Probing the concept of line tension down to the nanoscale

A novel mechanical approach is developed to explore by means of atom-scale simulation the concept of line tension at a solid-liquid-vapor contact line as well as its dependence on temperature, confinement, and solid/fluid interactions. More precisely, by estimating the stresses exerted along and normal to a straight contact line formed within a partially wet pore, the line tension can be estimated while avoiding the pitfalls inherent to the geometrical scaling methodology based on hemispherical drops. The line tension for Lennard-Jones fluids is found to follow a generic behavior with temperature and chemical potential effects that are all included in a simple contact angle parameterization. Former discrepancies between theoretical modeling and molecular simulation are resolved, and the line tension concept is shown to be robust down to molecular confinements. The same qualitative behavior is observed for water but the line tension at the wetting transition diverges or converges towards a finite value depending on the range of the solid/fluid interactions at play.Comment: 8 pages, 7 figure

Activated desorption at heterogeneous interfaces and long-time kinetics of hydrocarbon recovery from nanoporous media

Hydrocarbon recovery from unconventional reservoirs (shale gas) is debated due to its environmental impact and uncertainties on its predictability. But a lack of scientific knowledge impedes the proposal of reliable alternatives. The requirement of hydrofracking, fast recovery decay and ultra-low permeability—inherent to their nanoporosity—are specificities of these reservoirs, which challenge existing frameworks. Here we use molecular simulation and statistical models to show that recovery is hampered by interfacial effects at the wet kerogen surface. Recovery is shown to be thermally activated with an energy barrier modelled from the interface wetting properties. We build a statistical model of the recovery kinetics with a two-regime decline that is consistent with published data: a short time decay, consistent with Darcy description, followed by a fast algebraic decay resulting from increasingly unreachable energy barriers. Replacing water by CO₂ or propane eliminates the barriers, therefore raising hopes for clean/efficient recovery

Bottom-up model of adsorption and transport in multiscale porous media

We develop a model of transport in multiscale porous media which accounts for adsorption in the different porosity scales. This model employs statistical mechanics to upscale molecular simulation and describe adsorption and transport at larger time and length scales. Using atom-scale simulations, which capture the changes in adsorption and transport with temperature, pressure, pore size, etc., this approach does not assume any adsorption or flow type. Moreover, by relating the local chemical potential μ(r) and density ρ(r), the present model accounts for adsorption effects and possible changes in the confined fluid state upon transport. This model constitutes a bottom-up framework of adsorption and transport in multiscale materials as it (1) describes the adsorption-transport interplay, (2) accounts for the hydrodynamics breakdown at the nm scale, and (3) is multiscale.France. Investissements d'avenir (ICoME2/ANR-11-LABX-0053)France. Investissements d'avenir (A*NUDEX/ANR-11-IDEX-0001-02)Schlumberger FoundationShell Oil Compan

Adsorption in non interconnected pores open at one or at both ends: A reconsideration of the origin of the hysteresis phenomenon

We report on an experimental study of adsorption isotherme of nitrogen onto porous silicon with non interconnected pores open at one or at both ends in order to check for the first time the old (1938) but always current idea based on Cohan's description which suggests that the adsorption of gaz should occur reversibly in the first case and irreversibly in the second one. Hysteresis loops, the shape of which is usually associated to interconnections in porous media, are observed whether the pores are open at one or at both ends in contradiction with Cohan's model.Comment: 5 pages, 4 EPS figure

Influence of Elastic Strains on the Adsorption Process in Porous Materials. An Experimental Approach

The experimental results presented in this paper show the influence of the elastic deformation of porous solids on the adsorption process. With p+-type porous silicon formed on highly boron doped (100) Si single crystal, we can make identical porous layers, either supported by or detached from the substrate. The pores are perpendicular to the substrate. The adsorption isotherms corresponding to these two layers are distinct. In the region preceding capillary condensation, the adsorbed amount is lower for the membrane than for the supported layer and the hysteresis loop is observed at higher pressure. We attribute this phenomenon to different elastic strains undergone by the two layers during the adsorption process. For the supported layer, the planes perpendicular to the substrate are constrained to have the same interatomic spacing as that of the substrate so that the elastic deformation is unilateral, at an atomic scale, and along the pore axis. When the substrate is removed, tridimensional deformations occur and the porous system can find a new configuration for the solid atoms which decreases the free energy of the system adsorbate-solid. This results in a decrease of the adsorbed amount and in an increase of the condensation pressure. The isotherms for the supported porous layers shift toward that of the membrane when the layer thickness is increased from 30 to 100 microns. This is due to the relaxation of the stress exerted by the substrate as a result of the breaking of Si-Si bonds at the interface between the substrate and the porous layer. The membrane is the relaxed state of the supported layer.Comment: Accepted in Langmui

Enseigner les arts du cirque au collège : une dévolution artistique

L’article s’appuie sur une recherche d’ingénierie didactique qualifiée de coopérative, menée en Arts du cirque dans le cadre de l’EPS au collège. Les interactions verbales et corporelles transcrites à partir des neuf leçons du cycle d’enseignement sont analysées à l’aide de la théorie de l’action conjointe en didactique. Cette théorie vise à décrire les transactions qui s’observent lorsque le savoir se partage entre professeur et élèves (Sensevy & Mercier, 2007 ; Sensevy, 2011 ; Gruson, Forest & Loquet, 2013). Le « jeu didactique » considéré comme le jeu du professeur sur le jeu de l’élève, est identifié à la présence d’un contrat et d’un milieu didactiques (Brousseau, 1998) spécifiques des connaissances circassiennes étudiées. C’est à l’aide de ces deux notions, contrat et milieu, que nous analysons les transactions en classe lors d’une situation appelée « la machine à jongler complexe ». Les résultats montrent que le professeur met en œuvre un milieu favorable à l’activité créatrice et entretient, au sein du contrat qui le lie aux élèves, un subtil travail d’équilibration entre réticence et expression. Ce travail d’équilibration géré finement par le professeur permet à l’élève d’intégrer des savoirs utiles à la création artistique et le place en responsabilité de créer son propre spectacle.The article is based on a cooperative engineering as a specific design-based research, about circus arts as a part of physical education at school. Verbal and bodily interactions between teacher and students are analyzed, during a teaching session composed of nine lessons, thanks to the joint action theory in didactics. This theory aims to describe the transactions that are observed when knowledge is shared between teacher and students (Sensevy & Mercier, 2007; Sensevy, 2011; Gruson, Forest & Latch, 2013). The "didactics game", seen as the teacher’s game on the student’s game, is identified by both a didactic contract and a milieu (Brousseau, 1998) taking into account the specificity of circus knowledge studied. The transactions in class are analyzed during a particular situation called "the complex juggling machine". The results show that the teacher implements a favorable milieu which encourages creative activity and promotes, in the contract that binds him to the students, a subtle equilibration work between reticence and expression. This equilibration, finely worked by the teacher, allows students to integrate knowledge useful for artistic creation and gives them the responsibility to create their own spectacle

Subcontinuum mass transport of condensed hydrocarbons in nanoporous media

Although hydrocarbon production from unconventional reservoirs, the so-called shale gas, has exploded recently, reliable predictions of resource availability and extraction are missing because conventional tools fail to account for their ultra-low permeability and complexity. Here, we use molecular simulation and statistical mechanics to show that continuum description—Darcy’s law—fails to predict transport in shales nanoporous matrix (kerogen). The non-Darcy behaviour arises from strong adsorption in kerogen and the breakdown of hydrodynamics at the nanoscale, which contradict the assumption of viscous flow. Despite this complexity, all permeances collapse on a master curve with an unexpected dependence on alkane length. We rationalize this non-hydrodynamic behaviour using a molecular description capturing the scaling of permeance with alkane length and density. These results, which stress the need for a change of paradigm from classical descriptions to nanofluidic transport, have implications for shale gas but more generally for transport in nanoporous media.France. Investissements d'avenir (ICoME2 Labex ANR-11-LABX-0053)France. Investissements d'avenir (A*MIDEX ANR-11-IDEX-0001-02)Royal Dutch-Shell GroupSchlumberger Foundatio