arXiv subjects

Sergey Khrapak

Publications and source records attributed to Sergey Khrapak.

At least 19 recordsLinked to original sources

arXiv2026

Direct experimental measurement of ion properties in extreme plasma condition

We have demonstrated Laser Induced Fluorescence (LIF) in a Capacitively Coupled Plasma (CCP) discharge to directly measure the most crucial ion properties at a discharge regime suitable for a broad range of plasma research related to plasma processing and dusty plasma investigations that has been impossible for many years. The ion flow measurements in the presence and absence of dust particles show that ions move much faster directionally than expected from thermal motion, with reductions observed in the presence of dust particles. Ion temperatures are also found to exceed room temperature, contrary to a common assumption in the dusty plasma community. These findings represent a significant advancement in experimental plasma research, providing vital information to refine ion-driven process models with insights that span multiple research fields.

physics.plasm-ph↗

arXiv2026

Vibrational model of entropy in dense two-dimensional fluids

A vibrational paradigm of atomic dynamic in dense fluids is known to provide useful insight on the transport and thermodynamic properties of fluids in three dimensions. In this paper, a vibrational model is generalized to describe the excess entropy of two-dimensional (2D) fluids. A simple practical implementation of this model is demonstrated to deliver accurate results for various systems, such as one-component plasmas with Coulomb and logarithmic interactions, a 2D fluid of dipole particles, and a 2D Yukawa fluid. The applicability limits, relevance to three-dimensional fluids, relations to other 2D phenomena, and potential practical applications are briefly discussed.

cond-mat.soft↗

arXiv2026

Excess entropy scaling of the transverse sound speed in simple fluids

A calculation of the transverse sound velocity as a function of excess entropy is presented for several simple fluids, including the Lennard-Jones, Yukawa, one-component plasma, inverse-power law (soft sphere) and hard sphere models. A quasi-universal character of this dependence is established, extending Rosenfeld's excess-entropy scaling of transport coefficients to the transverse sound velocity. The results are discussed in terms of the soft- to hard-sphere crossover and the Frenkel crossover between gas-like and liquid-like dynamics.

cond-mat.soft↗

arXiv2025

Speed of sound in dense simple liquids

The speed of sound of simple dense fluids is shown to exhibit a pronounced freezing temperature scaling of the form $c_{\rm s}/v_{\rm T}\simeq \sqrtγ +α(T_{\rm fr}/T)^β$, where $c_s$ is the speed of sound, $v_{\rm T}$ is the characteristic thermal velocity, $γ$ is the ideal gas heat capacity ratio, $T$ is the temperature, $T_{\rm fr}$ is the freezing temperature, and $α$ and $β$ are dimensionless parameters. For the Lennard-Jones fluid we get $γ=5/3$, $α\simeq 7$ with a weak temperature dependence, and $β= 1/3$. Similar scaling works in several real liquids, such as argon, krypton, xenon, nitrogen, and methane. In this case, $α$ and $β$ are substance-dependent fitting parameters. A comparison between the prediction of this freezing temperature scaling and a recent experimental measurement of the speed of sound in methane under conditions of planetary interiors is presented and discussed. The results provide a simple practical tool to estimate the speed of sound in regimes where no experimental data are yet available.

cond-mat.soft↗

arXiv2024

Modified Bridgman formula for the thermal conductivity of complex (dusty) plasma fluids

A simple and popular Bridgman's formula predicts a linear correlation between the thermal conductivity coefficient and the sound velocity of dense liquids. Unfortunately, it cannot be applied to strongly coupled plasma-related fluids, because the sound velocity can greatly increase as screening weakens. We propose a modification of the Bridgman formula by correlating the thermal conductivity coefficient with the transverse (shear) sound velocity. This approach is demonstrated to work reasonably well in screened Coulomb (Yukawa) fluids and can be useful in the context of complex (dusty) plasmas.

physics.plasm-ph↗

arXiv2024

Excess entropy of strongly coupled Yukawa fluids

The entropy of strongly coupled Yukawa fluids is discussed from several perspectives. First, it is demonstrated that a vibrational paradigm of atomic dynamics in dense fluids can be used to obtain a simple and accurate estimate of the entropy without any adjustable parameters. Second, it is explained why a quasiuniversal value of the excess entropy of simple fluids at the freezing point should be expected, and it is demonstrated that a remaining very weak dependence of the freezing point entropy on the screening parameter in the Yukawa fluid can be described by a simple linear function. Third, a scaling of the excess entropy with the freezing temperature is examined, a modified form of the Rosenfeld-Tarazona scaling is put forward, and some consequences are briefly discussed. Fourth, the location of the Frenkel line on the phase diagram of Yukawa systems is discussed in terms of the excess entropy and compared with some predictions made in the literature. Fifth, the excess entropy scaling of the transport coefficients (self-diffusion, viscosity, and thermal conductivity) is reexamined using the contemporary datasets for the transport properties of Yukawa fluids. The results could be of particular interest in the context of complex (dusty) plasmas, colloidal suspensions, electrolytes, and other related systems with soft pairwise interactions.

cond-mat.soft↗

arXiv2024

Note: Shoving model and the glass transition in one-component plasma

A modified shoving model is applied to estimate the location of the glass transition in a one-component plasma. The estimated value of the coupling parameter $Γ\simeq 570$ at the glass transition is compared with other predictions available in the literature.

cond-mat.stat-mech↗

arXiv2024

On the system size dependence of the diffusion coefficients in MD simulations: A simple correction formula for pure dense fluids

A practical correction formula relating the self-diffusion coefficient of dense liquids from molecular dynamics simulations with periodic boundary conditions to the self-diffusion coefficient in the thermodynamic limit is discussed. This formula applies to pure dense fluids and has a very simple form $D=D_0(1-γN^{-1/3})$, where $D_0$ is the self-diffusion coefficient in the thermodynamic limit and $N$ is the number of particles in the simulation. The numerical factor $γ$ depends on the geometry of the simulation cell. Remarkably, $γ\simeq 1.0$ for the most popular cubic geometry. The success of this formula is supported by results from MD simulations, including very recent simulations with a ``magic'' simulation geometry.

cond-mat.stat-mech↗

arXiv2024

Elementary vibrational model for transport properties of dense fluids

A vibrational model of transport properties of dense fluids assumes that solid-like oscillations of atoms around their temporary equilibrium positions dominate the dynamical picture. The temporary equilibrium positions of atoms do not form any regular structure and are not fixed, unlike in solids. Instead, they are allowed to diffuse and this is why liquids can flow. However, this diffusive motion is characterized by much longer time scales compared to those of solid-like oscillations. Although this general picture is not particularly new, only in a recent series of works it has been possible to construct a coherent and internally consistent {\it quantitative} description of transport properties such as self-diffusion, shear viscosity, and thermal conductivity. Moreover, the magnitudes of these transport coefficients have been related to the properties of collective excitations in dense fluids. Importantly, the model is simple and no free parameters are involved. Recent achievements are summarized in this overview. Application of the vibrational model to various single-component model systems such as plasma-related Coulomb and screened Coulomb (Yukawa) fluids, the Lennard-Jones fluid, and the hard-sphere fluid is considered in detail. Applications to real liquids are also briefly discussed. Overall, good to excellent agreement with available numerical and experimental data is demonstrated. Conditions of applicability of the vibrational model and a related question concerning the location of the gas-liquid crossover are discussed.

cond-mat.soft↗

arXiv2023

Bridgman formula for the thermal conductivity of atomic and molecular liquids

A simple and popular Bridgman's model predicts a linear correlation between the thermal conductivity coefficient and the sound velocity of dense liquids. A proportionality coefficient proposed originally is fixed and independent of the liquid molecular structure. This work reports a systematic analysis of correlations between thermal conductivity and sound velocity in simple model systems (hard sphere and Lennard-Jones fluids), monoatomic liquids (argon and krypton), diatomic liquids (nitrogen and oxygen), and several polyatomic liquids (water, carbon dioxide, methane, and ethane). It is demonstrated that linear correlations are well reproduced for model fluids as well as real monoatomic and diatomic liquids, but seem less convincing in polyatomic molecular liquids. The coefficient of proportionality is not fixed; it is about unity for monoatomic liquids and generally increases with molecular complexity. Some implications for the possibility to predict the thermal conductivity coefficients are discussed.

cond-mat.soft↗

arXiv2023

Vibrational model of heat transfer in strongly coupled Yukawa fluids (dusty plasma liquids)

A concise overview of the vibrational model of heat transfer in simple fluids with soft pairwise interactions is presented. The model is applied to evaluate the thermal conductivity coefficient of the strongly coupled Yukawa fluid, which often serves as a simplest model of a real liquid-like dusty (complex) plasma. A reasonable agreement with the available data from molecular dynamics numerical simulations is observed. Universality of the properly reduced thermal conductivity coefficient with respect to the effective coupling parameter is examined. Relations between the vibrational model and the excess entropy scaling of the thermal conductivity coefficient are discussed.

physics.plasm-ph↗

arXiv2022

Note: Gas-liquid crossover in the Lennard-Jones system

It is demonstrated that the crossover between gas- and liquid-like regions on the phase diagram of the Lennard-Jones system occurs at a fixed value of the density divided by its value at the freezing point, $ρ/ρ_{\rm fr}\simeq 0.35$. This definition is consistent with other definitions proposed recently. As a result a very simple practical expression for the gas-to-liquid crossover line emerges.

cond-mat.stat-mech↗

arXiv2021

Excess entropy determines the applicability of Stokes-Einstein relation in simple fluids

The Stokes-Einstein (SE) relation between the self-diffusion and shear viscosity coefficients operates in sufficiently dense liquids not too far from the liquid-solid phase transition. By considering four simple model systems with very different pairwise interaction potentials (Lennard-Jones, Coulomb, Debye-Hückel or screened Coulomb, and the hard sphere limit) we identify where exactly on the respective phase diagrams the SE relation holds. It appears that the reduced excess entropy $s_{\rm ex}$ can be used as a suitable indicator of the validity of the SE relation. In all cases considered the onset of SE relation validity occurs at approximately $s_{\rm ex}\lesssim -2$. In addition, we demonstrate that the line separating gas-like and liquid-like fluid behaviours on the phase diagram is roughly characterized by $s_{\rm ex}\simeq -1$.

cond-mat.soft↗

arXiv2021

Note: Sound velocities of generalized Lennard-Jones ($n-6$) fluids near freezing

In a recent paper [S. Khrapak, Molecules {\bf 25}, 3498 (2000)] the longitudinal and transverse sound velocities of conventional Lennard-Jones systems at the liquid-solid coexistence were calculated. It was shown that the sound velocities remain almost invariant along the liquid-solid coexistence boundary lines and that their magnitudes are comparable with those of repulsive soft sphere and hard sphere models at the fluid-solid phase transition. This implies that attraction does not affect the magnitude of sound velocities at the fluid-solid phase transition. This paper provides further evidence to this by examining the generalized Lennard-Jones $n$-6 fluids with $n$ ranging from $12$ to $7$ and demonstrating that the steepness of repulsive term has only a minor effect on the magnitude of the sound velocities.

cond-mat.soft↗

arXiv2021

Thermal conductivity of strongly coupled Yukawa fluids

A vibrational model of heat conduction in liquids with soft pairwise interactions is applied to estimate the thermal conductivity coefficient of strongly coupled Yukawa fluids. A reasonable agreement with the available data from numerical simulations is observed. The results can be useful in the context of strongly coupled plasma and complex (dusty) plasma fluids, when Yukawa (or screened Coulomb) interaction potential is applicable.

physics.plasm-ph↗

arXiv2021

Thermal conduction in two-dimensional complex plasma layers

A simple vibrational model of heat transfer in two-dimensional (2D) fluids relates the heat conductivity coefficient to the longitudinal and transverse sound velocities, specific heat, and the mean interatomic separation. This model is demonstrated not to contradict the available experimental and numerical data on heat transfer in 2D complex plasma layers. Additionally, the heat conductivity coefficient of a 2D one-component plasma with a logarithmic interaction is evaluated.

physics.plasm-ph↗

arXiv2020

Sound velocities of Lennard-Jones systems near the liquid-solid phase transition

Longitudinal and transverse sound velocities of Lennard-Jones systems are calculated at the liquid-solid coexistence using the additivity principle. The results are shown to agree well with the ``exact'' values obtained from their relations to excess energy and pressure. Some consequences, in particular, in the context of the Lindemann's melting rule and Stokes-Einstein relation between the self-diffusion and viscosity coefficients are discussed. Comparison with available experimental data on the sound velocities of solid argon at melting conditions is provided.

cond-mat.soft↗

arXiv2020

Ion drift instability in a strongly coupled collisional complex plasma

We investigate the low-frequency wave mode associated with heavy particles and its instability in a collisional complex plasma with drifting ions. The effect of the ion drift on the sound velocity of this mode is discussed. The general condition of the instability is derived for subthermal ion drifts, taking into account strong coupling of the particle component. As a general tendency, strong coupling effects reduce the sound velocity and facilitate the occurrence of the ion drift instability. A wide parameter range is considered from the weakly collisional to strongly collisional regimes for the ion and particle components. The chosen plasma parameters are representative to the PK-4 experiment, currently operational on board the International Space Station.

physics.plasm-ph↗