Purpose. Experimental study of the density, thermal expansion and dynamic viscosity of halogenated arenes at temperatures from 293.15 to 343.15 K and atmospheric pressure 97.992 kPa.Methods. Density was determined by the pycnometric method, viscosity by the capillary method. A quartz pycnometer type PZh2 with a nominal capacity of 10 ml and a glass capillary viscometer type VPZh-2 with a nominal capillary diameter of 0.34 mm were used. The mass of liquids was measured using an electronic scale VSL-200/0.1A with a division value of 0.0001 g. Numerical methods were used to process the experimental results.Results. Experimental data were obtained on the density, coefficient of volumetric thermal expansion and dynamic viscosity of liquid o-xylene, ethylbenzene, fluorobenzene, chlorobenzene, bromobenzene, toluene, o-fluorotoluene, mfluorotoluene, p-fluorotoluene, o-chlorotoluene, m-chlorotoluene, p-chlorotoluene, 2,4-dichlorotoluene, 2,6-dichlorotoluene. Density and dynamic viscosity data are approximated by a third-degree polynomial. Data on the coefficient of volumetric thermal expansion are obtained from approximated measured density data. The maximum estimate of the measurement error for density was 0.13 %, for dynamic viscosity – 3.5 %. The maximum calculation error for the coefficient of thermal expansion is estimated at 3 %.Conclusion. Mathematical processing of the obtained experimental data on density and dynamic viscosity made it possible to obtain analytical relations in the form of power polynomials that allow calculating the values of these quantities at any temperature from the studied temperature range and atmospheric pressure with an error not exceeding the error of experimental determination. The calculated values are in good agreement with the data provided in the reference and scientific literature. The experimental data obtained complement the information on the properties of some of the liquids under consideration, for which the dependences of density and viscosity on temperature are poorly understood. The substances under study are used in various sectors of the economy and are therefore technically important liquids. The measurement results can be used in condensed matter physics, for the analysis of liquid hydrocarbons, and can be useful in the chemical industry.

Mathematical analysis of the motion of a piston in a fluid with density dependent viscosity

We study a free boundary value problem modelling the motion of a piston in a viscous compressible fluid. The fluid is modelled by 1D compressible Navier–Stokes equations with possibly degenerate viscosity coefficient, and the motion of the piston is described by Newton’s second law. We show that the initial boundary value problem has a unique global in time solution, and we also determine the large time behaviour of the system. Finally, we show how our methodology may be adapted to the motion of several pistons.

STUDY OF TEMPERATURE FIELDS NEAR A WELL DURING FILTRATION OF VISCOPLASTIC OIL

The work examines the time variation of fluid temperature during non-isothermal filtration of viscoplastic oil near a well. It is known that a characteristic feature of modern oil production is an increase in the share of hard-to-recover oil reserves, which include viscoplastic oils. According to their rheological characteristics, such oils are non-Newtonian liquids and, therefore, their filtration process is not described by Darcy’s linear law. When filtering fluids that are not described by Darcy's law, it is necessary to use nonlinear filtration laws, which make it possible to more adequately describe the processes occurring in productive formations when hydrocarbons are displaced from them. To describe the nonlinear filtration law, empirical formulas obtained by approximating data from laboratory experiments are usually used.In the works of V.V. Devlikamov and his co-authors provide experimental data on the dependence of viscosity on shear stress for high-viscosity oils. They have been shown to be approximated quite well by a sigmoid function. However, the problem then becomes significantly nonlinear. Subsequently, in order to simplify finding a solution to the problem of filtration of high-viscosity oil, experimental data were obtained by V.V. Devlikamov and his co-authors were approximated by broken lines.In this work, the experimental data are approximated by a sigmoid function, which allows for more accurate calculations. Assuming that the viscosity and density of oil do not depend on temperature, which is typical for natural development modes, the results of calculations of temperature changes over time near the well are presented. It is shown that the temperature of viscoplastic oil in the first hours after starting a well differs significantly from the temperature of Newtonian oil (oil with constant viscosity). Thus, if the temperature of Newtonian oil continuously increases, then the temperature of viscoplastic oil immediately after starting a well increases and coincides with the temperature of oil with constant viscosity, then drops and after some time begins to increase again. The work shows that such an unusual change in the temperature of viscoplastic oil over time is associated with a change in the zone of destruction of the formation structure.

The new rheological model for zirconia suspensions with long-term kinetic stability

For successful 3D printing based on stereolithography technologies, one of the most important factors influencing the success of printing is the dynamic viscosity of the prepared suspension. For the purposes of viscosity prediction at different volume fillings, two ZrO2 powders with different yttrium content (CY3Z-RS and CY8Z-RS) with volume filling 0–40 % in two different dispersing mediums at two temperatures were investigated in this paper. The dependence of relative viscosity on volumetric filling was fitted by existing models and a new semi-empirical model based on experimental data was proposed and successfully plotted. Fitted data for all zirconia photosensitive suspensions showed a high correlation to the proposed model (R2 from 97.02 % to 99.61 %). The model in addition to the possibilities of mathematical generalization of his prescription, introduces a fitting parameter C, which is thought to be thermodynamically influenceable and predictable. Furthermore, another key characteristic that is often neglected in rheological characterization, the stability of prepared ceramic suspensions, was verified either by rheological measurements but also by employing prospective technology based on analytical centrifuge. It was confirmed that all prepared suspensions were highly stable, and all instability indexes reached values less than 0.05.

ANALYSIS OF THE CHANGE IN THE RHEOLOGICAL CURVE OF THE FLOW OF NON-NEWTONIAN OILS IN THE FIELDS OF WESTERN KAZAKHSTAN

In order to improve oil recovery technologies, the task of studying the rheological properties of oil, which largely determine the development indicators of the development target, namely, the rate of oil withdrawal, the stability and nature of the movement of the front of displacement of oil by water or gas, and, as a result, final oil recovery, is becoming urgent. In this regard, there is a need for in-depth studies of the rheoviscidity properties of produced oils.This article presents the results of laboratory studies to study the rheological characteristics of both high-paraffin and heavy high-viscosity oils using the example of oil from fields in Western Kazakhstan. Since structured oils are thixotropic systems, all experiments were carried out with the same degree of structure destruction in oil, both anhydrous oils and those with different water cuts were studied. The authors also showed the nature of the change in the rheological curve of the flow of oil emulsion of various water cuts to identify the temperature interval where non-Newtonian properties of various oils are manifested. Their main physical and chemical properties are also given, and kinematic viscosities of oil with different densities are compared. The results of laboratory studies of viscosity change with temperature increase for Karazhanbas oil emulsion with different water cut are considered. Despite differences in viscosity values of different degrees of watered samples of highly paraffinic oil, the tendency of oil viscosity change depending on the shear gradient is the same. The formation of structured systems of resin and asphaltene particles is observed in flows with relatively low shear gradients. As a result, oil viscosity dependence on the shear gradient in the range of low values from 0 to 20 s-1 must be taken into account when solving tasks for high-paraffin production. The results obtained are of practical interest, since when creating a compositional model and technological modeling of the collection and transport system, the rheological characteristics of the oil of a particular field, as well as the presence of thixotropic properties, must be taken into account.

Nonlinear stability analysis of thermal convection in a fluid layer with slip flow and general temperature boundary condition

The current article presents a comprehensive analysis of the influence of inconstant viscosity into thermal convection, incorporating the influence of generalized velocity (slip boundary condition) and temperature boundary conditions (physically represent imperfectly conducting boundary) within a fluid layer. Slip boundary conditions are often considered more practical than no-slip conditions (Neto et al. (2003)). Therefore, in the present work, slip boundary condition is used to discuss the effect of temperature and pressure dependent viscosity. Both linear and nonlinear stability analyses are explored, revealing a noteworthy finding: the precise alignment of the nonlinear stability boundary with the linear instability threshold. Additionally, the exchange of stability is illustrated, indicating that convection exclusively manifests in a stationary mode. The findings are derived across a spectrum of boundary scenarios, ranging from free-free to rigid-free, and rigid-rigid boundaries, encompassing both isothermal and adiabatic conditions. Notably, the slip length parameter exhibits a destabilizing effect, while convection is observed to occur more swiftly under adiabatic boundary conditions compared to isothermal conditions. The Chebyshev pseudo-spectral technique is applied to solve the eigenvalue problems obtained from linear and nonlinear stability analysis.

Flow Activation Energy of High-Concentration Monoclonal Antibody Solutions and Protein-Protein Interactions Influenced by NaCl and Sucrose.

The solution viscosity and protein-protein interactions (PPIs) as a function of temperature (4-40 °C) were measured at a series of protein concentrations for a monoclonal antibody (mAb) with different formulation conditions, which include NaCl and sucrose. The flow activation energy (Eη) was extracted from the temperature dependence of solution viscosity using the Arrhenius equation. PPIs were quantified via the protein diffusion interaction parameter (kD) measured by dynamic light scattering, together with the osmotic second virial coefficient and the structure factor obtained through small-angle X-ray scattering. Both viscosity and PPIs were found to vary with the formulation conditions. Adding NaCl introduces an attractive interaction but leads to a significant reduction in the viscosity. However, adding sucrose enhances an overall repulsive effect and leads to a slight decrease in viscosity. Thus, the averaged (attractive or repulsive) PPI information is not a good indicator of viscosity at high protein concentrations for the mAb studied here. Instead, a correlation based on the temperature dependence of viscosity (i.e., Eη) and the temperature sensitivity in PPIs was observed for this specific mAb. When kD is more sensitive to the temperature variation, it corresponds to a larger value of Eη and thus a higher viscosity in concentrated protein solutions. When kD is less sensitive to temperature change, it corresponds to a smaller value of Eη and thus a lower viscosity at high protein concentrations. Rather than the absolute value of PPIs at a given temperature, our results show that the temperature sensitivity of PPIs may be a more useful metric for predicting issues with high viscosity of concentrated solutions. In addition, we also demonstrate that caution is required in choosing a proper protein concentration range to extract kD. In some excipient conditions studied here, the appropriate protein concentration range needs to be less than 4 mg/mL, remarkably lower than the typical concentration range used in the literature.

Определение вязкости и диэлектрической проницаемости жидкости методом эквивалентных схем с помощью резонатора с поперечным электрическим полем

The paper presents a method for determining the viscosity and permittivity of a liquid using the equivalent circuit method. The parameters of the Mason equivalent circuit were found for a piezoelectric resonator with a lateral electric field made of PZT-19 ceramics. The frequency dependences of the real and imaginary parts of the electrical impedance of a resonator loaded with liquid samples with different viscosity and permittivity were measured. The fitting of the theoretical frequency dependences of the real and imaginary parts of the electrical impedance of a resonator loaded with mixtures of “water - isopropyl alcohol” and “water - glycerin” to the measured dependences was carried out. As the result the dependence of the permittivity of the liquid on the additional capacitance (determined from the Mason scheme) and the dependence of the shear fluid viscosity on the imaginary part of the mechanical impedance of the fluid load (determined from the Mason scheme) were built. These dependences can be used as calibration curves to determine dielectric constant and viscosity.

(Digital Presentation) Physical and Chemical Properties of Lithium Tetrachloroaluminate Monosolvate with Sulfur Dioxide

In connection with the expansion of the areas of application of lithium-ion batteries and the increase in their energy characteristics, researchers are increasingly paying attention to the problems of battery safety. The use of non-flammable electrolyte systems will significantly improve the safety of lithium-ion batteries.Electrolyte systems based on solvate complexes of sulfur dioxide have high electrical conductivity and are non-flammable [1]. The most studied composition is LiAlCl4 × 1.6 SO2 [1] and LiAlCl4 × 3 SO2 [2]. The properties of LiAlCl4 x SO2 monosolvate have not been described in the literature.The presented work summarizes the results of studies of the physicochemical properties (electrical conductivity, viscosity, density) of lithium tetrachloroaluminate monosolvate with sulfur dioxide (table). The temperature dependences of viscosity, density and electrical conductivity are presented in the figure.This work was performed as part of the Ministry of Science and Higher Education of the Russian Federation [Government Order Theme No. 121111900148-3]. References Hartl, R., Fleischmann, M., Gschwind, R., Winter, M., & Gores, H. (2013). A Liquid Inorganic Electrolyte Showing an Unusually High Lithium-Ion Transference Number: A Concentrated Solution of LiAlCl4 in Sulfur Dioxide. Energies, 6(9), 4448–4464. DOI:10.3390/en6094448Chul Wan Park, & Oh, S. M. (1997). Performances of Li/LixCoO2 cells in LiAlCl4 · 3SO2 electrolyte. Journal of Power Sources, 68(2), 338–343. DOI:10.1016/s0378-7753(97)02518-4 Figure 1

The impact of rotation on the onset of cellular convective movement in a casson fluid saturated permeable layer with temperature dependent thermal conductivity and viscosity deviations

In this effort, we examined the impact of rotation on the arrival of cellular convective motion in a Casson fluid saturated permeable layer with temperature dependent thermal conductivity and viscosity deviations. The problem is important to cellular foams prepared from plastics, ceramics, and metallic where radiation conductivity is revealed as a power function of temperature. The altered Darcy model is used to characterize the rheological performance of Casson fluid flow in permeable medium. The approximate analytical solution and numerical solution correct to one decimal place are presented utilizing the Galerkin method. The analysis reveals that the influence of thermal conductivity disparity parameter and the rotation is to delay the convective motion whereas; the viscosity disparity parameter and the Casson parameter have dual impact on the convective motion in the presence of rotation. The range of the convective cell drops with increasing the thermal conductivity disparity parameter, the viscosity disparity parameter, the Casson parameter and rotation parameter. In the absence of rotation, the range of the convective cell does not depend on the Casson parameter and the viscosity disparity parameter. Further, the existing results are compared with the existing literature under the particular case of this study.

The Stability of a Dense Crust Situated on Small Bodies

Small planetary bodies in the solar system, including Io, Ganymede, and Callisto, may have a crust denser than their underlying mantle. Despite the inherent gravitational instability of such structures, we show that the growth timescale of the Rayleigh–Taylor (RT) instability can be as long as the age of the solar system, owing to the strong temperature dependence of viscosity. Even in cases where the instability timescale is shorter, the instability is confined to a thin layer at the base of the crust, making the foundering of the entire crust improbable in many scenarios. This study delineates the onset and aftermath of the RT instability, applying a quantitative framework to assess the stability of (i) rock-contaminated crust on icy satellites, and (ii) silicate crust floating on top of a subsurface magma ocean on Io. Notably, for Io the RT instability peels off only 10–100 m from the crust’s base, and thermal diffusion rapidly recovers the crustal thickness through solidification of a magma ocean. Despite recurrent delamination of the crustal base, the initial crustal thickness is maintained by thermal diffusion, virtually stabilizing a floating dense crust. Cracking of the crust also is unlikely to result in the foundering of the crust. A dense crust on a small body is therefore difficult to be overturned, suggesting the potential ubiquity of dense surface layers throughout the solar system.

Magnetic relaxation between ferromagnetic and helix spin configurations in holmium films

Multiple noncollinear spin structures and transitions between them initiated by field and temperature have attracted attention due to the extraordinary coexistence of complicated spin phases in Ho. We explore the magnetic relaxation dynamics within thin films undergoing transitions between ferromagnetic (FM) and helix states. When measuring susceptibility in a relatively high frequency range (∼100–1500 Hz) and magnetic viscosity at lower frequency (∼0.01 Hz), we focus on 400 nm thick Ho film. Notably, sharp variations in the real and imaginary components of magnetic susceptibility are discerned during the FM – Helix transition, occurring at temperatures between 15 and 30 K. Hysteresis effects in the magnetic susceptibility components are identified during cyclic variations in the external field, indicating a relatively rapid process with a timescale of approximately 10 ms accompanying the FM – Helix transition. The slow relaxation process is also found to exhibit sensitivity to this transition. Furthermore, the field dependence of magnetic viscosity displays a marked decline at the FM – Helix transition. The research methodologies proposed for the investigation of relaxation processes in materials with multiphase spin structures are deemed universally applicable and offer insights into transitions between spin states in materials manifesting non collinear spin structures.

Evaluation of the non-Newtonian lattice Boltzmann model coupled with off-grid bounce-back scheme: Wall shear stress distributions in Ostwald-de Waele fluids flow.

We present a comprehensive analysis of the non-Newtonian lattice Boltzmann method (LBM) when it is used to simulate the distribution of wall shear stress (WSS). We systematically identify sources of numerical errors associated with non-Newtonian rheological behavior of fluids in off-grid geometries. We implement the single relaxation time, Bhatnagar-Gross-Krook (BGK), and multiple relaxation time (MRT) collision operators and investigate flow in a two-dimensional channel aligned with lattice directions and off-grid Hagen-Poiseuille flow of Ostwald-de Waele (power-law) fluids. As for boundary conditions, we implement constant body force-driven and pressure-driven flows. These two boundary conditions have different numerical challenges, which include numerical stability, accuracy, mass conservation, and compressibility effects, which are inherent in the LBM method. Our results indicate that MRT, when the relaxation times are adequately tuned in the non-Newtonian case, significantly improves the WSS distribution accuracy and the numerical stability of the LBM. MRT also enhances the stability and accuracy for non-Newtonian fluids compared with the Newtonian case, meaning that it is questionable if a BGK collision operator is appropriate to use in a non-Newtonian case with off-grid boundaries. When analyzing the non-Newtonian LBM in the context of staircase walls and interpolated bounce-back (IBB) walls, a MRT collision operator with the appropriate choice of tunable relaxation times makes it possible to achieve numerically accurate results without a significant increase in grid resolution for matching to the analytical solution of WSS distributions. In analyzing the non-Newtonian flows, we show that the viscosity dependency of bounce-back walls in the BGK-LBM deviates from the results obtained under Newtonian assumptions. The power-law index further influences these discrepancies, and errors caused by the viscosity dependency of the bounce-back boundary conditions can be effectively mitigated by implementing the MRT procedure. Results show that non-Newtonian fluids, in contrast with the Newtonian assumption, encounter a greater mass imbalance when flowing through a periodic system with IBB walls. MRT can address this challenge, as it allows for independent adjustments of physical relaxation times and enhances mass conservation in the case of non-Newtonian fluids. In pressure-driven non-Newtonian flows, there is a significant impact of bulk viscosity. This aspect is often overlooked in Newtonian simulations but can significantly impact fluid adapting to rapid changes in local effective viscosity. One of our main conclusions is that the MRT collision operator with tuned relaxation times can effectively resolve numerical problems caused by non-Newtonian rheological properties and off-grid geometries. We also provide practical guidelines for selecting the most suitable simulation approach.