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S. A. Khrapak

Publications and source records attributed to S. A. Khrapak.

At least 19 recordsLinked to original sources
arXiv2026

Variational approach to Yukawa fluids. II. Instantaneous elastic moduli and sound velocities

The variational approach based on the Bogoliubov inequality using the fluid of hard spheres as a reference system is implemented to evaluate instantaneous shear, bulk and longitudinal elastic moduli, as well as related sound velocities of Yukawa fluids. The remarkable accuracy of this method is documented. In addition, we evaluate the adiabatic sound velocity from an appropriate equation of state and discuss its relation to the longitudinal and bulk sound velocities obtained from the corresponding instantaneous elastic moduli. The transition between weakly coupled and strongly coupled regimes is analyzed in detail.

physics.plasm-ph↗
arXiv2026

Variational approach to Yukawa fluids. I. Thermodynamics

The excess energy, entropy, and pressure of a strongly coupled Yukawa fluid are calculated from the variational approach using the fluid of hard spheres as a reference system. As in the case of the one-component plasma, the Percus-Yevick virial entropy is appropriate for such calculations and delivers remarkable agreement with available results from molecular dynamics simulations. The agreement with the molecular-dynamics results is particularly impressive in the strongly coupled regime, making this approach a useful predictive tool when numerical data are scarce or not yet available. As an application of the variational approach, we estimate the location of the melting curve in the regime of sufficiently strong screening.

physics.plasm-ph↗
arXiv2026

Excess energy of strongly coupled one-component plasma from variational approach

The excess energy of the one-component plasma fluid is calculated using the variational approach combined with different variants of the excess entropy of the hard-sphere fluid, which is used as a reference system. Our comparison with recent Monte Carlo results for the excess energy of the one-component plasma identifies the Percus-Yevick virial entropy as the most accurate entropy to be used in the variational calculation of this kind. The reason for this and potential developments of the present analysis are briefly discussed. We demonstrate that the original Rosenfeld-Tarazons scaling of the thermal component of the excess energy of the one-component plasma fluid is in excellent agreement with recent Monte Carlo results.

physics.plasm-ph↗
arXiv2025

Heat capacity of dense liquids: A link between two-phase model and melting temperature scaling

Generalized Rosenfeld-Tarazona scaling predicts the power-law dependence of the excess heat capacity of simple liquids on temperature. The two-phase model treats a liquid as a superposition of gas- and solid-like components whose relative abundance is quantified by a liquid rigidity parameter. We demonstrate here that the generalized Rosenfeld-Tarazona scaling emerges naturally in the two-phase model from the scale invariance of the liquid rigidity parameter.

cond-mat.soft↗
arXiv2024

Quasi-universal behaviour of shear relaxation times in simple fluids

We calculate the shear relaxation times in four important simple monatomic model fluids: Lennard-Jones, Yukawa, soft-sphere and hard-sphere fluids. It is observed that in properly reduced units, the shear relaxation times exhibit quasi-universal behaviour when the density increases from the gas-like low values to the high-density regime near crystallization. They first decrease with density at low densities, reach minima at moderate densities, and then increase toward the freezing point. The reduced relaxation times at the minima and at the fluid-solid phase transition are all comparable for the various systems investigated, despite more than ten orders of magnitude difference in real systems. Important implications of these results are discussed.

cond-mat.soft↗
arXiv2024

Freezing density scaling of transport coefficients in the Weeks-Chandler-Andersen fluid

It is shown that the transport coefficients (self-diffusion, shear viscosity, and thermal conductivity) of the Weeks-Chandler-Anderson (WCA) fluid along isotherms exhibit a freezing density scaling (FDS). The functional form of this FDS is essentially the same or closely related to those in the Lennard-Jones fluid, hard-sphere fluid, and some liquefied noble gases. This proves that this FDS represents a quasi-universal corresponding state principle for simple classical fluids with steep interactions. Some related aspects such as Stokes-Einstein relation without a hydrodynamic diameter and gas-to-liquid dynamical crossover are briefly discussed. Simple fitting formula for the transport coefficients of the dense WCA fluid are suggested.

cond-mat.stat-mech↗
arXiv2024

Elementary vibrational model for thermal conductivity of Lennard-Jones fluids: Applicability domain and accuracy level

Exact mechanisms of thermal conductivity in liquids are not well understood, despite rich research history. A vibrational model of energy transfer in dense simple liquids with soft pairwise interactions seems adequate to partially fill this gap. The purpose of the present paper is to define its applicability domain and to demonstrate how well it works within the identified applicability domain in the important case of the Lennard-Jones model system. The existing results from molecular dynamics simulations are used for this purpose. Additionally, we show that a freezing density scaling approach represents a very powerful tool to estimate the thermal conductivity coefficient across essentially the entire gas-liquid region of the phase diagram, including metastable regions. A simple practical expression serving this purpose is proposed.

cond-mat.soft↗
arXiv2022

Minima of shear viscosity and thermal conductivity coefficients of classical fluids

The shear viscosity and thermal conductivity coefficients of various liquids exhibit minima along certain trajectories on the phase diagram. These minima arise due to the crossover between the momentum and energy transport mechanisms in gas-like and liquid-like regimes. We demonstrate that the magnitudes of the minima are quasi-universal in appropriately reduced units, especially for the viscosity coefficients. Results presented in support of this observation concern the transport properties of three simple model systems with different pairwise interaction potentials (hard spheres, Lennard-Jones, and Coulomb) as well as seven important real atomic and molecular liquids (Ne, Ar, Kr, Xe, CH$_4$, CO$_2$, and N$_2$). The minima in viscosity and thermal conductivity represent useful reference points for fluid transport properties.

cond-mat.soft↗
arXiv2021

When do soft spheres become hard spheres?

The conventional (Zwanzig-Mountain) expressions for instantaneous elastic moduli of simple fluids predict their divergence as the limit of hard sphere (HS) interaction is approached. However, elastic moduli of a true HS fluid are finite. Here we demonstrate that this paradox reveals the soft- to hard-sphere crossover in fluid excitations and thermodynamics. With extensive \emph{in-silico} study of fluids with repulsive power-law interactions ($\propto r^{-n}$), we locate the crossover at $n\simeq 10-20$ and develop a simple and accurate model for the HS regime. The results open novel prospects to deal with the elasticity and related phenomena in various systems, from simple fluids to melts and glasses.

cond-mat.soft↗
arXiv2021

Transport properties of Lennard-Jones fluids: Freezing density scaling along isotherms

It is demonstrated that properly reduced transport coefficients (self-diffusion, shear viscosity, and thermal conductivity) of Lennard-Jones fluids along isotherms exhibit quasi-universal scaling on the density divided by its value at the freezing point. Moreover, this scaling is closely related to the density scaling of transport coefficients of hard-sphere fluids. The Stokes-Einstein relation without the hydrodynamic diameter is valid in the dense fluid regime. The lower density boundary of its validity can serve as a practical demarcation line between gas-like and liquid-like regimes.

cond-mat.soft↗
arXiv2021

Vibrational model of thermal conduction for fluids with soft interactions

A vibrational model of heat transfer in simple liquids with soft pairwise interatomic interactions is discussed. A general expression is derived, which involves an averaging over the liquid collective mode excitation spectrum. The model is applied to quantify heat transfer in a dense Lennard-Jones liquid and a strongly coupled one-component plasma. Remarkable agreement with the available numerical results is documented. A similar picture does not apply to the momentum transfer and shear viscosity of liquids.

cond-mat.soft↗
arXiv2019

Particle charge in PK-4 dc discharge from ground-based and microgravity experiments

The charge of microparticles immersed in the dc discharge of the Plasmakristall-4 experimental facility has been estimated using the particle velocities from experiments performed on Earth and under microgravity conditions on the International Space Station. The theoretical model used for these estimates is based on the balance of the forces acting on a single particle in the discharge. The model takes into account the radial dependence of the discharge parameters and describes reasonably well the experimental measurements.

physics.plasm-ph↗
arXiv2019

Particle dynamics in deposition of porous films with a pulsed radio-frequency atmospheric pressure glow discharge

Nanoparticles grown in a plasma are used to visualize the process of film deposition in a pulsed radio-frequency (rf) atmospheric pressure glow discharge. Modulating the plasma makes it possible to successfully prepare porous TiO2 films. We study the trapping of the particles in the sheath during the plasma-on phase and compare it with numerical simulations. During the plasma-off phase, the particles are driven to the substrate by the electric field generated by residual ions, leading to the formation of porous TiO2 film. Using video microscopy, the collective dynamics of particles in the whole process is revealed at the most fundamental "kinetic" level.

physics.plasm-ph↗
arXiv2018

Collective modes of two-dimensional classical Coulomb fluids

Molecular dynamics simulations have been performed to investigate in detail collective modes spectra of two-dimensional Coulomb fluids in a wide range of coupling. The obtained dispersion relations are compared with theoretical approaches based on quasi-crystalline approximation (QCA), also known as the quasi-localized charge approximation (QLCA) in the plasma-related context. An overall satisfactory agreement between theory and simulations is documented for the longitudinal mode at moderate coupling and in the long-wavelength domain at strong coupling. For the transverse mode, satisfactory agreement in the long-wavelength domain is only reached at very strong coupling, when the cutoff wave-number below which shear waves cannot propagate becomes small. The dependence of the cutoff wave-number for shear waves on the coupling parameter is obtained.

physics.plasm-ph↗
arXiv2016

Fingerprints of different interaction mechanisms on the collective modes in complex (dusty) plasmas

In this paper we discuss the relations between the exact shape of interparticle interactions in complex (dusty) plasmas and the dispersion relation of the longitudinal collective mode. Several representative repulsive potentials, predicted previously theoretically, are chosen and the corresponding dispersion relations are calculated using the quasi-crystalline approximation. Both weakly coupled and strongly coupled regimes are considered. It is shown that the long-wavelength portions of the dispersion relations are sensitive to the long-range asymptote of the interaction potential. This can be used to discriminate between different interaction mechanisms operational in complex plasmas experimentally. Main requirements are briefly discussed.

physics.plasm-ph↗
arXiv2016

Collective modes in two-dimensional one-component-plasma with logarithmic interaction

The collective modes of a familiar two-dimensional one-component-plasma with the repulsive logarithmic interaction between the particles are analysed using the quasi-crystalline approximation (QCA) combined with the molecular dynamic simulation of the equilibrium structural properties. It is found that the dispersion curves in the strongly coupled regime are virtually independent of the coupling strength. Arguments based on the excluded volume consideration for the radial distribution function allow us to derive very simple expressions for the dispersion relations, which show excellent agreement with the exact QCA dispersion over the entire domain of wavelengths. Comparison with the results of the conventional fluid analysis is performed and the difference is explained.

physics.plasm-ph↗
arXiv2016

Internal energy of the classical two- and three-dimensional one-component-plasma

We summarize several semi-phenomenological approaches to estimate the internal energy of one-component-plasma (OCP) in two (2D) and three (3D) dimensions. Particular attention is given to a hybrid approach, which reproduces the Debye-H$\ddot{\text{u}}$ckel asymptote in the limit of weak coupling, the ion sphere (3D) and ion disc (2D) asymptotes in the limit of strong coupling, and provides reasonable interpolation between these two limits. More accurate ways to estimate the internal energy of 2D and 3D OCP are also discussed. The accuracy of these analytic results is quantified by comparison with existing data from numerical simulations. The relevance of the KTHNY theory in locating melting transition in 2D OCP is briefly discussed.

physics.plasm-ph↗