Threshold criterion for wetting at the triple point

Grand canonical simulations are used to calculate adsorption isotherms of various classical gases on alkali metal and Mg surfaces. Ab initio adsorption potentials and Lennard-Jones gas-gas interactions are used. Depending on the system, the resulting behavior can be nonwetting for all temperatures studied, complete wetting, or (in the intermediate case) exhibit a wetting transition. An unusual variety of wetting transitions at the triple point is found in the case of a specific adsorption potential of intermediate strength. The general threshold for wetting near the triple point is found to be close to that predicted with a heuristic model of Cheng et al. This same conclusion was drawn in a recent experimental and simulation study of Ar on CO_2 by Mistura et al. These results imply that a dimensionless wetting parameter w is useful for predicting whether wetting behavior is present at and above the triple temperature. The nonwetting/wetting crossover value found here is w circa 3.3.Comment: 15 pages, 8 figure

Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations

We present numerical results obtained from Time-Dependent Density Functional calculations of the dynamics of liquid He-4 in different environments characterized by geometrical confinement. The time-dependent density profile and velocity field of He-4 are obtained by means of direct numerical integration of the non-linear Schrodinger equation associated with a phenomenological energy functional which describes accurately both the static and dynamic properties of bulk liquid He-4. Our implementation allows for a general solution in 3-D (i.e. no symmetries are assumed in order to simplify the calculations). We apply our method to study the real-time dynamics of pure and alkali-doped clusters, of a monolayer film on a weakly attractive surface and a nano-droplet spreading on a solid surface.Comment: q 1 tex file + 9 Ps figure

From nonwetting to prewetting: the asymptotic behavior of 4He drops on alkali substrates

We investigate the spreading of 4He droplets on alkali surfaces at zero temperature, within the frame of Finite Range Density Functional theory. The equilibrium configurations of several 4He_N clusters and their asymptotic trend with increasing particle number N, which can be traced to the wetting behavior of the quantum fluid, are examined for nanoscopic droplets. We discuss the size effects, inferring that the asymptotic properties of large droplets correspond to those of the prewetting film

To wet or not to wet: that is the question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H2 molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, water, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential's well-depth D is smaller than, or comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At the wetting temperature, Tw, the transition to wetting occurs, for entropic reasons, when the liquid's surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid- surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the "simple model", which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy

Optical pumping in 3He with a laser

Optical pumping of ground state 3He has several peculiar features because nuclear polarization is first obtained in the metastable 2 3S state and then transferred to the ground state (indirect optical pumping); this introduces non-linearities in the equation of evolution of the populations. Here we present a numerical calculation of the kinetics of optical pumping in 3 He, which allows a detailed study of the influence of the characteristics of the pumping source (intensity, frequency, polarization). These characteristics can be conveniently varied with a laser, and we describe experiments with a 3He gas at room or low temperatures, and compare the results obtained to the numerical predictions.Le pompage optique de l'hélium 3 dans l'état fondamental présente certaines particularités liées au fait que la polarisation nucléaire est d'abord obtenue dans l'état métastable 2 3S et ensuite transférée à l'état fondamental (pompage optique indirect). Ceci introduit des non-linéarités dans les équations d'évolution des populations. Nous présentons ici un modèle numérique de la cinétique du pompage optique de l'hélium 3, qui permet une étude détaillée de l'influence des caractéristiques de la source de pompage (intensité, fréquence, polarisation). On peut jouer d'une façon commode sur ces paramètres à l'aide d'un laser et nous décrivons des expériences faites sur un gaz d'hélium 3 à température ordinaire et à basse température ; leurs résultats sont comparés aux prédictions numériques

OPTICAL PUMPING OF 3He and 3He - 4He MIXTURES WITH A HIGH POWER INFRARED LMA LASER

No abstract availabl

Une équation cinétique pour les gaz quantiques

This article is the second of a series where we propose a theory of the dynamics of quantum gases with a precise treatment of the short range two body correlations. For this purpose, we use a variant of the Wigner transform, the « free transform », which has a useful property in this context : if, before collision, two particles are uncorrelated, the incoming part of the free Wigner transform remains always exactly factorized. Therefore, instead of asssuming a factorization of the two particle distribution, it is less restrictive to make a similar assumption on the free transform. Time symmetry is broken when the correlations of the outgoing part of the free transform are ignored ; this approximation, which is well in the spirit of Boltzmann, amounts to considering successive collisions as independent processes (the correlations present in the outgoing part could play a role if the same particles rapidly collided again, an unlikely event in dilute gases). Therefore, the succession of collisions is not treated exactly, but each collision is, even during interaction. One then obtains a closed set of equations which leads directly to a kinetic equation for the free distribution f. The equation includes retardation and quantum refraction effects, and provides a generalization of the Boltzmann equation. The reconstruction of the two particle distribution function fII from f reintroduces the effects of short range correlations and, more generally, gives the values of all one particle and two particle physical observables. The price to pay for not using the usual one particle distribution is that the relation between the local physical quantities (density of particles, energy density, etc.) becomes more complicated than in the usual theory, and includes terms which are non linear in f . We nevertheless show how this formalism automatically satisfies the local conservation of all hydrodynamic quantities : particle, momentum and energy densities. The pressure tensor includes quadratic terms which correspond to second virial corrections. The temperature of the gas is defined from the kinetic energy of the particles when they are far apart from each other. We finally briefly discuss the connections of this formalism with the classical Enskog theory, or mean field type theories such as the Landau theory (in particular, we note the presence of recoil effects in the « molecular field »).Ce travail sur l'étude de la dynamique hors d'équilibre des gaz quantiques s'appuie sur la définition et l'utilisation de la transformée de Wigner « libre », introduite dans un article précédent. Son but général est de tenir compte de façon précise des effets des collisions binaires, y compris les corrélations entre particules qu'elles introduisent à courtes distances (corrélations qui sont ignorées dans la théorie habituelle de Boltzmann) ; le point de départ est une étude exacte des effets d'une collision sur cette transformée libre, valable même durant l'interaction entre particules. Nous écrivons pour cette transformée libre une équation cinétique qui inclut les effets de retard et de réfraction dans les collisions, et constitue ainsi une généralisation de l'équation de Boltzmann. Nous montrons ensuite comment est satisfaite automatiquement dans ce formalisme la conservation des grandeurs hydrodynamiques : densité de particules, de quantité de mouvement et d'énergie. L'expression du tenseur des pressions fait apparaître des termes dépendant de la densité qui seront à l'origine de la seconde correction du viriel. La température du gaz est définie à partir de l'énergie cinétique des particules lorsqu'elles sont éloignées. On discute enfin les liens de cette théorie, de nature quantique, avec la théorie classique d'Enskog des gaz denses, ou des théories du type champ moyen comme celle de l'équation cinétique de Landau

A kinetic equation for quantum gases (spin and statistics)

We generalize our previous work on the compatibilty of kinetic equations with second virial corrections to the inclusion of spin and particle indistinguishability; the system is supposed to be sufficiently dilute for higher order virial density corrections (interactions and statistics) to be negligible. We show that the general idea of the “free Winger transform” can be extended to this situation; the function which appears in the kinetic equation becomes here a matrix which acts in the space of spin states of the particles. Assuming that the collisions are described by a hamiltonian which does not act on the spins (a very good approximation for nuclear spins), we write explicitly a kinetic equation which is valid for this case. The right hand side of the equation is an 18 dimension integral, as for spinless distinguishable particles, but here it contains an additional term due to statistics, which introduces commutators and anticommutators. We discuss the local conservation laws in this formalism and find, as expected, a total number of 8 conserved quantities for spin 1/2 particles (including three components of the magnetization). When the gas is at equilibrium, we obtain a pressure dependence which is in agreement with known calculations on spin polarized gases. We finally study the gradient expansion of the collision integral, and show that the zero-order (local) part is identical with the 4 terms (including identical spin rotation terms) obtained previously by Lhuillier et al. The first order (non-local) part contains many terms, wich we compare with those obtained by Silin in a context more closely related to the Landau theory

Dynamique des gaz quantiques (particules discernables sans spin)

We apply the results obtained in two preceding articles, in particular a kinetic equation which generalizes the Boltzmann equation, to the study of dilute quantum gases out of equilibrium. Extending the Chapman Enskog method to this equation, and taking into account the specific expressions of the locally conserved quantities in this theory, we calculate the hydrodynamical properties of the gas. To lowest order (Euler hydrodynamics), we obtain results which include second virial corrections to the pressure, sound velocity and specific heat, which are not accessible within the usual frame of the Boltzmann equation. To first order (viscous, or Navier Stokes, hydrodynamics), we get linear density corrections to transport coefficients. We also show that bulk viscosity may exist, being proportional to the square of density. This formalism gives access to the part of the second virial corrections to transport coefficients which correspond to transport by binary collisions, but ignores the modifications of the damping of currents due to three body interactions (although it does provide virial corrections to the damping of non hydrodynamical variables). It is purely quantum and, at equilibrium, allows one to recover the Beth Uhlenbeck expression of the second virial correction to the pressure in terms of the collision phase shifts. It will be extended to spin and statistics in a forthcoming article.Nous appliquons les résultats de deux articles précédents, en particulier une équation cinétique généralisant l'équation de Boltzmann, à l'étude des gaz dilués quantiques hors d'équilibre. Par une simple extension de la méthode de Chapman Enskog appliquée à cette équation cinétique, nous calculons les propriétés hydrodynamiques du gaz. A l'ordre le plus bas (hydrodynamique d'Euler), nous obtenons des résultats pour la pression, la vitesse du son et la capacité calorifique qui, contrairement à ceux de la théorie de Boltzmann, contiennent les secondes corrections du viriel. A l'ordre suivant (hydrodynamique visqueuse, ou de Navier Stokes), nous obtenons les corrections du premier ordre en densité aux coefficients de transport, et montrons qu'il peut exister un terme de seconde viscosité, quadratique en densité. Ce formalisme donne accès aux parties des corrections du viriel des coefficients de transport qui dépendent des collisions à deux corps, mais pas à celles qui sont dues aux collisions ternaires. Il est quantique et permet de réobtenir l'expression de Beth et Uhlenbeck de la seconde correction du viriel en fonction des déphasages

Relaxation nucléaire de 3He ↑ dans un champ magnétique inhomogène

This article gives a theoretical study of the effects of weak magnetic field gradients on the spin relaxation of a dilute atomic gas. The theory is valid for low or moderate values of the field (the relevant correlation time of the gas is then the diffusion time across the container) as well as for high fields (the intercollision time then plays the most important role). Quantum effects due to particle indistinguishability are included. No consequences of the divergence of the atomic mean free path in a polarized gas at zero temperature are found, in contrast to the situation for viscosity and heat conductivity. On the other hand, substantial effects due to « identical spin rotation effects » are predicted.On présente dans cet article une étude théorique de la relaxation de spin d'un gaz atomique dilué, produite par un gradient de champ magnétique de faible intensité. La théorie est valide à la fois en champ faible ou modéré (cas où le temps de corrélation du gaz qui joue un rôle est le temps de diffusion dans son récipient) et en champ fort (c'est alors le temps entre collisions qui joue le rôle essentiel). Elle tient compte des effets quantiques d'indiscernabilité des atomes. Si les effets de divergence du libre parcours moyen dans un gaz polarisé à température nulle ne jouent ici aucun rôle, contrairement à ce qui se produit pour la viscosité par exemple, les effets de « rotation des spins identiques » peuvent être importants