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E Lomba

Publications and source records attributed to E Lomba.

18 recordsLinked to original sources
PubMed2006

Phase behavior of a hard sphere Maier-Saupe nematogenic system in three dimensions.

We present a detailed computer simulation and integral equation study of the phase behavior of a nematogenic system composed of hard spheres with embedded three-dimensional Maier-Saupe spins. For this well-known system, we map the gas-liquid equilibrium, which is coupled to a first-order isotropic-nematic transition. The anisotropic integral equation theory is found to yield excellent agreement with the simulation data within the fluid regime. Additionally, we determine the fluid-solid equilibrium transition by means of computer simulation.

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PubMed2005

Simulation study of the phase behavior of a planar Maier-Saupe nematogenic liquid.

Using extensive Monte Carlo simulations and a simple approximation in density functional theory, we study the phase behavior of a fluid of nematogenic molecules with centers of mass constrained to lie in a plane but with axes free to rotate in any direction, both with and without an external disorienting field perpendicular to the plane. We find that simulation predicts the existence of an order-disorder phase transition belonging to the Berezinskii-Kosterlitz-Thouless type, along with a low temperature gas-liquid transition. In contrast to the simulation results, density functional theory predicts a first-order orientational phase transition coupled continuously with a first-order gas-liquid transition. The approximate theoretical approach qualitatively reproduces the field dependence of the order-disorder and gas-liquid transitions but is far from quantitative.

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PubMed2004

Determination of effective pair interactions from the structure factor.

In this work we present an efficient procedure to evaluate effective pair potentials, compatible with "experimental" structure factors, using a Monte Carlo simulation scheme. The procedure does not require the use of inverse Fourier transforms and is robust and rapidly convergent. As a test case the structure factor of liquid Selenium obtained from a Tight-Binding Molecular Dynamics simulation is inverted to obtain an effective pair potential and, as a by-product, the pair distribution function. The inversion procedure yields a pair structure in perfect agreement with the original molecular dynamics calculations and the analysis of the triplet structure and the dynamics also illustrates the limitations of the use of pair potentials in the description of liquids with strongly directional bonding, such as the covalent liquid Selenium.

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PubMed2003

Determination of the interaction potential from the pair distribution function: an inverse Monte Carlo technique.

In this work we present an efficient procedure to evaluate effective pair potentials compatible with "experimental" distribution functions using a Monte Carlo simulation scheme. Using computer simulation results for the pair distribution functions, we have applied the method to a Lennard-Jones fluid and to a model of liquid aluminum. In both cases the procedure was able to recover with high accuracy the actual interaction potential of the systems. Moreover, the procedure can easily incorporate additional information, for instance, thermodynamic properties, in order to improve the reliability of the results.

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PubMed2003

Local density approach for modeling fluids with density-dependent interactions.

In a recent paper [Phys. Rev. Lett. 86, 2038 (2001)] a simple fluid with a particular density-dependent pair potential was shown to exhibit, together with the vapor-liquid transition, a liquid-liquid phase separation and it was evidenced that, in order to adequately define the correct boundaries of stability, a simulation procedure based on the use of local densities had to be devised. It was found that for certain thermodynamic states the potential drives the system toward a phase separation that is otherwise frustrated by the change in the interactions induced by density fluctuations. Therefore, when integral equations or global density simulations are used, the critical points estimated from the thermodynamics are not associated with divergent correlations and vice versa. Here, we will explore in depth this fluid and introduce a detailed account of the proposed local density simulation technique. The results presented bear general significance for density-dependent potentials, like those of liquid metals or charge-stabilized colloids.

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PubMed2001

Adsorption of a diatomic molecular fluid into random porous media.

Structural and thermodynamic properties of a homonuclear hard dumbbell fluid adsorbed into a disordered hard sphere matrix are studied by means of integral equation techniques and computer simulation. In particular, we have rewritten the replica Ornstein-Zernike equations to deal with orientational degrees of freedom and we have solved them in two different approaches: the hypernetted chain equation and a semiempirical extension of Verlet's approximation. We have also derived direct expressions to calculate the chemical potential in these approximations. Comparison with grand canonical Monte Carlo results shows that both theoretical treatments describe adequately the physical behavior of the system, Verlet's approach being, however, clearly superior in accordance with previous findings for equilibrated hard core mixtures.

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PubMed2001

Evidence of double criticality in a fluid model with density-dependent interactions.

Evidence of a liquid-liquid equilibrium in simple fluids has recently been exposed for a density-dependent pair potential in the framework of a van der Waals theory. Here this double criticality is investigated by means of computer simulation, a perturbation theory, and integral equation theory. It is found that the critical point estimated from the integral equation thermodynamics is not associated with divergent correlations. To cope with these features, a special simulation procedure, based on the definition of local densities, is devised. Monte Carlo calculations confirm the existence of two critical points, in agreement with the predictions of perturbation theory.

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PubMed2000

Inhomogeneous integral equation approach to pair and triple correlations in a glass-forming simple liquid.

A simple model of glass-forming liquid modeled via Dzugutov's pair potential is studied by means of the triplet hypernetted chain approximation as formulated by Attard [P. Attard, J. Chem. Phys. 93, 7301 (1990)]. This system, which is known to be mostly dominated by microscopic icosahedral ordering, eludes a correct description in terms of classical two-body integral equation theories. However, it is found here that the simple hypernetted chain approximation when applied at the three-particle level can yield a correct semiquantitative description of both the pair and triplet structure of the supercooled state, capturing features which escape more sophisticated closures implemented on the two-particle level.

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PubMed2000

Structure and thermodynamics of a ferrofluid monolayer

We model a disordered planar monolayer of paramagnetic spherical particles, or ferrofluid, as a two-dimensional fluid of hard spheres with embedded three-dimensional magnetic point dipoles. This model, in which the orientational degrees of freedom are three dimensional while particle positions are confined to a plane, can be taken as a crude representation of a colloidal suspension of superparamagnetic particles confined in a water/air interface, a system that has recently been studied experimentally. In this paper, we propose an Ornstein-Zernike integral equation approach capable of describing the structure of this highly inhomogeneous fluid, including the effects of an external magnetic field. The method hinges on the use of specially tailored orthogonal polynomials whose weight function is precisely the one-particle distribution function that describes the surface- and field-induced anisotropy. The results obtained for various particle densities and external fields are compared with Monte Carlo simulations, illustrating the capability of the inhomogeneous Ornstein-Zernike equation and the proposed solution scheme to yield a detailed and accurate description of the spatial and orientational structure for this class of systems. For comparison, results from density-functional theory in the modified mean-field approximation are also presented; this latter approach turns out to yield at least qualitatively correct results.

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PubMed1991

[Brucellosis with nervous system involvement].

Cases of brucellosis with involvement of the nervous system which was diagnosed in the Miguel Servet Hospital during the period 1985-1987 are retrospectively studied. The total quantity of affected patients of brucellosis was 132. Of this quantity, 9 patients (6.8%) had neurological complications under the following clinical forms: epidural abscess (2), meningoencephalitis (1), meningitis (2), encephalitis (1), myelitis (1) and polyradiculitis (2). The most important epidemiological and clinical characteristics are analysed, pointing out the diagnostic difficulties we found when the neurological manifestations are predominant in the brucella infection.

Adolescent↗