Stability of cosmological detonation fronts

The steady state propagation of a phase transition front is classified, according to hydrodynamics, as a deflagration or a detonation, depending on its velocity with respect to the fluid. These propagation modes are further divided into three types, namely, weak, Jouguet, and strong solutions, according to their disturbance of the fluid. However, some of these hydrodynamic modes will not be realized in a phase transition. One particular cause is the presence of instabilities. In this work we study the linear stability of weak detonations, which are generally believed to be stable. After discussing in detail the weak detonation solution, we consider small perturbations of the interface and the fluid configuration. When the balance between the driving and friction forces is taken into account, it turns out that there are actually two different kinds of weak detonations, which behave very differently as functions of the parameters. We show that the branch of stronger weak detonations are unstable, except very close to the Jouguet point, where our approach breaks down.Comment: 34 pages, 11 figures. v2: typos corrected and minor change

Stability of cosmological deflagration fronts

In a cosmological first-order phase transition, bubbles of the stable phase nucleate and expand in the supercooled metastable phase. In many cases, the growth of bubbles reaches a stationary state, with bubble walls propagating as detonations or deflagrations. However, these hydrodynamical solutions may be unstable under corrugation of the interface. Such instability may drastically alter some of the cosmological consequences of the phase transition. Here, we study the hydrodynamical stability of deflagration fronts. We improve upon previous studies by making a more careful and detailed analysis. In particular, we take into account the fact that the equation of motion for the phase interface depends separately on the temperature and fluid velocity on each side of the wall. Fluid variables on each side of the wall are similar for weakly first-order phase transitions, but differ significantly for stronger phase transitions. As a consequence, we find that, for large enough supercooling, any subsonic wall velocity becomes unstable. Moreover, as the velocity approaches the speed of sound, perturbations become unstable on all wavelengths. For smaller supercooling and small wall velocities, our results agree with those of previous works. Essentially, perturbations on large wavelengths are unstable, unless the wall velocity is higher than a critical value. We also find a previously unobserved range of marginally unstable wavelengths. We analyze the dynamical relevance of the instabilities, and we estimate the characteristic time and length scales associated to their growth. We discuss the implications for the electroweak phase transition and its cosmological consequences.Comment: 45 pages, 13 figures. v2: Minor corrections, references added. v3: Typos corrected, minor modifications and references added (version accepted in PRD

Simulación computacional de la interacción con el campo electromagético del tejido adyacente un implante ocular

Se propone una microválvula activa implantable para el tratamiento de glaucoma, fabricada por tecnología MEMS, compuesta por un diafragma y por un circuito para controlarlo y alimentarlo externamente por radio frecuencia. Este circuito consta a su vez de una bobina sintonizada en 13.56 MHz y de un microchip que utiliza la energía de la señal inducida en la bobina para autoalimentarse, y para alimentar el diafragma. Este chip incluye un potenciostato que permite controlar la apertura del actuador. En este trabajo se simuló la interacción del campo electromagnético de RF con el tejido que circunda al implante. Este último fue modelado como la integración de una microbobina y un microchip montados en un sustrato flexible (kaptón), recubiertos de un polímero biocompatible (PDMS). Se analizó el SAR (Specific absorption rate), la densidad de corriente, y el aumento de temperatura inducidos en el tejido cercano al implante.Fil: Schaumburg, Federico. Universidad Nacional de Entre Rios. Facultad de Ingeniería. Departamento de Bioingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Guarnieri, Fabio Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones En Metodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones En Metodos Computacionales; Argentina. Universidad Nacional de Entre Rios. Facultad de Ingeniería. Departamento de Bioingeniería; Argentin

Regulación del sistema educativo y desigualdades de aprendizaje en el Uruguay

En este artículo se muestra cómo determinados aspectos del sistema uruguayo de educación secundaria pública inciden en rendimientos desiguales de los alumnos. Al utilizar la edición 2006 del Programa Internacional de Evaluación de Estudiantes (pisa) (ocde, 2006a) resaltan tres aspectos clave de las instituciones reguladoras de la educación secundaria que contribuyen a reproducir las desigualdades iniciales, inhibiendo el papel igualador que orienta al sistema educativo. En primer lugar, el mecanismo de asignación de docentes produce un doble efecto de alta rotación de profesores jóvenes en establecimientos de enseñanza de contextos socioculturales desfavorables, así como un anquilosamiento de aquellos docentes más experimentados en establecimientos de contextos favorables. En segundo lugar, el sistema de distribución de alumnos basado en el radio escolar reproduce tel proceso de segregación residencial existente. Finalmente, con el sistema centralizado de provisión de materiales educativos y tecnológicos no se logra cubrir las necesidades de los establecimientos

Translating Metainferences Into Formulae: Satisfaction Operators and Sequent Calculi

In this paper, we present a way to translate the metainferences of a mixed metainferential system into formulae of an extended-language system, called its associated σ-system. To do this, the σ-system will contain new operators (one for each standard), called the σ operators, which represent the notions of "belonging to a (given) standard". We first prove, in a model-theoretic way, that these translations preserve (in)validity. That is, that a metainference is valid in the base system if and only if its translation is a tautology of its corresponding σ-system. We then use these results to obtain other key advantages. Most interestingly, we provide a recipe for building unlabeled sequent calculi for σ-systems. We then exemplify this with a σ-system useful for logics of the ST family, and prove soundness and completeness for it, which indirectly gives us a calculus for the metainferences of all those mixed systems. Finally, we respond to some possible objections and show how our σ-framework can shed light on the “obeying” discussion within mixed metainferential context

Education system institutions and educational inequalities in Uruguay

This article shows how certain aspects at the secondary level of Uruguay’s public school system produce inequalities in student achievement. The 2006 edition of the Programme for International Student Assessment (pisa) (oecd, 2006a) points to three key aspects of the institutions that regulate secondary education that play a part in reproducing inequalities of origin, hindering the equalizing role that guides the education system. First, the teacher assignment mechanism has the dual effect of sending a revolving door of young and inexperienced teachers to schools in unfavourable sociocultural contexts as well as concentrating teachers with more experience in schools in favourable contexts. Second, the geography-based system for assigning students to schools reproduces the residential segregation process. Lastly, the centralized system for supplying educational and technological materials is inadequate to the needs of the schools

Bubble wall correlations in cosmological phase transitions

We study statistical relationships between bubble walls in cosmological first-order phase transitions. We consider the conditional and joint probabilities for different points on the walls to remain uncollided at given times. We use these results to discuss space and time correlations of bubble walls and their relevance for the consequences of the transition. In our statistical treatment, the kinematics of bubble nucleation and growth is characterized by the nucleation rate and the wall velocity as functions of time. We obtain general expressions in terms of these two quantities, and we consider several specific examples and applications.Fil: Megevand, Ariel Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; ArgentinaFil: Membiela, Federico Agustin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; Argentin