Shear Viscosity of Liquid Potassium and Cesium: a Simulation Study

The density and temperature dependences of the shear viscosity of liquid potassium and cesium are studied. The stress autocorrelation function is calculated from equilibrium molecular dynamics simulations. Using the Green-Kubo formula, the shear viscosity is obtained. Interionic interactions are calculated by Fiolhais potential and are validated by comparison between simulation and experimental data along the liquid-gas coexistence curve for K and Cs. For both metals, three isochors and one isotherm are investigated. The recently proposed relation in [Phys. Rev. B 93, 214203 (2016)] is tested in the cases of K and Cs and it appears that this function reproduces qualitatively and quantitatively well the behavior of each element

Détermination de la structure des métaux liquides : comparaison entre théories analytiques, simulation numérique et expérience pour les alcalins

Physics of the structure of liquid metals boasts about a double diversity. Firstly, numerous potentials exist to describe the interactions between particles. Secondly, a large number of approaches have been proposed to deduce the structure from the effective potential. In this work, we study the structure of liquid alkali metals. It is developed around two ideas, attributing a central role to molecular dynamics results. In a first part, the quality of the potential implemented is discussed through a comparison between experimental and simulation results. We used Shaw's model potential and both the Vashishta-Singwi and the Ichimaru-Utsumi local field corrections. It appears that this ab initio potential describes correctly the structure of each alkali metal, including lithium. Molecular dynamic's results seem quite insensitive to the choice of the dielectric function. This confirms the predominant role played by short range forces in determining the structure. The second idea, this study is built on, is an evaluation of different methods available for the description of the structure. By comparison with molecular dynamics, qualities and defaults of both perturbation schemes (ORPA-WCA, ORPA-JA) and integral equations (SMSA) are discussed. In the cases of Na, K, Rb and Cs, these methods produce results near simulation ones. However, the SMSA equation does not show the characteristic drawbacks of perturbation methods. Lithium is particular since any of these analytical methods achieves in matching, even approximately, simulation results. The reasons are not clearly understood. Screening influences S(o) and we underline that its value depends on the way long range interactions are taken into accountLa physique de la structure des métaux liquides est riche à double titre, d'abord par la diversité des potentiels qui permettent de décrire l'interaction entre les particules, ensuite par celle des méthodes employées pour déduire la structure du potentiel effectif. Au travers du cas des métaux alcalins et tout en attribuant un rôle central aux résultats de dynamique moléculaire (DM), cette étude s'articule autour de deux axes principaux. Le premier discute de la qualité du potentiel mis en oeuvre par comparaison des résultats expérimentaux et de DM. Nous utilisons le potentiel de Shaw et les fonctions diélectriques de Vashishta-Singwi et d'Ichimaru-Utsumi. L'étude révèle que ce potentiel de type premier principe décrit correctement la structure de tous les alcalins, lithium y compris. L'insensibilité des résultats de DM par rapport à la fonction diélectrique confirme l'influence prédominante des forces à courte portée sur la structure. Le second axe de l'étude est une évaluation des propriétés structurales par différentes méthodes semi-analytiques. Par comparaison avec les résultats de DM, les aptitudes des schémas perturbatifs (ORPA-WCA, ORPA-JA et des méthodes intégrales (SMSA) sont examinées. Sur Na,K, Rb et Cs, toutes ces méthodes approchent honorablement les résultats de simulation. L'équation SMSA ne possède cependant pas les défauts inhérents aux méthodes de perturbation thermodynamiques. Le cas du lithium, qu'aucune de ces méthodes analytiques ne décrit correctement avec le potentiel employé, n'est pas élucidé. L'influence de l'écrantage sur la valeur de S(o) est confirmée et il apparait que la nature du traitement des interactions à longue distance détermine sa valeu

Analyzing the dynamic structure of liquid metals and alloys

Experimental and numerical improvements have stimulated a great interest in the dynamic structure of liquids during the last decades. Many unexpected features have been unveiled among which fast sound, positive dispersion and possible coupling between transverse and longitudinal excitations can be mentioned. Models used to analyze these data have to be sound and more and more rigorous. In this study, we discuss the capability of a recently proposed fitting scheme (Wax J.-F. and Bryk T. J. Phys.: Condens. Matter 25 325104 (2013); 26 168002 (2014). Wax J.-F., Johnson M.R., and Bryk T. J. Phys.: Condens. Matter 28 185102 (2016).) to interpret these features of the dynamic structure of liquid metals and alloys

Analyzing the dynamic structure of liquid metals and alloys

Experimental and numerical improvements have stimulated a great interest in the dynamic structure of liquids during the last decades. Many unexpected features have been unveiled among which fast sound, positive dispersion and possible coupling between transverse and longitudinal excitations can be mentioned. Models used to analyze these data have to be sound and more and more rigorous. In this study, we discuss the capability of a recently proposed fitting scheme (Wax J.-F. and Bryk T. J. Phys.: Condens. Matter 25 325104 (2013); 26 168002 (2014). Wax J.-F., Johnson M.R., and Bryk T. J. Phys.: Condens. Matter 28 185102 (2016).) to interpret these features of the dynamic structure of liquid metals and alloys

Contribution à l'étude des propriétés dynamiques des métaux liquides simples par simulation numérique et modèles analytiques

METZ-SCD (574632105) / SudocSudocFranceF

Shear Viscosity of Liquid Potassium and Cesium: a Simulation Study

The density and temperature dependences of the shear viscosity of liquid potassium and cesium are studied. The stress autocorrelation function is calculated from equilibrium molecular dynamics simulations. Using the Green-Kubo formula, the shear viscosity is obtained. Interionic interactions are calculated by Fiolhais potential and are validated by comparison between simulation and experimental data along the liquid-gas coexistence curve for K and Cs. For both metals, three isochors and one isotherm are investigated. The recently proposed relation in [Phys. Rev. B 93, 214203 (2016)] is tested in the cases of K and Cs and it appears that this function reproduces qualitatively and quantitatively well the behavior of each element

Universality of the shear viscosity of alkali metals

International audienceThe universality of the shear viscosity of alkali metals is studied up to high pressure. Equilibrium moleculardynamics simulations are used to calculate the stress autocorrelation function, which allows us to obtain the valueof shear viscosity using the Green-Kubo formula. Atomic interactions are computed from Fiolhais pseudopotentialand are validated by comparison between pair distribution functions and mean-squared displacements obtainedfrom classical andab initiomolecular dynamics simulations. The description of the interactions is accurate atleast up to 12 GPa, 9.4 GPa, 6.6 GPa, and 3 GPa for Na, K, Rb, and Cs, respectively, and to a lesser extent upto 4.8 GPa for Li. A good agreement between simulation and experimental viscosity results along the liquid-gascoexistence curve is found. The viscosity appears to be a universal property over a wide range of the liquid phaseof the phase diagram, between 0.85 and 1.5 times the ambient melting density and up to seven times the ambientmelting temperature. Scaling laws are proposed following relations formulated in [Meyer, Xu, and Wax,Phys.Rev. B93,214203(2016)] so that it is possible to predict the viscosity value of any alkali metal with an accuracybetter than 10% over the corresponding density and temperature range

Viscosity of Lennard-Jones mixtures: A systematic study and empirical law

International audienceA systematic study of the viscosity of the binary Lennard-Jones (LJ) mixtures is carried out by equilibrium molecular dynamics simulations via the Green-Kubo relation. The effects of mass, size, and energy-parameter asymmetries on the viscosity and the self-diffusion coefficients are examined separately, both in equimolar mixtures and by varying the molar fractions. The systems are mapped into an effective one-component model according to the van der Waals one-fluid (vdW1) model. Furthermore, using an empirical law for pure LJ liquids, similar to the one proposed recently for liquid sodium, it is shown that the viscosity of the mixtures studied here are well-predicted by the combination of vdW1 fluid and empirical law. The Stokes-Einstein relation in the mixtures has also been investigated. A possible simple extension of this relation, from pure liquids to mixtures, has been proposed and tested

Shear Viscosity of Liquid Potassium and Cesium: a Simulation Study

The density and temperature dependences of the shear viscosity of liquid potassium and cesium are studied. The stress autocorrelation function is calculated from equilibrium molecular dynamics simulations. Using the Green-Kubo formula, the shear viscosity is obtained. Interionic interactions are calculated by Fiolhais potential and are validated by comparison between simulation and experimental data along the liquid-gas coexistence curve for K and Cs. For both metals, three isochors and one isotherm are investigated. The recently proposed relation in [Phys. Rev. B 93, 214203 (2016)] is tested in the cases of K and Cs and it appears that this function reproduces qualitatively and quantitatively well the behavior of each element

Pressure-induced effects in the spectra of collective excitations in pure liquid metals

International audienceCollective dynamics of metallic melts at high pressures is one of the open issues of condensed matter physics. By means of ab initio molecular dynamics simulations, we examine features of dispersions of collective excitations through transverse current spectral functions, as a function of pressure. Typical metallic melts, such as Li and Na monovalent metals as well as Al, Pb and In polyvalent metals are considered. We firmly establish the emergence of a second branch of high-frequency transverse modes with pressure in these metals, that we associate with the pronounced high-frequency shoulder in the vibrational density of states. Similar correlation also exist with the low frequency modes. The origin of the pressure-induced evolution of transverse excitations in liquid metals is discussed