Kinetic Theory Research Articles

Damping of gravitational waves in f(R) gravity

We investigate the damping of gravitational waves (GW) in f(R) gravity by matter. By applying the kinetic theory, we examine the first-order approximation of the relativistic Boltzmann equation. In the flat spacetime, we derive the evolution equations for waves in f(R) gravity and demonstrate that Landau damping is absent while collision damping is present. In the Friedmann–Robertson–Walker (FRW) cosmology, we also examine the dynamical equations for the two modes. Furthermore, in the model f(R)=R+αR2, we investigate the effect of the mass term on wave amplitude decay within the neutrino system. We observe that the tensor mode with m=1eV exhibits faster decay compared to other cases, while the scalar mode with m=1eV appears to suppress decay.

Calculation Schemes for Determining Contact Stresses in Railway Rails

One of the tasks of ensuring the safe and sustainable operation of railway transport is to assess the life cycle of the railway track and its elements—in particular, rails. It is known that the main cause of their failure is the development of defects that arise as a result of contact of rails with the wheels of rolling stock—contact fatigue defects. Modern approaches to predicting the contact-fatigue endurance of railway rails and wheels of rolling stock are based on the use of the kinetic theory of damage. The basis of such predictions is the calculation of the stress–strain state of rails under the action of combinations of external force and temperature influences, which is associated with the need to solve spatial boundary value problems of contact interaction. The complexity of such problems necessitates the use of numerical methods, such as the finite element method in particular, for their solution. This paper considers the features of constructing calculation schemes for such problems. Attention is primarily paid to assessing the influence of some design parameters of the rail track and wheels on the magnitude and distribution of stresses in the contact zone. The results will be useful for understanding the physical processes of damage accumulation, the occurrence and development of defects in rails and wheels, as well as for developing methods for predicting the contact fatigue endurance of important elements of railway infrastructure.

Nanowire-Activated Formation of Coaxial Core-Shell Nanostructure Encapsulated by Carbon Nanotube.

Molecular dynamics simulations have unveiled the atomic-scale mechanisms underlying the formation of core-shell nanostructures involving nanowires (NWs) and graphene. Our study identifies two pivotal steps in this process: the self-scrolling of graphene around NWs and the subsequent edge connection to form carbon nanotubes (CNTs). The tendency of graphene to scroll is governed by the complex interplay of van der Waals forces. Stronger interactions between the NW and graphene act as a driving force, significantly promoting the scrolling process. We find that the initial velocity of NWs significantly influences CNT formation, with optimal velocities enabling the creation of defect-free CNTs. Increasing NW velocity enhances the graphene self-scrolling and wrapping dynamics. NW diameter plays a critical role; larger diameters minimize bending deformation, favoring defect-free CNT formation, but also augment transverse wave amplitudes, increasing relative sliding resistance between graphene and NWs and decelerating van der Waals interactions, thereby reducing scrolling speed. We determine that graphene length must meet the condition L = π(D+2r0) and the width-to-length ratio (W/L) must surpass a threshold for effective NW/CNT core-shell structure formation. Multilayer graphene, experiencing reduced van der Waals adsorption from the substrate, is more prone to self-scrolling. Lower temperatures enhance graphene wrapping around NWs, facilitating edge connection and system stabilization, whereas higher temperatures disrupt core-shell integrity by increasing atomic thermal motion, preventing edge connection. These findings establish a solid theoretical foundation for the design and fabrication of advanced NW/CNT core-shell nanostructures.

Mathematical Modeling of Self‐Heating and Self‐Ignition Behavior of Lignocellulosic Biomass Fuel in a Rectangular Stockpile: A Spectral Approach

ABSTRACTThe adoption of lignocellulosic biomass fuels as substitutes for fossil fuels has been known to lower greenhouse gas emissions and enhance the quality of life. However, their tendency for self‐heating, which could result in explosions, presents a significant challenge. To overcome this challenge, a mathematical model for the flow of biomass fuel in a rectangular stockpile is considered in this study. It is assumed that lignocellulosic material behaves like a Bingham fluid with a large yield stress, and the chemical reaction in the biomass fuel particles follows Arrhenius's kinetic theory. The governing equations of the problem are strongly nonlinear. Hence, a numerical method (Chebyshev spectral collocation method) is adopted to provide a solution to the governing equations. Our results indicate that the delay of self‐ignition can be achieved by enhancing the thermal Biot number and activation energy while reducing the Rayleigh number and mass Biot number. This study encourages the wider use of biomass fuels by addressing safety issues, advocating for sustainable energy practices, and improving environmental conservation.

Investigating the kinetics of single-chain expansion upon release in theta conditions

The free expansion of a confined chain in theta solvents following a sudden removal of the confining constraint is investigated using Langevin dynamics simulations in both two- and three-dimensional spaces. The average evolution of the chain size exhibits a sigmoidal transition between the confined and the free states on a logarithmic timescale, indicating a two-stage expansion, each characterized by its own timescale. A kinetic theory is developed by applying Onsager’s variational principle, which balances the change in free energy with energy dissipation. Through scaling analysis, the characteristic time τ1 for the first expansion stage is shown to scale as the cube of the initial chain size, while the chain size increases according to a power law with an exponent α1=1/3, independent of the spatial dimension. In the second stage, the timescale τ2 is found to be proportional to the square of the chain length, and the evolution of the chain size follows an exponential recovery function powered by an exponent α2=1/4. These results are further validated by a direct analysis of the kinetic equations via simulations. Moreover, the general forms of the free energy for the two expansion stages are established through the integration of the kinetic equations. Finally, physical interpretations are proposed, employing a radial expansion model and a diffusive mechanism to explain the observed scaling behaviors. This work explores a model system under the specific solvent condition, providing foundational theory and enhancing our understanding of the expansion-upon-release phenomenon.

Simulation Research on Factors of Deep-Sea Glass Floating Processing Technology

Abstract The coupling problem of thermal and mechanical motion in the process of droplet forming has been analyzed, a mathematical model for droplet forming has been established, and the interaction law between material weight, shape, and droplet temperature obtained. By analyzing the influence of droplet shape, weight, and temperature on the quality of deep-sea glass hemispheres through experiments, the required droplet parameters for forming deep-sea glass hemispheres have been obtained. By using exponential simulation to simulate the cooling rate of glass droplets in the mold, the optimal wall thickness and mold material of the mold have been determined, reducing the forming defects of the glass hemisphere and improving its quality. The actual process situation through the heat conduction inside the glass hemisphere and between the hemisphere and the mold have been simulated, the distribution of glass temperature and stress fields during the molding of the glass hemisphere has been calculated, the layout of the cooling system adjusted, and the process conditions optimized, based on the calculation results, thus improving the quality of the glass hemisphere. Combined with actual experimental results, the above three influencing factors have been demonstrated, and, finally, simulation was used to guide actual production, obtaining a reliable deep-sea glass hemisphere.

Kinetic theory of parametric instabilities in magnetized plasmas and its application to analyzing decay waves during the injecton of lower hybrid waves

Abstract In this paper, a kinetic formalism of parametric decay instabilities (PDI) with electromagnetic waves in magnetized plasmas is compared with its simplified models under the quasi-linear treatment and under the electrostatic limit, as well as a kinetic-fluid hybrid and electrostatic model. Further iteration beyond the quasi-linear approach is shown necessary to deal with the cases of quasi-mode decay; while the kinetic effect and the electromagnetic effect are shown less important in nonlinear coupling physics of PDI during lower hybrid current drive (LHCD) with typical parameters in the scrape-off layer (SOL) of tokamak plasmas. Therefore, the hybrid electrostatic model can be used in the typical scenarios of LHCD induced PDI provided that the kinetic effect in linear response and the electromagnetic effect in the linear dispersion relation of the pump wave are kept. Also, necessary numerical techniques are applied to address the common problems that usually exist in calculating the nonlinear dispersion relations of PDI. Efforts in physical models and numerical techniques allow us to present detailed investigations of decay channels of the PDI during LHCD. It is shown that the PDI can occur for the decay waves with large refractive index parallel to the static magnetic field (typically N_z>10 and much larger than that of the pump), which are easy to be damped and therefore might be ignored to some extent. Parameter dependencies of the different decay channels are also displayed.

Investigation of Pyrrolidinium based Superoxides and Ozonides

Pyrrolidinium‐based cations were systematically modified to investigate their influence on ozonide and superoxide anions and to understand the scattering of experimentally determined dimensions of these radical anions. Hence, 1‐ethyl‐1‐methyl‐pyrrolidinium, 1‐propyl‐1‐methyl‐pyrrolidinium and 1‐butyl‐1‐methyl‐pyrrolidinium superoxide, as well as 1‐methyl‐1‐methyl‐pyrrolidinium and 1‐propyl‐1‐methyl‐pyrrolidinium ozonide have been synthesized via ion exchange in liquid ammonia, and characterized by single crystal X‐ray diffraction. Attempts to crystallize the 1‐ethyl‐1‐methyl‐pyrrolidinium and 1‐butyl‐1‐methyl‐pyrrolidinium ozonide in the same way failed. Two fundamental causes for the variation in the dimensions of the radical anions were revealed: firstly, the reduced accuracy of the experimental data due to static disorder and increased thermal motion, and secondly, the effects of the environments to which the anions are exposed in the crystal matrix. Quantum chemical calculations of the isolated or embedded anions were also performed, showing a significant influence of hydrogen bonding and crystal field effects.

27Al NMR spectroscopic and DFT computational study of the quadrupole coupling of aluminum in two polymorphs of the complex aluminum hydride CsAlH4.

The quadrupole coupling constant CQ and the asymmetry parameter η of the aluminum nuclei in two polymorphs of the complex aluminum hydride CsAlH4 are determined from both 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra and 27Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The accuracy with which these parameters can be determined from the static spectra [CsAlH4(o): CQ = (1.42 ± 0.01) MHz, η = (0.62 ± 0.01) and CsAlH4(t): CQ = (1.43 ± 0.02) MHz, η < 0.03] seems to be slightly higher than via the MAS approach. The experimentally determined parameters (δiso, CQ, and η) are compared with those obtained from DFT-GIPAW (density functional theory-gauge-including projected augmented wave) calculations. When using DFT-optimized structures, the magnitude of the quadrupole coupling constant is overestimated by about 45% for both polymorphs. Further calculations in which the geometry of the AlH4 tetrahedra is varied show a high sensitivity of CQ to the H-Al-H angles. Modest changes in the angles on the order of one to three degrees are sufficient to achieve a near-perfect agreement between GIPAW calculations and experiments. The deviations found for the DFT-optimized structures are explained with the neglect of thermal motion, which typically leads to a reduction in the distortion of the AlH4 tetrahedra. From a broader perspective, the uncertainty in the positions of the hydrogen atoms renders the accurate reproduction or prediction of quadrupole coupling constants for aluminum hydrides challenging.

Unveiling the Microscopic Origin of Ion Transference Numbers in Molten Salt Systems: A Kinetic Theory Approach to an Accurate “Golden Rule”

Unveiling the Microscopic Origin of Ion Transference Numbers in Molten Salt Systems: A Kinetic Theory Approach to an Accurate “Golden Rule”

Experimental and computational study of phase space dynamics in strongly coupled plasmas with steep density gradients

Understanding how plasmas thermalize when density gradients are steep remains a fundamental challenge in plasma physics, with direct implications for fusion experiments and astrophysical phenomena. Standard hydrodynamic models break down in these regimes, and kinetic theories make predictions that have never been directly tested. Here, we present the first detailed phase-space measurements of a strongly coupled plasma as it evolves from sharp density gradients to thermal equilibrium. Using laser-induced fluorescence imaging of an ultracold calcium plasma, we track the complete ion distribution function f(x,v,t). We discover that commonly used kinetic models (Bhatnagar–Gross–Krook and Lenard–Bernstein) overpredict thermalization rates, even while correctly capturing the initial counterstreaming plasma formation. Our measurements reveal that the initial ion acceleration response scales linearly with electron temperature, and that the simulations underpredict the initial ion response. In our geometry we demonstrate the formation of well-controlled counterpropagating plasma beams. This experimental platform enables precision tests of kinetic theories and opens new possibilities for studying plasma stopping power and flow-induced instabilities in strongly coupled systems.

Self-diffusive dynamics of active Brownian particles at moderate densities

The Active Brownian Particle (ABP) model has become a prototype of self-propelled particles. ABPs move persistently at a constant speed V along a direction that changes slowly by rotational diffusion, characterized by a coefficient Dr. Persistent motion plus random reorientations generate a random walk at long times with a diffusion coefficient that, for isolated ABPs in two dimensions, is given by D0=V2/(2Dr). Here, we study the density effects on the self-diffusive dynamics using a recently proposed kinetic theory for ABPs, in which persistent collisions are described as producing a net displacement on the particles. On intermediate timescales, where many collisions have taken place but the director of the tracer particle has not yet changed, it is possible to solve the Lorentz kinetic equation for a tracer particle. It turns out that, as a result of collisions, the tracer follows an effective stochastic dynamics, characterized by an effective reduced streaming velocity Veff and anisotropic diffusion, with coefficients explicitly depending on the density. Based on this result, an effective theoretical and numerical approach is proposed in which the particles in a bath follow stochastic dynamics with mean-field interactions based on the local density. Finally, on time scales larger than Dr−1, studying the van Hove function at small wavevectors, it is shown that the tracer particle presents an effective diffusive motion with a coefficient D=Veff2/(2Dr). The dependence of Veff on the density indicates that the kinetic theory is limited to area fractions smaller than 0.42, and beyond this limit, unphysical results appear.

Constructing robust multifunctional coating on wearable nylon/cotton blend fabrics for human thermal management, motion monitoring, fire safety, and water repellency

Constructing robust multifunctional coating on wearable nylon/cotton blend fabrics for human thermal management, motion monitoring, fire safety, and water repellency

Far-from-equilibrium attractors in kinetic theory for a mixture of quark and gluon fluids

Far-from-equilibrium attractors in kinetic theory for a mixture of quark and gluon fluids

Comment on "A decade of thermostatted kinetic theory models for complex active matter living systems" by Carlo Bianca.

Comment on "A decade of thermostatted kinetic theory models for complex active matter living systems" by Carlo Bianca.

Micro-mechanism of basal stresses in steady rock-ice mixture flows: Insights from 2D DEM simulations

Micro-mechanism of basal stresses in steady rock-ice mixture flows: Insights from 2D DEM simulations

On a Critical Acceleration Scale of Dark Matter in ΛCDM and Dynamical Dark Energy

Abstract Universal acceleration a 0 emerges in various empirical laws, yet its fundamental nature remains unclear. Using Illustris and Virgo N-body simulations, we focus on the velocity and acceleration fluctuations in collisionless dark matter involving long-range gravity. For comparison, in the kinetic theory of gases, molecules undergo random elastic collisions involving short-range interactions, where only velocity fluctuations are relevant. Hierarchical structure formation proceeds through the merging of smaller halos to form larger halos, which facilitates a continuous energy cascade from small to large halos at a constant rate ε u ≈ −10−7 m2 s−3. Velocity fluctuations involve a critical velocity u c ∝ (1 + z)−3/4. Acceleration fluctuations involve a critical acceleration a c ∝ (1 + z)3/4. Two critical quantities are related by the rate of energy cascade ε u ≈ −a c u c /[2(3π)2], where factor 3π is from the angle of incidence during merging. With critical velocity u c on the order of 300 km s−1 at z = 0, the critical acceleration is determined to be a c0 ≡ a c (z = 0) ≈ 10−10 m s−2, suggesting a c might explain the universal acceleration a 0 ≈ 10−10 m s−2 in the empirical Tully–Fisher relation or modified Newtonian dynamics. The redshift evolution a c ∝ (1 + z)3/4 is in good agreement with Magneticum and EAGLE simulations and in reasonable agreement with limited observations. This suggests a larger a 0 at a higher redshift such that galaxies of fixed mass rotate faster at a higher redshift. Note that for dark energy (DE) density ρ DE 0 ≈ a c 0 2 / G = 1 0 − 10 J m−3, we postulate an entropic origin of the DE from acceleration fluctuations of dark matter, analogous to the gas pressure from velocity fluctuations. This leads to a dynamical DE coupled to the structure evolution involving a relatively constant DE density followed by a slow weakening phase, suggesting possible deviations from the standard ΛCDM paradigm.

Scaling of pre-equilibrium dilepton production in QCD kinetic theory

We use quantum chromodynamics (QCD) kinetic theory to compute dilepton production coming from the pre-equilibrium phase of the quark-gluon plasma created in high-energy heavy-ion collisions. We demonstrate that the dilepton spectrum exhibits a simple scaling in terms of the specific shear viscosity η/s and entropy density dS/dζ∼(Tτ1/3)∞3/2, which can be derived from dimensional analysis in the presence of a pre-equilibrium attractor. Based on this scaling we perform event-by-event calculations of in-medium dilepton production and determine the invariant mass range where the pre-equilibrium yield is the leading contribution. Published by the American Physical Society 2025

Kinetic theory analysis of microscale lubrication of a gas between eccentric circular cylinders: Effect of rotation of the outer cylinder

Kinetic theory analysis of microscale lubrication of a gas between eccentric circular cylinders: Effect of rotation of the outer cylinder

Effects of external gravitational field on highly rarefied gases: analysis based on stochastic soft-sphere collision models

This study examines the effects of an external gravitational field on highly rarefied gases in the transitional-flow regime near the free-molecular-flow regime. In our theoretical study, we rederive the classical kinetic theory for an ideal gas in terms of the kinetics of the constituent particles to account for the effect of particle acceleration by an external gravitational field. Subsequently, we derive an extended expression for the virial pressure equation as a generic description of the dynamics under an external gravitational field. We employ the soft-sphere model for the following reasons: In highly rarefied gases, short-range and instantaneous collisional interactions are dominant. Thus, by expanding the asymmetric two-body potential in the virial pressure equation and retaining only the contribution of the short-range interaction, we can obtain a soft-sphere model that represents the interaction in the collision direction as a harmonic oscillation. In the absence of dissipation, the soft-sphere model has been confirmed to reproduce fully elastic collisions. In our collision simulations, we define two parameters. The first parameter represents the collision probability between each pair of approaching particles, and the second represents the ratio of the magnitude of the external potential energy to the total kinetic energy of the particles. The behavior of the system is analyzed by varying the values of these two parameters. Our analysis shows that if the external potential energy is sufficiently small (1%–5%) compared with the total kinetic energy, then a pressure difference emerges between the walls. However, the system retains the properties of equilibrium statistical mechanics, as indicated by the Maxwell–Boltzmann (MB) distribution. In conclusion, highly rarefied gases obey the MB distribution even when placed under weak gravitational fields.