1,396 research outputs found
Simple model for the phase coexistence and electrical conductivity of alkali fluids
We report the first theoretical model for the alkali fluids which yields a
liquid-vapor phase coexistence with the experimentally observed features and
electrical conductivity estimates which are also in accord with observations.
We have carried out a Monte Carlo simulation for a lattice gas model which
allows an integrated study of the structural, thermodynamic, and electronic
properties of metal-atom fluids. Although such a technique is applicable to
both metallic and nonmetallic fluids, non-additive interactions due to valence
electron delocalization are a crucial feature of the present model.Comment: RevTex, 11 pages, 2 ps figure files appended, submitted to PR
Study of theoretical models for the liquid-vapor and metal-nonmetal transitions of alkali fluids
Theoretical models for the liquid-vapor and metal-nonmetal transitions of
alkali fluids are investigated. Mean-field models are considered first but
shown to be inadequate. An alternate approach is then studied in which each
statistical configuration of the material is treated as inhomogeneous, with the
energy of each ion being determined by its local environment. Nonadditive
interactions, due to valence electron delocalization, are a crucial feature of
the model. This alternate approach is implemented within a lattice-gas
approximation which takes into account the observed mode of expansion in the
materials of interest and which is able to treat the equilibrium density
fluctuations. We have carried out grand canonical Monte Carlo simulations, for
this model, which allow a unified, self-consistent, study of the structural,
thermodynamic, and electronic properties of alkali fluids. Applications to Cs,
Rb, K, and Na yield results in good agreement with observations.Comment: 13 pages, REVTEX, 10 ps figures available by e-mail
Direcció d'organitzacions: Una visió estratègica
Es una introducción al problema de la dirección en las organizaciones actuales, que toma como punto de partida la evolución de los conceptos fundamentales hasta llegar a las definiciones más ajustadas a la nueva visión estratégica.
Con estas premisas se describe un proceso de dirección completo e integrado que nos permite comprender las competencias directivas necesarias para afrontar todos los problemas de este proceso: Tener la información adecuada para tomar decisiones de todo tipo en cada área organizativa; además de ser capaces de ponerlas en práctica y evaluar los resultados para reajustar el proceso con toda esta información.
Así pues, describimos el proceso de dirección estratégica; distinguimos los tipos de decisiones y indicamos los diversos métodos de toma de decisiones, complementado con un apartado sobre las características del sistema de información necesario para “soportar” todo el proceso, en cada momento.El text forma part dels materials docents programats mitjançant l'ajut del Servei de Política Lingüística de la Universitat de València
Las sociedades científicas de estudiantes de odontología: una fuerza motriz para la investigación.
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Diffusion at the liquid-vapor interface
Recently, the intrinsic sampling method has been developed in order to
obtain, from molecular simulations, the intrinsic structure of the liquid-vapor
interface that is presupposed in the classical capillary wave theory. Our
purpose here is to study dynamical processes at the liquid-vapor interface,
since this method allows tracking down and analyzing the movement of surface
molecules, thus providing, with great accuracy, dynamical information on
molecules that are "at" the interface. We present results for the coefficients
for diffusion parallel and perpendicular to the liquid-vapor interface of the
Lennard-Jones fluid, as well as other time and length parameters that
characterize the diffusion process in this system. We also obtain statistics of
permanence and residence time. The generality of our results is tested by
varying the system size and the temperature; for the later case, an existing
model for alkali metals is also considered. Our main conclusion is that, even
if diffusion coefficients can still be computed, the turnover processes, by
which molecules enter and leave the intrinsic surface, are as important as
diffusion. For example, the typical time required for a molecule to traverse a
molecular diameter is very similar to its residence time at the surface.Comment: 25 pages, 7 figures. Submitted to J. Chem. Phy
Lattice density-functional theory of surface melting: the effect of a square-gradient correction
I use the method of classical density-functional theory in the
weighted-density approximation of Tarazona to investigate the phase diagram and
the interface structure of a two-dimensional lattice-gas model with three
phases -- vapour, liquid, and triangular solid. While a straightforward
mean-field treatment of the interparticle attraction is unable to give a stable
liquid phase, the correct phase diagram is obtained when including a suitably
chosen square-gradient term in the system grand potential. Taken this theory
for granted, I further examine the structure of the solid-vapour interface as
the triple point is approached from low temperature. Surprisingly, a novel
phase (rather than the liquid) is found to grow at the interface, exhibiting an
unusually long modulation along the interface normal. The conventional
surface-melting behaviour is recovered only by artificially restricting the
symmetries being available to the density field.Comment: 16 pages, 6 figure
Liquid drops on a surface: using density functional theory to calculate the binding potential and drop profiles and comparing with results from mesoscopic modelling
The contribution to the free energy for a film of liquid of thickness on
a solid surface, due to the interactions between the solid-liquid and
liquid-gas interfaces is given by the binding potential, . The precise
form of determines whether or not the liquid wets the surface. Note that
differentiating gives the Derjaguin or disjoining pressure. We develop a
microscopic density functional theory (DFT) based method for calculating
, allowing us to relate the form of to the nature of the molecular
interactions in the system. We present results based on using a simple lattice
gas model, to demonstrate the procedure. In order to describe the static and
dynamic behaviour of non-uniform liquid films and drops on surfaces, a
mesoscopic free energy based on is often used. We calculate such
equilibrium film height profiles and also directly calculate using DFT the
corresponding density profiles for liquid drops on surfaces. Comparing
quantities such as the contact angle and also the shape of the drops, we find
good agreement between the two methods. We also study in detail the effect on
of truncating the range of the dispersion forces, both those between the
fluid molecules and those between the fluid and wall. We find that truncating
can have a significant effect on and the associated wetting behaviour of
the fluid.Comment: 16 pages, 13 fig
Bending modulus of lipid membranes from density correlation functions.
The bending modulus κ quantifies the elasticity of biological membranes in terms of the free energy cost of increasing the membrane corrugation. Molecular dynamics (MD) simulations provide a powerful approach to quantify κ by analyzing the thermal fluctuations of the lipid bilayer. However, existing methods require the identification and filtering of non-mesoscopic fluctuation modes. State of the art methods rely on identifying a smooth surface to describe the membrane shape. These methods introduce uncertainties in calculating κ since they rely on different criteria to select the relevant fluctuation modes. Here, we present a method to compute κ using molecular simulations. Our approach circumvents the need to define a mesoscopic surface or an orientation field for the lipid tails explicitly. The bending and tilt moduli can be extracted from the analysis of the density correlation function (DCF). The method introduced here builds on the Bedeaux and Weeks (BW) theory for the DCF of fluctuating interfaces and on the coupled undulatory (CU) mode introduced by us in previous work. We test the BW-DCF method by computing the elastic properties of lipid membranes with different system sizes (from 500 to 6000 lipid molecules) and using coarse-grained (for POPC and DPPC lipids) and fully atomistic models (for DPPC). Further, we quantify the impact of cholesterol on the bending modulus of DPPC bilayers. We compare our results with bending moduli obtained with X-ray diffraction data and different computer simulation methods
Density Functional for Anisotropic Fluids
We propose a density functional for anisotropic fluids of hard body
particles. It interpolates between the well-established geometrically based
Rosenfeld functional for hard spheres and the Onsager functional for elongated
rods. We test the new approach by calculating the location of the the
nematic-isotropic transition in systems of hard spherocylinders and hard
ellipsoids. The results are compared with existing simulation data. Our
functional predicts the location of the transition much more accurately than
the Onsager functional, and almost as good as the theory by Parsons and Lee. We
argue that it might be suited to study inhomogeneous systems.Comment: To appear in J. Physics: Condensed Matte
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