Vibrational Modes of ⁴He and H₂ Gases Adsorbed on Carbon Nanotubes

We present calculations of the breathing mode phonon frequencies of 4He and H2 physically adsorbed on the outside surface of one or more carbon nanotubes. Two geometries are considered. The first is a single, isolated nanotube, upon which the gas is adsorbed as a commensurate phase. The second is a quasi-one-dimensional groove phase nestled between two nanotubes. While the computed breathing mode frequencies depend on nanotube radius and the adsorbate, in general, they are of the same order of magnitude as those of the bare nanotubes

Simple Model of Capillary Condensation in porous media

We employ a simple model to describe the phase behavior of 4He and Ar in a hypothetical porous material consisting of a regular array of infinitely long, solid, parallel cylinders. We find that high porosity geometries exhibit two transitions: from vapor to film and from film to capillary condensed liquid. At low porosity, the film is replaced by a ``necking'' configuration, and for a range of intermediate porosity there are three transitions: from vapor to film, from film to necking and from necking to a capillary condensed phase.Comment: 14 pages, 7 figure

Gas Adsorption in Novel Environments, Including Effects of Pore Relaxation

Adsorption experiments have been interpreted frequently with simplified model geometries, such as ideally flat surfaces and slit or cylindrical pores. Recent explorations of unusual environments, such as fullerenes and metal-organic-framework materials, have led to a broadened scope of experimental, theoretical and simulation investigations. This paper reviews a number of such studies undertaken by our group. Among the topics receiving emphasis are these: universality of gas uptake in pores, relaxation of a porous absorbent due to gas uptake and the novel phases of gases on a single nanotube, all of which studies have been motivated by recent experiments

Social behavior modeling based on Incremental Discrete Hidden Markov Models

12 pagesInternational audienceModeling multimodal face-to-face interaction is a crucial step in the process of building social robots or users-aware Embodied Conversational Agents (ECA). In this context, we present a novel approach for human behavior analysis and generation based on what we called "Incremental Discrete Hidden Markov Model" (IDHMM). Joint multimodal activities of interlocutors are first modeled by a set of DHMMs that are specific to supposed joint cognitive states of the interlocutors. Respecting a task-specific syntax, the IDHMM is then built from these DHMMs and split into i) a recognition model that will determine the most likely sequence of cognitive states given the multimodal activity of the in- terlocutor, and ii) a generative model that will compute the most likely activity of the speaker given this estimated sequence of cognitive states. Short-Term Viterbi (STV) decoding is used to incrementally recognize and generate behav- ior. The proposed model is applied to parallel speech and gaze data of interact- ing dyads

Dynamic Analysis of Unidirectional Pressure Infiltration of Porous Preforms by Pure Metals

Unidirectional pressure infiltration of porous preforms by molten metals is investigated numerically. A phenomenological model to describe fluid flow and transport phenomena during infiltration of fibrous preforms by a metal is formulated. The model describes the dynamics of the infiltration process, the temperature distribution, and solid fraction distribution. The numerical results are compared against classical asymptotic analyses and experimental results. This comparison shows that end effects may become important and render asymptotic results unreliable for realistic samples. Fiber volume fraction and initial temperature appear as the factors most strongly influencing infiltration. Metal superheating affects not only the length of the two-phase zone but also the solid fraction distribution in the two-phase zone. The effect of constant applied pressure, although significant on the infiltration velocity, is almost negligible on the two-phase zone length and on solid fraction distribution. When the initial preform temperature is below the metal melting point, and constant pressure is applied under adiabatic conditions, the flow ceases when sufficient solidification occurs to obstruct it. A comparison with literature experiments proves the model to be an efficient predictive tool in the analysis of infiltration processes for different preform/melt systems

What are the best indicators to measure sustainable development? Top-down vs bottom-up approach

Las herramientas de mayor aceptación que posibilitan determinar el estado actual del ambiente y monitorear los cambios en la disponibilidad y calidad de recursos son los indicadores de sustentabilidad, los cuales comúnmente se desarrollan bajo dos enfoques metodológicos: el top-down (indicadores «técnicos») y el bottom-up (indicadores «locales» o «participativos»). Estos enfoques tienen fundamentos claramente diferentes, lo cual ha dado origen a numerosos debates sobre la necesidad de contar con indicadores híbridos construidos a partir de la integración de ambas perspectivas. Los avances logrados hasta el momento no son contundentes, registrándose una falta de consenso sobre cómo abordar esta integración de una manera eficaz. El objetivo de este trabajo es ayudar a encontrar el equilibrio entre los enfoques top-down y bottom-up, identificando sus fortalezas y debilidades mediante un análisis crítico de casos de estudio y reportes metodológicos disponibles en la bibliografía. Del análisis realizado se desprende que la principal fortaleza que se le atribuye al enfoque top-down es la posibilidad de exponer tendencias espaciales y temporales (en base a herramientas estadísticas) que podrían no evidenciarse al aplicar un método participativo. Mientras que una de las limitaciones más significativas es que los indicado-res son elegidos por técnicos y/o investigadores y por lo tanto reflejan su percepción del concepto de sustentabilidad, la cual muchas veces pasa por alto aspectos que son prioritarios para la comunidad involucrada, limitando esto su participación. Por el contrario, la principal fortaleza del enfoque bottom-up es la posibilidad de empoderar a la comunidad local mediante su participación activa en los procesos de selección, evaluación y monitoreo de indicadores. No obstante, esta fortaleza a veces es vista como una debilidad. Por ejemplo, algunos trabajos analizados reportan conflictos en cuanto al logro de la participación comunitaria, asociados con la falta de claridad en los potenciales beneficios a corto, mediano y largo plazo. Desde otra perspectiva, otros casos mencionan que la participación de la comunidad local durante el proceso de selección y monitoreo de indicadores se convierte en un obstáculo en el camino hacia la sustentabilidad, cuando continúan vigentes valores y prácticas de producción que han propiciado situaciones actuales insostenibles. Es evidente que in-tegrar los enfoques top-down y bottom-up para el desarrollo y la aplicación de indicadores de sustentabilidad no es tarea sencilla. Para lograrlo será necesario fortalecer el vínculo entre el sector académico y la comunidad local, y fomentar el intercambio de perspectivas y el aprendizaje integrado. Este trabajo constituye los primeros pasos de un proyecto de investigación iniciado recientemente que pretende construir un marco metodológico integrado que capitalice las fortalezas de ambos enfoques, para generar indicadores de uso sustentable de tierras agrícolas aplicables a la provincia de Mendoza. Se espera que al concluir el proyecto se logren indicadores que ayuden a mejorar la comprensión de los problemas ambientales y sociales vinculados con el uso agrícola de la tierra y faciliten el desarrollo local sustentable, percibido como tal por la comunidad involucrada

Capillary condensation in cylindrical nanopores

Using grand canonical Monte Carlo simulations, we have explored the phenomenon of capillary condensation (CC) of Ar at the triple temperature inside infinitely long, cylindrical pores. Pores of radius R= 1 nm, 1.7 nm and 2.5 nm have been investigated, using a gas-surface interaction potential parameterized by the well-depth D of the gas on a planar surface made of the same material as that comprising the porous host. For strongly attractive situations, i.e., large D, one or more (depending on R) Ar layers adsorb successively before liquid fills the pore. For very small values of D, in contrast, negligible adsorption occurs at any pressure P below saturated vapor pressure P0; above saturation, there eventually occurs a threshold value of P at which the coverage jumps from empty to full, nearly discontinuously. Hysteresis is found to occur in the simulation data whenever abrupt CC occurs, i.e. for R>= 1.7 nm, and for small D when R=1nm. Then, the pore-emptying branch of the adsorption isotherm exhibits larger N than the pore-filling branch, as is known from many experiments and simulation studies. The relation between CC and wetting on planar surfaces is discussed in terms of a threshold value of D, which is about one-half of the value found for the wetting threshold on a planar surface. This finding is consistent with a simple thermodynamic model of the wetting transition developed previously.Comment: 13 pages, 7 figure