Dislocation screening in crystals with spherical topology

Whereas disclination defects are energetically prohibitive in two-dimensional flat crystals, their existence is necessary in crystals with spherical topology, such as viral capsids, colloidosomes or fullerenes. Such a geometrical frustration gives rise to large elastic stresses, which render the crystal unstable when its size is significantly larger than the typical lattice spacing. Depending on the compliance of the crystal with respect to stretching and bending deformations, these stresses are alleviated by either a local increase of the intrinsic curvature in proximity of the disclinations or by the proliferation of excess dislocations, often organized in the form of one-dimensional chains known as "scars". The associated strain field of the scars is such to counterbalance the one resulting from the isolated disclinations. Here, we develop a continuum theory of dislocation screening in two-dimensional closed crystals with genus one. Upon modeling the flux of scars emanating from a given disclination as an independent scalar field, we demonstrate that the elastic energy of closed two-dimensional crystals with various degrees of asphericity can be expressed as a simple quadratic function of the screened topological charge of the disclinations, both at zero and finite temperature. This allows us to predict the optimal density of the excess dislocations as well as the minimal stretching energy attained by the crystal

Faceting and flattening of emulsion droplets

When cooled down, emulsion droplets stabilized by a frozen interface of alkane molecules and surfactants have been observed to undergo a spectacular sequence of morphological transformations: from spheres to faceted icosahedra, down to flattened platelet-like prisms. While generally ascribed to the interplay between the elasticity of the frozen interface and surface tension, the physical mechanisms underpinning these transitions have remained elusive, despite different theoretical pictures having been proposed in recent years. In this article, we introduce a comprehensive mechanical model of morphing emulsion droplets, able to quantitatively account for various experimental observations, including the scaling behavior of the faceting transition. Our analysis highlights the role of gravity and the spontaneous curvature of the frozen interface in determining the specific transition pathway

Dynamical behavior of martensite-austenite transitions: simulations and experiments

The dynamical behavior of the reverse martensitic transformation has been numerically simulated with an atomistic model and compared with experiments in Cu-Zn-Al alloys. Starting from different configurations of the martensitic variants (varying mainly their mean size), the transformation to austenite was studied as a function of the heating speed. Both, experimental and numerical results show that at low velocities there is no dependence of the transition temperatures, whereas at higher speeds they gradually increase. Simulations allow us to have an insight of the underlying processes during the transition to austenite. They also show that a heating speed independent transition can only be obtained when a microstructure of very small variants is present.Fil: Laguna, Maria Fabiana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); ArgentinaFil: García Aguilar, Ireth. Universidad de Costa Rica; Costa RicaFil: Arneodo Larochette, Pierre Paul. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Pelegrina, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentin

Becas de Verano 2012 del Instituto Balseiro – Informes

El programa de Becas de Verano del Instituto Balseiro ofrece a estudiantes universitarios avanzados o recientemente egresados de carreras de grado en Ciencias o Ingenierías la posibilidad de realizar una pasantía durante el mes de febrero. El objetivo de la misma es familiarizarse con técnicas experimentales y colaborar en tareas de investigación en laboratorios del Centro Atómico Bariloche. Anualmente, 15 estudiantes son seleccionados y reciben ayuda económica completa. Cada participante elige un único tema de investigación entre una serie de propuestas y es guiado por un grupo de investigadores del Centro Atómico Bariloche. La pasantía finaliza con la entrega de un informe y la presentación de un póster en el que se detallan los resultados obtenidos a lo largo del mes de trabajo