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18 pages, 2251 KiB

Open AccessArticle

Isomorph Invariance of Higher-Order Structural Measures in Four Lennard–Jones Systems

by Mahajabin Rahman, Benjamin M. G. D. Carter, Shibu Saw, Ian M. Douglass, Lorenzo Costigliola, Trond S. Ingebrigtsen, Thomas B. Schrøder, Ulf R. Pedersen and Jeppe C. Dyre

Molecules 2021, 26(6), 1746; https://doi.org/10.3390/molecules26061746 - 20 Mar 2021

Cited by 4 | Viewed by 2120

Abstract

In the condensed liquid phase, both single- and multicomponent Lennard–Jones (LJ) systems obey the “hidden-scale-invariance” symmetry to a good approximation. Defining an isomorph as a line of constant excess entropy in the thermodynamic phase diagram, the consequent approximate isomorph invariance of structure and [...] Read more.

In the condensed liquid phase, both single- and multicomponent Lennard–Jones (LJ) systems obey the “hidden-scale-invariance” symmetry to a good approximation. Defining an isomorph as a line of constant excess entropy in the thermodynamic phase diagram, the consequent approximate isomorph invariance of structure and dynamics in appropriate units is well documented. However, although all measures of the structure are predicted to be isomorph invariant, with few exceptions only the radial distribution function (RDF) has been investigated. This paper studies the variation along isomorphs of the nearest-neighbor geometry quantified by the occurrence of Voronoi structures, Frank–Kasper bonds, icosahedral local order, and bond-orientational order. Data are presented for the standard LJ system and for three binary LJ mixtures (Kob–Andersen, Wahnström,

{NiY}_{2}

). We find that, while the nearest-neighbor geometry generally varies significantly throughout the phase diagram, good invariance is observed along the isomorphs. We conclude that higher-order structural correlations are no less isomorph invariant than is the RDF. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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6 pages, 241 KiB

Open AccessArticle

Sound Velocities of Generalized Lennard-Jones (n − 6) Fluids Near Freezing

by Sergey Khrapak

Molecules 2021, 26(6), 1660; https://doi.org/10.3390/molecules26061660 - 16 Mar 2021

Cited by 3 | Viewed by 1496

Abstract

In a recent paper [S. Khrapak, Molecules 25, 3498 (2020)], the longitudinal and transverse sound velocities of a conventional Lennard–Jones system at the liquid–solid coexistence were calculated. It was shown that the sound velocities remain almost invariant along the liquid–solid coexistence boundary [...] Read more.

In a recent paper [S. Khrapak, Molecules 25, 3498 (2020)], the longitudinal and transverse sound velocities of a conventional Lennard–Jones system 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 considerably affect the magnitude of the 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 the repulsive term has only a minor effect on the magnitude of the sound velocities. Nevertheless, these minor trends are identified and discussed. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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23 pages, 9392 KiB

Open AccessArticle

Phase-Transition Thermal Charging of a Channel-Shape Thermal Energy Storage Unit: Taguchi Optimization Approach and Copper Foam Inserts

by Mohammad Ghalambaz, Seyed Abdollah Mansouri Mehryan, Ahmad Hajjar, Obai Younis, Mikhail A. Sheremet, Mohsen Saffari Pour and Christopher Hulme-Smith

Molecules 2021, 26(5), 1235; https://doi.org/10.3390/molecules26051235 - 25 Feb 2021

Cited by 11 | Viewed by 1906

Abstract

Thermal energy storage is a technique that has the potential to contribute to future energy grids to reduce fluctuations in supply from renewable energy sources. The principle of energy storage is to drive an endothermic phase change when excess energy is available and [...] Read more.

Thermal energy storage is a technique that has the potential to contribute to future energy grids to reduce fluctuations in supply from renewable energy sources. The principle of energy storage is to drive an endothermic phase change when excess energy is available and to allow the phase change to reverse and release heat when energy demand exceeds supply. Unwanted charge leakage and low heat transfer rates can limit the effectiveness of the units, but both of these problems can be mitigated by incorporating a metal foam into the design of the storage unit. This study demonstrates the benefits of adding copper foam into a thermal energy storage unit based on capric acid enhanced by copper nanoparticles. The volume fraction of nanoparticles and the location and porosity of the foam were optimized using the Taguchi approach to minimize the charge leakage expected from simulations. Placing the foam layer at the bottom of the unit with the maximum possible height and minimum porosity led to the lowest charge time. The optimum concentration of nanoparticles was found to be 4 vol.%, while the maximu possible concentration was 6 vol.%. The use of an optimized design of the enclosure and the optimum fraction of nanoparticles led to a predicted charging time for the unit that was approximately 58% shorter than that of the worst design. A sensitivity analysis shows that the height of the foam layer and its porosity are the dominant variables, and the location of the porous layer and volume fraction of nanoparticles are of secondary importance. Therefore, a well-designed location and size of a metal foam layer could be used to improve the charging speed of thermal energy storage units significantly. In such designs, the porosity and the placement-location of the foam should be considered more strongly than other factors. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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25 pages, 3349 KiB

Open AccessArticle

Theoretical Estimate of the Glass Transition Line of Yukawa One-Component Plasmas

by Federico Lucco Castello and Panagiotis Tolias

Molecules 2021, 26(3), 669; https://doi.org/10.3390/molecules26030669 - 28 Jan 2021

Cited by 4 | Viewed by 2100

Abstract

The mode coupling theory of supercooled liquids is combined with advanced closures to the integral equation theory of liquids in order to estimate the glass transition line of Yukawa one-component plasmas from the unscreened Coulomb limit up to the strong screening regime. The [...] Read more.

The mode coupling theory of supercooled liquids is combined with advanced closures to the integral equation theory of liquids in order to estimate the glass transition line of Yukawa one-component plasmas from the unscreened Coulomb limit up to the strong screening regime. The present predictions constitute a major improvement over the current literature predictions. The calculations confirm the validity of an existing analytical parameterization of the glass transition line. It is verified that the glass transition line is an approximate isomorphic curve and the value of the corresponding reduced excess entropy is estimated. Capitalizing on the isomorphic nature of the glass transition line, two structural vitrification indicators are identified that allow a rough estimate of the glass transition point only through simple curve metrics of the static properties of supercooled liquids. The vitrification indicators are demonstrated to be quasi-universal by an investigation of hard sphere and inverse power law supercooled liquids. The straightforward extension of the present results to bi-Yukawa systems is also discussed. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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8 pages, 1959 KiB

Open AccessArticle

Dynamics of Active Brownian Particles in Plasma

by Kyaw Arkar, Mikhail M. Vasiliev, Oleg F. Petrov, Evgenii A. Kononov and Fedor M. Trukhachev

Molecules 2021, 26(3), 561; https://doi.org/10.3390/molecules26030561 - 21 Jan 2021

Cited by 27 | Viewed by 3276

Abstract

Experimental data on the active Brownian motion of single particles in the RF (radio-frequency) discharge plasma under the influence of thermophoretic force, induced by laser radiation, depending on the material and type of surface of the particle, are presented. Unlike passive Brownian particles, [...] Read more.

Experimental data on the active Brownian motion of single particles in the RF (radio-frequency) discharge plasma under the influence of thermophoretic force, induced by laser radiation, depending on the material and type of surface of the particle, are presented. Unlike passive Brownian particles, active Brownian particles, also known as micro-swimmers, move directionally. It was shown that different dust particles in gas discharge plasma can convert the energy of a surrounding medium (laser radiation) into the kinetic energy of motion. The movement of the active particle is a superposition of chaotic motion and self-propulsion. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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11 pages, 394 KiB

Open AccessArticle

Possible Mechanisms of String Formation in Complex Plasmas at Elevated Pressures

by Victoria Yaroshenko and Mikhail Pustylnik

Molecules 2021, 26(2), 308; https://doi.org/10.3390/molecules26020308 - 9 Jan 2021

Cited by 15 | Viewed by 2062

Abstract

Possible mechanisms of particle attraction providing formation of the field aligned microparticle strings in complex plasmas at elevated gas pressures are theoretically investigated in the light of the Plasmakristall-4 (PK-4) experiment on board the International Space Station. The particle interaction energy is addressed [...] Read more.

Possible mechanisms of particle attraction providing formation of the field aligned microparticle strings in complex plasmas at elevated gas pressures are theoretically investigated in the light of the Plasmakristall-4 (PK-4) experiment on board the International Space Station. The particle interaction energy is addressed by two different approaches: (i) using the dynamically screened wake potential for small Mach numbers derived by Kompaneets et al., in 2016, and (ii) introducing effect of polarization of the trapped ion cloud by discharge electric fields. Is is found that both approaches yield the particle interaction energy which is independent of the operational discharge mode. In the parameter space of the performed experiments, the first approach can provide onset of the particle attraction and string formation only at gas pressures higher than 40–45 Pa, whilst the mechanism based on the trapped ion effect yields attraction in the experimentally important pressure range 20–40 Pa and may reconcile theory and observations. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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11 pages, 428 KiB

Open AccessArticle

Sound Velocities of Lennard-Jones Systems Near the Liquid-Solid Phase Transition

by Sergey A. Khrapak

Molecules 2020, 25(15), 3498; https://doi.org/10.3390/molecules25153498 - 31 Jul 2020

Cited by 16 | Viewed by 2538

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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8 pages, 3616 KiB

Open AccessArticle

Dynamic Effects of Laser Action on Quasi-Two-Dimensional Dusty Plasma Systems of Charged Particles

by Mikhail M. Vasiliev, Oleg F. Petrov, Anastasiya A. Alekseevskaya, Alexander S. Ivanov and Elena V. Vasilieva

Molecules 2020, 25(15), 3375; https://doi.org/10.3390/molecules25153375 - 25 Jul 2020

Cited by 9 | Viewed by 2058

Abstract

We present the results of an experimental study of the behavior of a colloidal plasma system formed by copper-coated and uncoated polymer particles under the action of laser irradiation. A comparative study of particle velocity distribution profiles depending on the power of the [...] Read more.

We present the results of an experimental study of the behavior of a colloidal plasma system formed by copper-coated and uncoated polymer particles under the action of laser irradiation. A comparative study of particle velocity distribution profiles depending on the power of the pushing laser was conducted. In the case of uncoated melamine-formaldehyde (MF) particles, we observed the well-known action of light pressure, causing shear stress in the colloidal plasma structure and leading to the occurrence of a laminar flow within the affected area. For the copper-coated MF particles, we revealed some additional patterns of behavior for the dust particles, i.e., kinetic temperature growth due to laser radiation absorption by the copper coating, as well as the appearance of chaotic particle motion. We believe that this happens due to the existence of defects in the coating, causing asymmetric heating of the particles, which in turn leads to chaotic deviations of the photophoretic force pushing the particles in different directions. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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7 pages, 266 KiB

Open AccessBrief Report

Prandtl Number in Classical Hard-Sphere and One-Component Plasma Fluids

by Sergey Khrapak and Alexey Khrapak

Molecules 2021, 26(4), 821; https://doi.org/10.3390/molecules26040821 - 5 Feb 2021

Cited by 5 | Viewed by 1974

Abstract

The Prandtl number is evaluated for the three-dimensional hard-sphere and one-component plasma fluids, from the dilute weakly coupled regime up to a dense strongly coupled regime near the fluid-solid phase transition. In both cases, numerical values of order unity are obtained. The Prandtl [...] Read more.

The Prandtl number is evaluated for the three-dimensional hard-sphere and one-component plasma fluids, from the dilute weakly coupled regime up to a dense strongly coupled regime near the fluid-solid phase transition. In both cases, numerical values of order unity are obtained. The Prandtl number increases on approaching the freezing point, where it reaches a quasi-universal value for simple dielectric fluids of about ≃1.7. Relations to two-dimensional fluids are briefly discussed. Full article

(This article belongs to the Special Issue Thermodynamics and Transport Properties of Fluids)

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