Exact results and mean field approximation for a model of molecular aggregation

We present a simple one-dimensional model with molecular interactions favouring the formation of clusters with a defined optimal size. Increasing the density, at low temperature, the system goes from a nearly-ideal gas of independent molecules to a system with most of the molecules in optimal clusters, in a way that resembles the formation of micelles in a dilution of amphiphilic molecules, at the critical micellar concentration. Our model is simple enough to have an exact solution, but it contains some basic features of more realistic descriptions of amphiphilic systems: molecular excluded volume and molecular attractions which are saturated at the optimal cluster. The comparison between the exact results and the mean field density functional approximation suggests new approaches to study the more complex and realistic models of micelle formation; in particular it addresses the long-standing controversy surrounding separation of internal degrees of freedom in the formulation of cluster association phenomena.Comment: 7 pages, 5 figures, some minor correction

Dynamic Density Functional theory for steady currents: Application to colloidal particles in narrow channels

We present the theoretical analysis of the steady state currents and density distributions of particles moving with Langevin dynamics, under the effects of an external potential displaced at constant rate. The Dynamic Density Functional (DDF) formalism is used to introduce the effects of the molecular interactions, from the equilibrium Helmholtz free energy density functional. We analyzed the generic form of the DDF for one-dimensional external potentials and the limits of strong and weak potential barriers. The ideal gas case is solved in a closed form for generic potentials and compared with the numerical results for hard-rods, with the exact equilibrium free energy. The results may be of relevance for microfluidic devices, with colloidal particles moving along narrow channels, if external driving forces have to compete with the brownian fluctuations and the interaction forces of the particles

Crystallization of hard spheres under gravity

We present a simple argument to account for crystallization of hard spheres under the action of a gravitational field. The paper attempts to bridge the gap between two communities of scientists, one working on granular materials and the other on inhomogeneous liquid state theory.Comment: Physica A (in press

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

A critical view to current economic topics through an Online-Forum activity

[EN] In order to address the critical thinking and the knowledge of contemporary problems an online forum activity is proposed in this paper. The activity was applied within the World Economy subject in the degree of Business Administration.  The main aim of this teaching proposal is to discuss some current issues and events in the international economic context which likely are not addressed in the syllabus. Through an online forum both teacher and pupils posted comments about some relevant topics related with the subject content. Students have the opportunity to express their own opinion and provide additional information and resources such as news links, documentaries, Youtube videos and other kind of audio-visual material. Despite of this activity was non mandatory, the response was positive, high proportion of pupils (67.1%) were involved in it and 877 comments were published. Thus, the result was satisfactory but there was also some aspects to improve. In addition, this activity is suitable for other subjects or even other university degrees as well and can be easily adapted and modificated.Acknowledges to the project EstadísTIC@ i MatemàTIC@, Servei de Formació Permanent i Innovacio Educativa (SFPIE) de la Universitat de València.http://ocs.editorial.upv.es/index.php/HEAD/HEAD18Caballer Tarazona, V.; Caballer Tarazona, M. (2018). A critical view to current economic topics through an Online-Forum activity. Editorial Universitat Politècnica de València. 221-227. https://doi.org/10.4995/HEAD18.2018.7953OCS22122

Deconstructing temperature gradients across fluid interfaces: the structural origin of the thermal resistance of liquid-vapor interfaces

The interfacial thermal resistance determines condensation-evaporation processes and thermal transport across material-fluid interfaces. Despite its importance in transport processes, the interfacial structure responsible for the thermal resistance is still unknown. By combining non-equilibrium molecular dynamics simulations and interfacial analyses that remove the interfacial thermal fluctuations we show that the thermal resistance of liquid-vapor interfaces is connected to a low density fluid layer that is adsorbed at the liquid surface. This thermal resistance layer (TRL) defines the boundary where the thermal transport mechanism changes from that of gases (ballistic) to that characteristic of dense liquids, dominated by frequent particle collisions involving very short mean free paths. We show that the thermal conductance is proportional to the number of atoms adsorbed in the TRL, and hence we explain the structural origin of the thermal resistance in liquid-vapor interfaces.Comment: 4 pages, 4 figures, and supplementary informatio