Kinetic Theory Research Articles

Revisiting shear stress tensor evolution: Nonresistive magnetohydrodynamics with momentum-dependent relaxation time

This study aims to develop second-order relativistic viscous magnetohydrodynamics (MHD) derived from kinetic theory within an extended relaxation time approximation (momentum/energy dependent) for the collision kernel. The investigation involves a detailed examination of shear stress tensor evolution equations and associated transport coefficients. The Boltzmann equation is solved using a Chapman-Enskog-like gradient expansion for a charge-conserved conformal system, incorporating a momentum-dependent relaxation time. The derived relativistic nonresistive, viscous second-order MHD equations for the shear stress tensor reveal significant modifications in the coupling with dissipative charge current and magnetic field due to the momentum dependence of the relaxation time. By utilizing a power law parametrization to quantify the momentum dependence of the relaxation time, the anisotropic magnetic field-dependent shear coefficients in the Navier-Stokes limit have been investigated. The resulting viscous coefficients are seen to be sensitive to the momentum dependence of the relaxation time. Published by the American Physical Society 2024

Analysis of Dynamic Wetting and Dewetting on a Hydrophobic Surface Using Molecular Kinetic Theory

Analysis of Dynamic Wetting and Dewetting on a Hydrophobic Surface Using Molecular Kinetic Theory

Second-order spectral line shift comparisons

The second-order spectral line width formulae from the projection operator and kinetic theory methods were recently compared. It was shown that a systematic expansion of the projection operator width expression including initial correlations formally agrees with the second-order kinetic theory result. It is now shown that the second-order dynamic shifts are also formally the same. The static shifts, however, differ due to an ad hoc treatment of electron-electron correlations in the projection operator method. The approximation is necessary in order to screen the radiator-electron interactions. The differences, however, are expected to be small. The results suggest using the rigorous and more compact second-order width and shift expressions from the kinetic theory method as the starting point for spectral line shape calculations. At line center, however, the projection operator second-order expression for the width and shift without initial correlations is simpler and reduces to the kinetic theory result.

Accelerated 3D CFD modeling of multichannel flat grooved heat pipes

Flat grooved heat pipes (HPs) have become essential in advanced thermal management solutions across various engineering applications. Modeling these devices, especially multichannel flat grooved HPs, involves significant challenges due to complex phenomena such as phase-change heat transfer and free-surface flow, requiring substantial computational resources, time and expertise. These constraints often limit the full exploration and optimization of HPs’ potential in diverse applications. To address this gap, an accelerated 3D computational fluid dynamics (CFD) modeling approach is presented in this study. This novel method begins with a detailed 3D modeling of a single groove, developed using kinetic theory and facilitated by CFD software. The results from this model are then applied as boundary conditions to simulate the entire HP in a multichannel configuration. The importance of this methodology is further highlighted by the alignment of simulation results with experimental observations. The approach significantly enhances computational efficiency by reducing the number of iterations by 10% and computational time by 80%, resulting in a five-fold speed-up. The methodology enables accelerated, comprehensive modeling of multichannel variations and delivers critical insights for optimizing the design of multichannel flat grooved HPs for various engineering applications.

Prediction of hydrodynamic characteristics of a 3D liquid-solid tapered fluidized bed using kinetic theory of rough spheres

Prediction of hydrodynamic characteristics of a 3D liquid-solid tapered fluidized bed using kinetic theory of rough spheres

Ti-Mo microalloyed medium Mn steels: Precipitation and strengthening mechanism

Medium-Mn steels possess an exceptional strength-ductility balance but normally relatively low yield strength, which hinders their practical applications especially in ultra-high strength structural components. In this study, the simultaneous addition of Ti and Mo was proposed to improve the yield strength of medium-Mn steels, together with the systematic experimental and theoretical investigations on the precipitation behavior of (Ti, Mo)C carbides and its effects on the microstructure-properties relationship. The addition of Ti and Mo increases yield strength by 150∼350 MPa and ultimate tensile strength by 55∼320 MPa without a significant loss of ductility. Based on the intercritically annealed microstructure close to equilibrium as well as the nucleation and growth kinetic theory, the Ti–Mo addition can reduce the fraction of austenite, but increase the mechanical stability of austenite by refining the mean grain size of austenite. Mo was found to increase the precipitate density of (TixMo1-x)C by lowering the interfacial energy between the nano-precipitates and the matrix, and reduce the mean size of precipitates. A maximum increment in yield strength (∼350 MPa) was achieved at 610 °C, which was mainly attributed to the combined effects of grain refinement and precipitation hardening due to the addition of Ti and Mo.

Chiral phosphoric acid-catalyzed asymmetric epoxidation of alkenyl aza-heteroarenes using hydrogen peroxide

The synthesis of chiral α-azaheteroaryl oxiranes via enantioselective catalysis is a formidable challenge due to the required complex stereoselectivity and diverse N-heterocyclic structures. These compounds play a crucial role in developing bioactive molecules, where precise chirality significantly influences biological activity. Here we show that using chiral phosphoric acid as a catalyst, our method efficiently addresses these challenges. This technique not only achieves high enantio- and diastereoselectivity but also demonstrates superior chemo- and stereocontrol during the epoxidation of alkenyl aza-heteroarenes. Our approach leverages a synergistic blend of electrostatic and hydrogen-bonding interactions, enabling the effective activation of both substrates and hydrogen peroxide. The resulting chiral oxiranes exhibit enhanced diversity and functionality, aiding the construction of complex chiral azaaryl compounds with contiguous stereocenters. Kinetic and density functional theory studies elucidate the mechanism, highlighting chiral phosphoric acid’s pivotal role in this intricate enantioselective process.

Spatial and temporal dynamics of thermal motion of a chain of liquid lead nanoinclusions attached to a fixed dislocation segment in an aluminum matrix

Thermal motion of a chain of liquid lead nanoinclusions attached to a dislocation segment fixed at its ends in the aluminum-based alloy was studied in situ using TEM in the temperature range from 442 °C to 497 °C. The projections of points of the trajectories of the inclusions onto the dislocation line were analyzed. The evidence for the collective interaction of all the inclusions and their spatially correlated collective thermal motion was obtained. Also, moderate temporal autocorrelation of thermal motion of the inclusions and a very weak temporal cross-correlation of thermal motion of the inclusions in all pairs was shown. The FFT phase spectra of the time dependencies of the inclusions positions on the dislocation segment give the evidence for the phase synchronization of thermal oscillations of the inclusions. It is shown that the thermal oscillations of the terminal inclusion attached to the dislocation node and elevation of temperature lead to decrease in the level of synchronization of thermal oscillations of the inclusions in the chain. It is indicated that the development of the process of desynchronization of thermal oscillations of the inclusions manifests itself by an increase in the density of phase steps in the initially linear unwrapped phase spectra accompanying by an increase in the interaction of the steps.

Mode analysis of spin field of thermal atomic ensembles

The spin dynamics in a thermal atomic vapor cell have been investigated thoroughly over the past decades and have proven to be successful in quantum metrology and memory owing to their long coherent time and manipulation convenience. The existing mean field analysis of spin dynamics among the whole cell is sometimes inaccurate due to the non-uniformity of the ensemble and spatial coupling of multi-physical fields interacting with the ensembles. Here we perform mode analysis onto the quasi-continuous spin field including atomic thermal motion to derive Bloch mode equations and obtain corresponding analytical solutions in diffusion regime. We demonstrate that the widely used mean field dynamics of thermal gas is a particular case in our solution, corresponding to the uniform spatial mode. This mode analysis approach offers a precise method for analyzing the dynamics of the spin ensemble in greater detail from a field perspective, enabling the effective determination of spatially non-uniform multi-physical fields coupling with the spin ensembles, which cannot be accurately analyzed by the mean field method. Furthermore, this work paves the way to address quantum noises and relaxation mechanisms associated with non-uniform fields and inter-atomic interactions, which limit further improvement of ultra-sensitive spin-based sensors.

A single crystal diffuse scattering study of structural relaxations arising from dopants in the semiconductor Cd0.9Zn0.1Te

We have measured diffuse scattering in a single crystal of Cd0.9Zn0.1Te using a state-of-the-art laboratory diffractometer. A large-box atomistic simulation of a model crystal is used in conjunction with Monte Carlo modeling and the Kirkwood potential. A combination of structural relaxation in the presence of the dopant and thermal motion results in good qualitative agreement between the computed diffraction patterns of the model crystal and the measured x-ray patterns. This is shown to be rather distinct from the diffuse scattering arising from purely structural relaxations or thermal motion only. The atoms are shown to displace predominantly in ⟨1,1,1⟩ and ⟨1,0,0⟩ type directions. Our approach to Monte Carlo modeling can easily be extended to more complex defect structures to incorporate, e.g., chemical ordering on the Cd/Zn sublattice, more than one species of dopant or vacancies.

Exploring electron transfer: Bioinspired, biomimetics, and bioelectrochemical systems for sustainable energy and Value-Added compound synthesis

Electron transfer (ET) is a fundamental process that underlies various phenomena in physics, chemistry, and biology. Understanding ET mechanisms is crucial for developing sustainable energy solutions and synthesizing value-added compounds efficiently. In this context, the present review provides the fundamental aspects of ET involving bioinspired, biomimetics, and biological entities and its significance for sustainable energy and green electrosynthesis fields. Among the theoretical and experimental cornerstones, Marcus Theory, electronic conductance, computational modeling, biomolecular thermodynamics, electrochemical and kinetic theories, protein film voltammetry, and the emergence of in situ and operando techniques are explored. Theoretical modeling is vital for understanding and predicting ET processes. Additionally, the significance of experimental techniques for investigating the ET process in biological entities and interfaces is discussed. Protein film voltammetry is a valuable and consolidated technique for studying ET processes at the protein-electrode interface, whereas in situ and operando techniques for interrogating ET processes in real time provide insights into the dynamics and mechanisms of ET. The concept of quantum conductance in biological structures is addressed, evidencing a trend and power of single-entity analysis. Aspects of extracellular and interfacial ET processes are presented and discussed in the electrochemical energy conversion systems. A deep understanding of these processes can improve the design of efficient bioinspired catalysts. Therefore, this multidisciplinary work aims to fill the gaps between different scientific fields related to ET involving bioentities to develop innovative energy and value-added compound synthesis solutions.

Classical and quantum theory of fluctuations for many‐particle systems out of equilibrium

AbstractCorrelated classical and quantum many‐particle systems out of equilibrium are of high interest in many fields, including dense plasmas, correlated solids, and ultracold atoms. Accurate theoretical description of these systems is challenging both, conceptionally and with respect to computational resources. While for classical systems, in principle, exact simulations are possible via molecular dynamics, this is not the case for quantum systems. Alternatively, one can use many‐particle approaches such as hydrodynamics, kinetic theory, or nonequilibrium Green functions (NEGF). However, NEGF exhibit a very unfavorable cubic scaling of the CPU time with the number of time steps. An alternative is the G1–G2 scheme [N. Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020)] which allows for NEGF simulations with time linear scaling, however, at the cost of large memory consumption. The reason is the need to store the two‐particle correlation function. This problem can be overcome for a number of approximations by reformulating the kinetic equations in terms of fluctuations – an approach that was developed, for classical systems, by Yu.L. Klimontovich [JETP 33, 982 (1957)]. Here, we present an overview of his ideas and extend them to quantum systems. In particular, we demonstrate that this quantum fluctuations approach can reproduce the nonequilibrium GW approximation [E. Schroedter et al., Cond. Matt. Phys. 25, 23401 (2022)] promising high accuracy at low computational cost which arises from an effective semiclassical stochastic sampling procedure. We also demonstrate how to extend the approach to the two‐time exchange‐correlation functions and the density response properties. [E. Schroedter et al., Phys. Rev. B 108, 205109 (2023)]. The results are equivalent to the Bethe–Salpeter equation for the two‐time exchange‐correlation function when the generalized Kadanoff‐Baym ansatz with Hartree‐Fock propagators is applied [E. Schroedter and M. Bonitz, phys. stat. sol. (b) 2024, 2300564].

Simulation Study on Defect Damage Behavior of Fe under Irradiation Environment

Abstract As a commonly used structural material in nuclear power plants, Reactor pressure vessel (RPV) steel is subjected to high-energy neutron irradiation from the reactor core for a long time, and the service environment is harsh. The accumulation of point defects (interstitial atoms and vacancies) generated by irradiation over a long period can seriously affect the microstructure of the material. The macroscopic properties of the material changed until the material failed, which threatened the safety and operation stability of nuclear power plants. This paper used the method of molecular dynamics to simulate the cascade collision process of Fe in the irradiation environment. The relationship between different factors and radiation damage defects was studied. Firstly, the quantity of the Frenkel pairs increases rapidly in the irradiation environment, and then recombination occurs after reaching the peak, which makes the quantity of the Frenkel pairs decrease rapidly. Finally, the quantity of Frenkel pairs is in a stable trend. When PKA energy and temperature increase, the higher the quantity of Frenkel pairs at the peak is, the higher the recombination rate of defects is. Meanwhile, the larger the cluster size of interstitial atoms and vacancies is, the greater the corresponding quantity of clusters is. As PKA energy increases, the quantity of residual Frenkel pairs at stability increases, while the number of residual Frenkel pairs at stability decreases with temperature. This is attributed to the intense thermal motion of molecules at high temperatures, resulting in the quantity of Frenkel pairs decreasing at the stable stage. Therefore, by simulating the irradiation damage process of Fe, the prediction of material irradiation damage under a neutron irradiation environment is provided, and the service life of materials can be provided with theoretical guidance.

A Novel Model for the MeV Emission Line in GRB 221009A

Gamma-ray bursts (GRBs) have long been considered potential sources of ultrahigh-energy cosmic rays (UHECRs; with energy ≳1018 eV). In this work, we propose a novel model generating MeV emission lines in GRBs, which can constrain the properties of heavy nuclei that potentially exist in GRB jets. Specifically, we find that relativistic hydrogen-like high-atomic-number ions originating from the β decay of unstable nuclei and/or the recombination entrained in the GRB jet can generate narrow MeV emission lines through the de-excitation of excited electrons. This model can successfully explain the MeV emission line observed in the most luminous GRB ever recorded, GRB 221009A, with suitable parameters including a Lorentz factor γ ∼ 820–1700 and a total mass of heavy nuclei M tot ∼ 1023–1026 g. Especially, the emission line broadening can be reasonably attributed to both the expansion of the jet shell and the thermal motion of nuclei, naturally resulting in a narrow width (σ line/E line ≲ 0.2) consistent with the observation. Furthermore, we predict that different GRBs can exhibit lines in different bands with various evolving behaviors, which might be confirmed with further observations. Finally, our model provides indirect evidence that GRBs may be one of the sources of UHECRs.

Mutual Diffusivities of Mixtures of Carbon Dioxide and Hydrogen and Their Solubilities in Brine: Insight from Molecular Simulations.

H2-CO2 mixtures find wide-ranging applications, including their growing significance as synthetic fuels in the transportation industry, relevance in capture technologies for carbon capture and storage, occurrence in subsurface storage of hydrogen, and hydrogenation of carbon dioxide to form hydrocarbons and alcohols. Here, we focus on the thermodynamic properties of H2-CO2 mixtures pertinent to underground hydrogen storage in depleted gas reservoirs. Molecular dynamics simulations are used to compute mutual (Fick) diffusivities for a wide range of pressures (5 to 50 MPa), temperatures (323.15 to 423.15 K), and mixture compositions (hydrogen mole fraction from 0 to 1). At 5 MPa, the computed mutual diffusivities agree within 5% with the kinetic theory of Chapman and Enskog at 423.15 K, albeit exhibiting deviations of up to 25% between 323.15 and 373.15 K. Even at 50 MPa, kinetic theory predictions match computed diffusivities within 15% for mixtures comprising over 80% H2 due to the ideal-gas-like behavior. In mixtures with higher concentrations of CO2, the Moggridge correlation emerges as a dependable substitute for the kinetic theory. Specifically, when the CO2 content reaches 50%, the Moggridge correlation achieves predictions within 10% of the computed Fick diffusivities. Phase equilibria of ternary mixtures involving CO2-H2-NaCl were explored using Gibbs Ensemble (GE) simulations with the Continuous Fractional Component Monte Carlo (CFCMC) technique. The computed solubilities of CO2 and H2 in NaCl brine increased with the fugacity of the respective component but decreased with NaCl concentration (salting out effect). While the solubility of CO2 in NaCl brine decreased in the ternary system compared to the binary CO2-NaCl brine system, the solubility of H2 in NaCl brine increased less in the ternary system compared to the binary H2-NaCl brine system. The cooperative effect of H2-CO2 enhances the H2 solubility while suppressing the CO2 solubility. The water content in the gas phase was found to be intermediate between H2-NaCl brine and CO2-NaCl brine systems. Our findings have implications for hydrogen storage and chemical technologies dealing with CO2-H2 mixtures, particularly where experimental data are lacking, emphasizing the need for reliable thermodynamic data on H2-CO2 mixtures.

The influence of small molecule products on natural ester impregnated cellulose insulation paper under electric-thermal coupling field

Transformer oil-paper insulation materials are vulnerable to insulation failure due to the influence of voltage, high temperature, and other complex conditions. Additionally, during the operation of the transformer, the generation of harmful substances such as small molecule acids and water becomes a significant concern. In this study, we employed molecular simulation technology to investigate the effect of small molecule products on natural ester-impregnated cellulose insulation paper under electric-thermal coupling field. The results show that: under the action of electric-thermal coupling, the introduction of the electric field can weaken the molecular thermal motion to a certain extent, and the higher the temperature, the more pronounced the weakening effect of the electric field. When subjected to an electric-thermal coupling field, the mean square displacement of the cellulose molecular chains inside the models doped with water and formic acid increases compared to the oil-paper composite model, with an increase of 60.13 % and 98.25 %, respectively, at 700 K. The fraction of free volume in the model increases and the cohesive energy density decreases, the diffusion ability of molecules inside the model remains strong. The weakening effect of the electric field does not diminish the hydrolysis reaction between the small molecular products and the cellulose molecular chains. This leads to a decrease in model stability and ultimately to the breakdown of the insulation paper.

Ca2+-dependent thermal sensitivity of bacterial MreB assemblies

AbstractIn the bacterial cells, the actin homolog MreB manages the cellular motions and morphology. MreB polymers are important for cell-wall growing and cell shape determination. Fluorescence microscopy studies investigated that in bacterial cells the MreB polymer forms ribbon-like structures that likely helical nearby of the cell wall at the periphery of the cell. As we presented earlier, the thermal motion of the ribbon-like MreB polymers was slowed down by the addition of millimolar Ca2+. The rapid Ca2+ depletion, via EGTA treatment, reordered the polymers into extensive sheets in the presence of magnesium, and further treatment with calcium led to fissured monolayer sheets and the dissociation of filaments into web-like structures which attached to the glass surface. The heat denaturation of MreB assemblies, under varying Ca2+ concentrations, was investigated by DSC, and the Ca2+-dependent MreB polymer rearrangement rates were assessed by isoperibol calorimetry. Here, we measured Ca2+-dependent thermodynamics of prokaryotic MreB assemblies. Under high ionic strength, the MreB polymers show multiple thermal components around 60 °C and 82 °C, generated by less and more stable structures. MreB polymers with a relatively slow exothermic kinetics turned to be more stable due to adding millimolar Ca2+. However, changing the calcium concentration from micromolar to nanomolar and subsequently recovered it to micromolar initialized endothermic remodeling of MreB assemblies and the majority of them showed higher stability than before the treatment. Presumably, the final cell shape depends on the assembling of MreB polymers and the ionic milieu. Calcium concentration-induced changes of MreB structure makes sense in membrane remodeling during prokaryotic cell division or osmotic adaptation.

Far-from-equilibrium slow modes and momentum anisotropy in an expanding plasma

The momentum distribution of particle production in heavy-ion collisions encodes information about thermalization processes in the early-stage quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium evolution of an expanding plasma with an anisotropic momentum-space distribution. We identify slow and fast degrees of freedom in the far-from-equilibrium plasma from the evolution of moments of this distribution. At late times, the slow modes correspond to hydrodynamic degrees of freedom and are naturally gapped from the fast modes by the inverse of the relaxation time, τR−1. At early times, however, there are an infinite number of slow modes with a gap inversely proportional to time, τ−1. From the evolution of the slow modes we generalize the paradigm of the far-from-equilibrium attractor to vector and tensor components of the energy-momentum tensor, and even to higher moments of the distribution function that are not part of the hydrodynamic evolution. We predict that initial-state momentum anisotropy decays slowly in the far-from-equilibrium phase and may persist until the relaxation time. Published by the American Physical Society 2024

Analysis of the Motion Postures in Equestrian Sports Based on Multi-Sensor Data Fusion

Horse riding, in its essence, is both an art and a captivating sport. In these events, riders showcase their refined skills and deep bond with their horses, most notably through changes in physical positioning. This research aims to accurately capture and analyze the spatial postures of riders in equestrian sports, exploring the dynamic differences between professional and amateur riders. To achieve this, we employed a multi-sensor data fusion approach based on human kinetics theories, enhanced by an extended Kalman filter. This method, combined with an optical tracking system, enabled us to intricately compare and analyze the 3D postures of riders during walking and trotting phases. Our research captured these nuanced postural shifts using an inertial sensor network, yielding nine-axis motion data of riders during their performance. The accuracy of our fusion technique was validated using the optical tracking system. By analyzing the motion data, we discerned posture variations among the riders of differing expertise levels, even when executing the same riding technique. The insights from this research offer a quantifiable metric for refining equestrian training and contribute to understanding the complex dynamics of rider–horse interaction.

Very stable open form of 2H-naphthopyran: A theoretical study

Using DFT calculations, it was shown that the intermediate stacked form of the ground state of 2H-naphthopyran can be formed not only during the relaxation of the intramolecular S1 excimer but also during the thermal recovery of the closed form from the trans–trans and cis–trans isomers of the open form. The potential well corresponding to the intermediate form is an additional factor in the stability of the open form, since from this well the molecule can return to the open cis–trans form using thermal motion. The cis–trans and trans–trans isomers of the open form are separated from each other by potential barriers with a height of 1.25 eV. Their overcoming by thermal motion is less likely than the transition of the cis–trans form to the intermediate stacked form. The absorption of conformers in the visible region, which determines the color of the open form, was calculated using TD-DFT; the resulting vibronic absorption band of the trans–trans form coincided well with the experimental one.