Forma??o de padr?es em meios granulares densos.

Os sistemas granulares s?o conhecidos por apresentarem um comportamento em certos aspectos compar?vel aos observados nos fluidos e alguns outros aos observados em s?lidos como, por exemplo, a forma??o de padr?es e a segrega??o de part?culas. Neste trabalho, realizamos simula??o em larga escala e obtemos padr?es observados experimentalmente na inje??o de gr?os de diferentes tamanhos numa c?lula de Hele-Shaw. Na simula??o utilizamos um modelo de Automata Celular Probabil?stico de G?s de Rede (PLGA) definido numa rede triangular bi-dimensional e observamos que a forma??o dos padr?es est? relacionada tanto com o tamanho dos gr?os injetados, quanto com a probabilidade de movimento desses gr?os. Dos resultados da simula??o, fomos capazes de propor um modelo te?rico simples para o fen?meno, que ? produzido pelas altas tens?es impostas pela fric??o entre os gr?os e entre estes e a c?lula. Como pode ser visto ao longo desse trabalho, com esse modelo te?rico, fomos capazes de predizer a dimens?o fractal dos padr?es formados e mostrar que ela est? de acordo com os modelos de DLA (diffusion-limited aggregation). Recentemente, foi comprovado experimentalmente a dimens?o fractal prevista por nosso modelo, num experimento que se obt?m padr?es formados por esferas de vidros dispostas entre as placas da c?lula de Helle-Shaw.Granular systems are known to present dynamical behavior which can be compared with that of fluids or that observed in solids. Pattern formation and segregation are some of the observable phenomena. In this work, we perform large scale simulations to obtain numerically the major features observed in grain-grain invasion experiment. A probabilistic lattice gas automata (PLGA) was used as model. Defined on a 2-dimensional triangular lattice, we observed that the patterns formed depends on the type of introduced grains, as well as on the probabilities of movement. From the results of these simulations, we were able to propose a theoretical framework for the phenomenon, which is produced by the high stress gradient regime imposed by friction between the grains and/or the plates. By means of such model, we obtain the fractal dimension of the patterns, which approaches the exponent characteristic of DLA (diffusion-limited aggregation). Recently, it was found, in an experiment with glass balls in a Helle-Shaw cells, a fractal dimension similar to that previously predict by our model

Vibrational G peak splitting in laterally functionalized single wall carbon nanotubes : theory and molecular dynamics simulations.

We present a theoretical study of the vibrational spectrum, in the G band region, of laterally hydrogenated single wall carbon nanotubes through molecular dynamics simulations. We find that bilateral hydrogenation e which can be induced by hydrogenation under lateral strain e causes permanent oval deformations on the nanotubes and induces the splitting of vibrational states in the G-band region. We propose that such splitting can be used as a Raman fingerprint for detecting nanotubes that have been permanently modified due to bilateral hydrogenation. In particular, our results may help to clarify the recent findings of Araujo and collaborators [Nano Lett. 12, 4110 (2012)] which have found permanent modifications in the Raman G peaks of locally compressed carbon nanotubes. We have also developed an analytical model for the proposed phenomenon that reproduces the splitting observed in the simulations