Kinetic Theory Research Articles

Collision integrals within the Chapman-Enskog theory for a generalized Lennard-Jones potential.

We report the values of the collision integrals, needed for the calculation of the macroscopic transport properties such as viscosity (η) and diffusion coefficient (D) of gases within the Chapman-Enskog kinetic gas theory, for a generalized Lennard-Jones potential (gLJ), a more general potential with an adjustable long range 1/r dependence that can describe a wide range of intermolecular interactions.

Spatial Kinetic Modeling of Crowd Evacuation: Coupling Social Behavior and Infectious Disease Contagion

This paper introduces a kinetic model of crowd evacuation from a bounded domain, integrating social behavior and contagion dynamics. The model describes the spatial movement of individuals in a crowd, taking into account interactions with other people and the geometry of the environment. Interactions between healthy and infectious individuals can lead to disease transmission and are considered. The approach is grounded in the kinetic theory of active particles, where the activity variable represents both the infectious disease status of individuals (e.g., susceptible, infected) and the psychological state of pedestrians, including contagion awareness. Varying awareness levels influence individual behavior, leading to more cautious movement patterns, potentially reducing the overall infection rate. The performance of the model is evaluated through a series of numerical simulations. Different scenarios are examined to investigate the impact of awareness levels on pedestrian behavior, infectious disease spread, and evacuation times. Additionally, the effects of population immunization and individual contagion awareness are assessed to determine the most effective strategy for reducing infections. The results provide valuable insights into targeted strategies to mitigate contagion.

Kinetic theory of discontinuous shear thickening of a moderately dense inertial suspension of frictionless soft particles

Kinetic theory of discontinuous shear thickening of a moderately dense inertial suspension of frictionless soft particles

Molecular-Scale Simulation of Wetting of Actin Filaments by Protein Droplets.

Liquid phase-separating proteins can form condensates that play an important role in spatial and temporal organization of biological cells. The understanding of the mechanisms that lead to the formation of protein condensates and their interactions with other biomolecules may lead to processing routes for soft materials with tailored geometry and function. Fused in sarcoma (FUS) is an example of a nuclear protein that forms stable complexes, and recent studies have highlighted its ability to wet actin filaments and bundle them into networks. We perform coarse-grained molecular dynamics simulations to investigate the wetting and spreading of FUS droplets on actin filaments. We employ the Martini model and rescale the protein-protein and protein-actin interactions to tune the interfacial and wetting properties of FUS droplets. By measuring the molecular displacements in the three-phase region, we are able to relate contact angle, contact line velocity, and contact line friction in terms of a linear approximation of molecular kinetic theory. The results show that the rescaled Martini model can be used to study the molecular mechanisms of dynamic wetting at the nanoscale and to obtain quantitative predictions of the contact line friction and contact angles during dynamic wetting.

Resummed spin hydrodynamics from quantum kinetic theory

Resummed spin hydrodynamics from quantum kinetic theory

Solvability of a system of nonlinear integral equations on the entire line

A system of singular integral equations with monotone and concave nonlinearity in the subcritical case is investigated. The specified system and its scalar analogy have direct applications in various areas of physics and biology. In particular, scalar and vector equations of this nature are encountered in the dynamic theory of p-adic strings, in the theory of radiative transfer, in the kinetic theory of gases, and in the mathematical theory of the spread of epidemic diseases. A constructive theorem of existence in the space of continuous and bounded vector functions is proved. The integral asymptotic of the constructed solution is studied. An effective iterative process for constructing an approximate solution to this system is proposed. In a certain class of bounded vector functions, the uniqueness of the solution is proved. In the class of bounded vector functions (with non-negative coordinates) with a zero limit at infinity, the absence of an identically zero solution is proved. Examples of the matrix kernel and nonlinearities are given to illustrate the importance of the obtained results. Some of the examples have applications in the above-mentioned branches of natural science.

Advancing dynamic quantum crystallography: enhanced models for accurate structures and thermodynamic properties.

X-ray diffraction (XRD) has evolved significantly since its inception, becoming a crucial tool for material structure characterization. Advancements in theory, experimental techniques, diffractometers and detection technology have led to the acquisition of highly accurate diffraction patterns, surpassing previous expectations. Extracting comprehensive information from these patterns necessitates different models due to the influence of both electron density and thermal motion on diffracted beam intensity. While electron-density modelling has seen considerable progress [e.g. the Hansen-Coppens multipole model and Hirshfeld Atom Refinement (HAR)], the treatment of thermal motion has remained largely unchanged. We have developed a novel method that combines the strengths of the advanced charge-density models [Aspherical Atom Models (AAMs), such as HAR or the Transferable Aspherical Atom Model (TAAM)] and the thermal motion model (normal modes refinement, NoMoRe). We denote this approach AAM_NoMoRe, wherein instead of refining routine anisotropic displacement parameters (ADPs) against single-crystal X-ray diffraction data, we refine the frequencies obtained from periodic density functional theory (DFT) calculations. In this work, we demonstrate the effectiveness of this model by presenting its application to model compounds, such as alanine, xylitol, naphthalene and glycine polymorphs, highlighting the influence of our method on the H-atom positions and shape of their ADPs, which are comparable with neutron data. We observe a significant decrease in the similarity index for H-atom ADPs after AAM_NoMoRe in comparison to only AAM, aligning more closely with neutron data. Due to the use of aspherical form factors (AAM), our approach demonstrates better fitting performance, as indicated by consistently lower wR2 values compared to the Independent Atom Model (IAM) refinement and a significant decrease compared to the traditional NoMoRe model. Furthermore, we present the estimation of a key thermodynamic property, namely, heat capacity, and demonstrate its alignment with experimental calorimetric data.

Linear and nonlinear resonant properties of electron gas in n-InSb and graphene layers in terahertz range in bias magnetic fields

Abstract The nonlinear conductivity of 2D electron gas in graphene and 3D electron gas in the narrow-gap n-InSb semiconductor has been simulated in terahertz (THz) range by various methods including the direct quantum approach, the quasi-classical kinetics, and the quasi-relativistic hydrodynamics. These methods yield the same results under the electron temperatures ≤100 K when the kinetic electron effective mass used in the nonlinear hydrodynamics is equal to the effective mass in n-InSb and it is equal to some nonzero mass that depends on 2D electron concentration in the graphene. The linear resonant dependencies of the complex electron conductivity are simulated both from the kinetic theory and from the hydrodynamic one. The kinetic dependencies of the resonant conductivity on frequency coincide with ones obtained from the hydrodynamic theory under realistic electron concentrations and collision frequencies. Thus, the nonlinear hydrodynamic equations are valid to describe the nonlinear dynamics of the electron gas in graphene and in n-InSb. The nonlinear hydrodynamics has been applied for simulations of nonlinear propagation of THz electromagnetic waves through the multilayer structures dielectric - graphene or n-InSb - dielectric placed in a bias magnetic field. The simplest three-layer structures demonstrate the sharp nonlinear switching of the transparency of THz waves and the bistability under relatively low values of amplitudes of the incident electromagnetic wave.

CORROSÃO LOCALIZADA DO ALUMÍNIO EM MEIOS AERADOS E EM MEIOS COM BAIXO TEOR OXIGÊNIO: ESTUDO E COMPARAÇÃO POR MEIO DE CURVAS DE POLARIZAÇÃO

LOCALIZED CORROSION OF ALUMINUM IN AERATED ENVIRONMENTS AND IN ENVIRONMENTS WITH LOW OXYGEN CONTENT: STUDY AND COMPARISON THROUGH POLARIZATION CURVES. In this study, the influence of deaeration on the electrochemical behavior of aluminum was studied by polarization techniques. The advantages of deaeration for the evaluation of localized corrosion on aluminum are discussed. The influence of the corrosive environment with low oxygen contents on the electrochemical response of aluminum was evaluated by anodic and cathodic potentiodynamic polarization curves. The polarization tests were carried out in chloride solutions followed by optical and scanning electron microscopy characterization. It was observed that deaeration of the solution significantly influences the electrochemical behavior of aluminum, as predicted by corrosion kinetics theories. For the cathodic curves, a decrease in the limiting current density for the oxygen reduction reaction (ORR) was observed for low oxygen solutions, while the “apparent” initiation of the hydrogen evolution reaction (HER) was shifted to higher potentials. In the anodic curves, a common passive behavior was not observed in aerated conditions, however, at the corrosion potential, localized pitting was observed indicating that the material was already above the breakdown potential (Ebr). In the low oxygen content solution, however, a passive region with a breakdown potential, was observed on the anodic curves.

Why do newer degrees of freedom appear in higher-order truncated hydrodynamic theory?

An exact derivation of relativistic hydrodynamics from an underlying microscopic theory has been shown to be an all-order theory. From the relativistic transport equation of kinetic theory, the full expressions of hydrodynamic viscous fluxes have been derived which turn out to include all orders of out-of-equilibrium derivative corrections. It has been shown, that for maintaining causality, it is imperative that the temporal derivatives must include all orders, which can be resummed in non-local, relaxation operator-like forms and finally ‘integrated in’ introducing newer degrees of freedom. The theory can of course be truncated at any higher spatial orders, but the power over the the infinite temporal sum increases correspondingly such that the causality is respected. As a result, the theory truncated at any higher order of spatial gradient, requires newer degrees of freedom for each increasing order.

New insights into the analytic structure of correlation functions via kinetic theory

The way a relativistic system approaches fluid dynamical behavior can be understood physically through the signals that will contribute to its linear response to perturbations. What these signals are is captured in the analytic structure of the retarded correlation function. The nonanalyticities can be grouped into three types based on their dimension in the complex frequency plane. In this paper, we will use kinetic theory in the (momentum-dependent) relaxation time approximation to find how we can calculate their corresponding signals. In the most general case of a system with particles that have a continuum of thermalization rates, we find that a nonanalytic region appears. To calculate its signal, we introduce the “nonanalytic area density” that describes the properties of this region, and we construct a method to calculate it. Further, to take into account the ambiguity present in signal analysis, following from manipulations of the nonanalyticities, we will identify two specific choices called “pictures” with interesting analytic properties and compare in what scenarios each picture is most useful. Published by the American Physical Society 2024

Toward a microscopic picture of superfluid Helium-4

Taking inspiration from F. Bloch’s seminal work (PRA 7, 2187 (1973)), we investigate the quantum many-body states of superfluid 4He, unveiling a novel characteristic in the system’s energy levels. Below the transition temperature, the thermally active low-energy levels exhibit a distinctive grouping behavior, with each level belonging exclusively to a single group. In a superflow state, the system establishes thermal equilibrium with its surroundings on a group-specific basis. Specifically, the levels within a chosen group, initially populated, undergo thermal redistribution, while the remaining groups of levels stay vacant due to absence of transitions between groups. The macroscopic properties of the system, such as its superflow velocity and thermal energy density, are statistically determined by the thermal distribution of the occupied group. Additionally, we infer that the thermal energy of a superflow has an unusual relationship with flow velocity, such that the larger the flow velocity, the smaller the thermal energy. This relationship is responsible for a range of intriguing phenomena, including the mechano-caloric effect and the fountain effect, which demonstrate a fundamental coupling between the thermal motion and hydrodynamic motion of the system. Furthermore, we present experimental evidence of a self-heating effect in 4He superflows, confirming that a 4He superflow carries significant thermal energy related to its velocity.

Computational Fluid Dynamics Simulation and Analysis of Non-Newtonian Drilling Fluid Flow and Cuttings Transport in an Eccentric Annulus

This study examines the flow behavior as well as the cuttings transport of non-Newtonian drilling fluid in the geometry of an eccentric annulus, accounting for what impacts drill pipe rotation might have on fluid velocity, as well as annular eccentricity on axial and tangential distributions of velocity. A two-phase Eulerian–Eulerian model was developed by using computational fluid dynamics to simulate drilling fluid flow and cuttings transport. The kinetic theory of granular flow was used to study the dynamics and interactions of cuttings transport. Non-Newtonian fluid properties were modeled using power law and Bingham plastic formulations. The simulation results demonstrated a marked improvement in efficiency, as much as 45%, in transport by increasing the fluid inlet velocity from 0.54 m/s to 2.76 m/s, reducing the amount of particle accumulation and changing axial and tangential velocity profiles dramatically, particularly at narrow annular gaps. At a 300 rpm rotation, the drill pipe brought on a spiral flow pattern, which penetrated tangential velocities in the narrow gap that had increased transport efficiency to almost 30% more. Shear-thinning behavior characterizes fluid of which the viscosity, at nearly 50% that of the central core low-shear regions, was closer to the wall high-shear regions. Fluid velocity and drill pipe rotation play a crucial role in optimizing cuttings transport. Higher fluid velocities with controlled drill pipe rotation enhance cuttings removal and prevent particle build-up, thereby giving very useful guidance on how to clean the wellbore efficiently in drilling operations.

Identifying determinants of γ’ phase coarsening behavior in Co/CoNi-based superalloys with explainable artificial intelligence (XAI)

The coarsening of γ’ phases in superalloys is influenced by multiple factors and significantly impacts mechanical properties, such as strength and creep resistance. While classical kinetic theory delineates the coarsening mechanisms of γ’ phases, it has limitations when excessive coarsening occurs. The challenge of identifying the pivotal factors that markedly affect the kinetic behavior of γ’ phase coarsening remains largely unresolved. This study utilized eXtreme Gradient Boosting (XGBoost) models to identify the CP and CPAE features that characterize the coarsening behavior of γ’ phases in Co/CoNi-based superalloys. Through meticulous analysis of CP features [aging temperature (T ), the atomic fraction of the Ti element (c Ti), the atomic fraction of the V element (c V), and the atomic fraction of the Ta element (c Ta)] and CPAE features [T , Young’s modulus (E ), electronegativity (EN ), atomic radius (r ), c Ti, the differences of atomic radius (σr ), and the differences of electronegativity (σEN )], we derived explicit mathematical expressions using symbolic classification approach, which offers improved interpretability compared to traditional black-box models. Our findings indicate that T , c Ti, c Ta, E and σr are dominant factors influencing γ’ coarsening behavior across different Co/CoNi-based superalloys. Notably, T , c Ti, c Ta and σr exhibit an inverse correlation with the γ’ coarsening behavior, whereas E shows a positive correlation. These insights provide valuable guidance for determining whether coarsening occurs in newly designed alloys. Furthermore, they facilitate the design of innovative anti-coarsening Co/CoNi-based superalloys. This knowledge is instrumental in enhancing alloy formulations, ensuring their performance and longevity in demanding environments where thermal stability is paramount.

Dissecting mode-coupling theory for supercooled liquids

The mode-coupling theory (MCT) of the glass transition ranks among the most successful first-principles kinetic theories to describe glassy dynamics. However, MCT does not fully account for crucial aspects of the dynamics near the glass transition. To facilitate improving the theory, we critically test the approximations inherent in MCT for a supercooled mixture using Brownian dynamics simulations. Although each MCT approximation significantly impacts the predicted dynamics, our findings show that long-time cancellations of errors occur due to static and dynamic approximations of four-point correlation functions, with the validity of these approximations remaining relatively constant across different temperatures. Notably, the MCT form of the memory functional maintains remarkably high accuracy even in the supercooled regime when evaluated with the intermediate scattering function from simulations. The primary discrepancies between theoretical predictions and experimental results arise from the self-consistent nature of the MCT equations, which amplify minor errors in the memory kernel. This suggests that minor corrections to the memory functional could substantially enhance the theory's predictive accuracy. Published by the American Physical Society 2024

Coupling the thermal acoustic modes of a bubble to an optomechanical sensor.

Optomechanical sensors provide a platform for probing acoustic/vibrational properties at the micro-scale. Here, we used cavity optomechanical sensors to interrogate the acoustic environment of adjacent air bubbles in water. We report experimental observations of the volume acoustic modes of these bubbles, including both the fundamental Minnaert breathing mode and a family of higher-order modes extending into the megahertz frequency range. Bubbles were placed on or near optomechanical sensors having a noise floor substantially determined by ambient medium fluctuations, and which are thus able to detect thermal motions of proximate objects. Bubble motions could be coupled to the sensor through both air (i.e., with the sensor inside the bubble) and water, verifying that sound is radiated by the high-order modes. We also present evidence for elastic-Purcell-effect modifications of the sensor's vibrational spectrum when encapsulated by a bubble, in the form of cavity-modified linewidths and line shifts. Our results could increase the understanding of bubble acoustics relevant to a variety of fields such as lab-on-a-chip microfluidics and biosensing, and could also inform future efforts to optimize the properties of micro-mechanical oscillators.

Kinetic Theory with Casimir Invariants—Toward Understanding of Self-Organization by Topological Constraints

A topological constraint, characterized by the Casimir invariant, imparts non-trivial structures in a complex system. We construct a kinetic theory in a constrained phase space (infinite-dimensional function space of macroscopic fields), and characterize a self-organized structure as a thermal equilibrium on a leaf of foliated phase space. By introducing a model of a grand canonical ensemble, the Casimir invariant is interpreted as the number of topological particles.

A Synergistic Dual‐Atom Sites Nanozyme Augments Immunogenic Cell Death for Efficient Immunotherapy

AbstractInducing immunogenic cell death (ICD) is a promising approach to elicit enduring antitumor immune responses. Hence, extensive efforts are being made to develop ICD inducers. Herein, a cascaded dual‐atom nanozyme with Fe and Cu sites (FeCu‐DA) as an efficient ICD inducer is presented. The Fe and Cu dual‐atom sites synergistically enhance peroxidase (POD) and catalase activities, effectively converting intratumoral hydrogen peroxide (H2O2) to hydroxyl radicals (·OH) and oxygen (O2). Moreover, FeCu‐DA exhibits superior glutathione‐oxidase (GSH‐OXD) activity, catalyzing GSH oxidation to generate H2O2, enabling cascaded catalysis for sustainable ∙OH generation and reducing reactive oxygen species (ROS) resistance by consuming GSH. Steady–state kinetic analysis and density functional theory calculations indicate that FeCu‐DA exhibits a higher catalytic rate and efficiency than Fe single‐atom nanozymes (Fe‐SA) because of its stronger interactions with H2O2. Its POD activity is 948.05 U mg−1, which is 2.8‐fold greater than that of Fe‐SA. Furthermore, FeCu‐DA exhibits impressive photothermal effects and photothermal‐enhanced cascaded catalysis kinetics for ROS generation, thereby inducing potent ICD. Combined with anti‐PD‐L1 antibody (αPD‐L1) blockade, FeCu‐DA shows synergistic enhancement in treatment under near‐infrared irradiation. This study provides insights for designing efficient dual‐atom nanozymes and demonstrates their potential in ICD‐induced cancer immunotherapy.

Thermodynamic bounce effect in quantum BTZ black hole

A novel thermodynamic phenomenon has been observed in the quantum Bañados-Teitelboim-Zanelli (qBTZ) black hole, utilizing generalized free energy and Kramers escape rate. This phenomenon also reveals the unique property of the quantum black hole. The stochastic thermal motion of various thermodynamic states within the black hole system induces phase transitions, under the influence of generalized free energy which obtained by extending Maxwell’s construction. Through the analysis of Kramers escape rate, it is discovered that the qBTZ black hole thermodynamic system exhibits a bounce effect. It originates from the non-monotonicity of entropy in black hole thermodynamic systems. Furthermore, the overall thermodynamic picture of the qBTZ black hole has been obtained under different quantum backreactions.

Molecular Dynamics Simulations on Heat Transport of Nanoconfined Water under Electric Fields: Effect of Nanochannel Size.

When water is confined in a nanochannel, its thermodynamic and kinetic properties change dramatically compared to the macroscale. To investigate these phenomena, we conducted nonequilibrium molecular dynamics simulations on the heat transfer in copper-water nanochannels with lengths ranging from 12 to 20 nm in the absence and presence of an electric field. The results indicate that in the absence of an electric field (Lz = 12-20 nm), the binding force between water molecules in the 20 nm nanochannel is the weakest, facilitating thermal motion in the liquid phase. When compared to the 12 nm nanochannel, the enhancement rate of the thermal conductivity is 19.53%. In the presence of a uniform electric field in the positive z-direction (Lz = 12-16 nm), water molecules in the 16 nm nanochannel are more readily frozen into ice crystal structures. This change in the mode of heat transfer shifts from the thermal diffusion of water molecules to the vibrations between copper atoms and the ice crystal, resulting in a significant increase in the thermal conductivity of water.