PHASE CHANGES

Author: PÉREZ RECHE FRANCISCO JOSÉ.
Year: 2004.
University: BARCELONA.
Place of defense: FACULTAT DE FÍSICA.
Place of preparation: FACULTAT DE FÍSICA DE LA UNIVERSITAT DE BARCELONA.

Summary: In this thesis has studied some aspects of systems that present phase transitions, and in particular, those that transitions involves a dynamic avalanche (discontinuous evolution of some observable during the transition). Such dynamics can be seen in many of phase transitions (Barkhausen effect in magnetic systems or transition martensítica solids) that the effects of thermal fluctuations are not very important. The aspects studied in the thesis can be classified into two major groups: kinetics of structural phase transitions (basically transition martensítia in shape memory alloys) and the effect of disorder in systems that present phase transitions of the first order. As for the study of the structural transitions, it has been analyzed: 1 - The effect of temperature fluctuations. 2 - The effect of inducing transitions repeatedly. 3, - The effect of the rate parameter control of the transition on avalanches are observed. With regard to the study of systems with disorder, it has analyzed in detail the behavior of the Ising model with random fields. In particular, it has studied the evolution of this system assuming that remains a dynamic metaestable (thermal and adiabática) to vary an external field. This has been made and the numerical simulations s have analyzed data obtained with sophisticated techniques of finite-size scaling. This model presents a continuous phase transition induced disorder in the diagram phases metaestable. Our study has led, among other things, identify the order parameter of the transition as well as the geometric properties of the avalanches that are observed to vary the external field.

Author: GALÁN GONZÁLEZ CARLOS ALBERTO.
Year: 2004.
University: EXTREMADURA.
Place of defense: FACULTAD DE CIENCIAS.
Place of preparation: FACULTAD DE CIENCIAS.

Summary: The objective of this thesis is to provide valid methods for obtaining property interfaciales fluids industrial interest from molecular models. Among these properties highlight the surface energy and surface tension, property for which there are several methods available, although not all of them can be implemented successfully. It is divided into six chapters, the first introductory chapter of a very general nature, followed by a chapter where there are more equations of state used to describe the thermodynamic properties of fluids simple (hard disks and spheres, and reference system for WCA Lennard-Jones fluid), comparing the predictions of these equations with molecular simulations of literature and conducting a test of validity to them. These equations provide the properties of phases in equilibrium essential to the knowledge of the thermodynamic behavior of the system, a preliminary step toward understanding the properties interfaciales, the primary objective of this thesis. In Chapter Three describes the interface líquido Lennard-Jones fluid, and discussed relevant previous work done on the topic indicating deficiencies and / or benefits from them. Chapters four and five focus on the calculation of the surface energy and surface tension of Lennard-Jones fluid from different theoretical approaches, using either the radial distribution function in the liquid phase or the radial distribution function at the interface líquido, using different possibilities. After verifying the validity of various theoretical methods and approaches employed, Chapter six focuses on the proposition and implementation of a model for calculating the surface tension of fluid are polar compared to previous results obtained by other empirical models.

Author: Caballero Moraleda José Baldomero.
Year: 2005.
University: ALMERÍA.
Place of defense: Departamento de Física Aplicada.
Place of preparation: Universidad de Almería.

Summary: In this thesis combining simulations and experimetnos to study the phase diagram of colloidal electrolyte, mixtures equimolares of spherical colloidal particles of the same size and same load. In other words, the analogy symmetrical ionic colloidal fluid. The first chapter introduces the concepts and basic knowledge of charts stages in mono systems, which will be useful in the course of this work. Following is the behavior of the fluid phasic ion, which due to the analogy that our model presents expect qualitatively similar results. Finally, the chapter concludes with the presentation of colloidal electrolyte, as well as the theoretical model that we have continued to perform simulations. Specifically, the electrostatic interactions are modeled through a potential cash rate DLVO, or electric potential shielded by the presence of contraiones in the middle. The second chapter examines the equilibrium phase diagram of this system using Monte Carlo simulations. Mainly, he examines the transition phase gas-líquido and its dependence on the extent of interaction (controlled by the concentration saline environment). It is found that the critical temperature evolves in a non-monotonous regarding the degree of shielding of the charges, this is the extent of the interaction. In particular, the critical temperature increases as decreases the extent of the interaction, reaches a maximum and then decreases in a monotone with increasing concentration saline environment. This comportamietno can be explained according to the strong correlations emerging in the region of phase coexistence between particles of different sign. Each particle is surrounded by a layer of locally particles opposite to which it remains a second layer of particles of the same sign that the particle core. Thus, the balance between repulsive interactions and attractive porvoca this evolution of the critical temperature. In addition, this chapter will look different crystalline structures coexisting with the fluid phases. In conclusion, it was found that the phase diagram of colloidal electrolyte is similar, qualitatively, the fluid ion when critical temperature is stable with respect to crystallization. In chapter 3 are located transitions not ergódicas this system regarding the transition phase gas-líquido using Molecular Dynamics simulations. With the results of the phase diagram of colloidal electrolyte is divided into three regions: i) at high temperatures are systems homogeneous ergódicos ii) to moderate temperatures systems are in coexistence phase gas-líquido iii) in the region low temperatures of the samples are no longer ergódicas; here the phase separation is interrupted by the appearance of not ergódicas transitions. In the latter regime were found two types of comportamietnos very different. In the first one, it forms a dense compact phase reaches the thermodynamic equilibrium. This situation is interpreted as a consequence of a glass transition at high densities, it has been found typical features of these transitions in the vicinity of the state identified as cutting between the two transitions. Precisely in this region, the collective dynamic system is reproduced by the coupled mode theory (known as TCM theory). Thus, we assume that the origin of the glass transition is dynamic and we can entnder changing the cut points with the extent of the interaction. The second behavior is not ergódico appears in the region of very low temperatures, where is that the systems are not crystalline solids very low density, ie gels, regardless of the extent of the interaction. The state gel has been characterized dynamically, we found that the dynamics depends on the initial density of the sample once the temperature has been fixed. However, despite differences dynamic gel states have some structural similarities over 8 of the i 72b soterma that have been investigated. At this point presents the theoretical phase diagram has been calculated using different modeling techniques. This will be compared with experimental results in section 4, where two particles are used latex similar sizes and similar charges but opuestaas. To avoid irreversible aggregation, but the least potential interactions associated with the dispersion, colloidal particles are coated with a non-ionic surfactant and studying the behavior of phases of this complex system varying the concentration saline and colloidal. Later, he examines each of the different behaviors found, and finally, the results are analyzed with the phase diagram generally found with the simulations, found enough similarities between theoretical predictions and experiments. Even discusses some of the comportamietnos that found in other systems of macromolecules of opposite charges on the basis of the results obtained in this work. The work ends with Chapter 5, which summarizes the findings more relevant.