Simon Gravelle

Nanofluidique : une investigation théorique et numérique du transport fluidique dans les nanocannaux: Cette thèse décrit diverses situations liées au transport fluidique aux nano-échelles. Le premier chapitre est une introduction à la nanofluidique qui contient une revue des longueurs caractéristiques, des forces et des phénomènes présents aux nano-échelles. Le deuxième chapitre est une étude de l'impact de la géométrie sur la perméabilité hydrodynamique d'un nanopore. Inspirée par la forme des aquaporines, cette étude suggère une optimisation possible pour des canaux biconiques. Le troisième chapitre est une étude du remplissage capillaire dans des canaux sub-nanométriques en carbone. Cette étude montre l'importance de la pression de disjonction induite par la structure du fluide sur le remplissage. Le quatrième chapitre est une étude d'une diode nanofluidique, un composant connu pour imiter le comportement d'une diode à semi-conducteur. On montre qu'un fort couplage entre l'eau et la dynamique des ions entraîne une rectification du flux d'eau à l'intérieur de la diode. Le cinquième et dernier chapitre est une étude de l'origine du bruit rose (1=f) communément observé lors des mesures de courant ionique dans les nanopores

Salt precipitation, a phenomenon central to processes such as soil salinization, water treatment, and energy storage, is dictated by nanoscale hydration and electrostatic interactions at crystal interfaces. Understanding how their interplay gives rise to salt-specific growth behaviors and morphologies remains a critical challenge. Combining large-scale molecular dynamics simulations with free energy calculations, we investigate the precipitation behavior of three common salts, namely, NaCl, KCl, and Na2SO4, on their respective crystal surfaces. While all crystals grow steadily under controlled supersaturation, their surface morphologies differ markedly. KCl grows in a nearly ideal layer-by-layer mode, whereas NaCl and Na2SO4 develop increasingly rough, defect-rich surfaces. This roughening is accompanied by an early-stage charge imbalance at the crystal surface, characterized by preferential cation adsorption. The resulting local electrostatic environment modifies the structuring and dynamics of interfacial water, which in turn facilitates the premature growth of subsequent layers. We further find that ion adsorption is strongly defect-dependent: kinks provide stabilizing environments, whereas steps are energetically unfavorable due to their distinct hydration. Our study offers molecular-level insight into the coupled roles of ion-specific adsorption and interfacial hydration in shaping crystal morphology for different salts.

The sliding motion of aqueous droplets on hydrophobic surfaces leads to charge separation at the trailing edge, with implications from triple-line friction to hydrovoltaic energy generation. Charges deposited on the solid surface have been attributed to ions or electrons ripped off from the liquid drop. However, the dynamics and exact physicochemical nature of these surface-trapped charges remains poorly explored. Here, we take advantage of a scanning-based electrostatic mapping technique, to directly quantify the spatiotemporal dynamics of surface deposited charges in the wake of droplets sliding on hydrophobic surfaces. We confirm the ionic nature of these interfacially trapped charges, and evidence that they undergo very fast bidimensional diffusive transport, gliding with low friction at the solid/gas interface. We interpret our observations in the framework of molecular dynamics simulation of hydrated ions adsorbed on solid surfaces, revealing a peculiar transport mechanism limited by purely interfacial friction of the ionic solvation shell with the solid surface. By uncovering the unexpected dynamics of these ionic puddles—a distinct state of interfacial ionic matter—our findings have general implications for molecular-scale ionic transport, electrified matter, and wetting dynamics at interfaces.

The structure and dynamics of fluids confined in nanoporous media differ from those in bulk, which can be probed using NMR relaxation measurements. We here show, using atomistic molecular dynamics simulations of water in a slit nanopore, that the behavior of the NMR relaxation rate, R_{1}, with varying surface interaction and confinement strength can be estimated from the exchange statistics of fluid molecules between the adsorbed surface layer and the bulk region. In detail, we test the applicability of intermittent molecular water dynamics, a theoretical framework that relates NMR relaxation to the statistics of molecular exchange between these two states. We confront this framework with the direct evaluation of dipolar NMR relaxation rates from atomistic molecular dynamics simulations of water confined in a slit nanopore, allowing to assess its accuracy across varying surface interactions. We employ first return passage time calculations to quantify the molecular exchange statistics, thereby linking microscopic parameters of the confined fluid-such as adsorption time, pore size, and diffusion coefficient-to the NMR relaxation rate. Our results show that the theoretical framework proposed by Levitz et al. captures the qualitative trends of NMR relaxation rates observed in our molecular simulations. This approach allows to predict, interpret, and analyze the molecular relaxation of fluids at interfaces using merely concepts of statistical mechanics that can be generalized to closed and open geometries and thus yields direct insights into the experimental study of NMR relaxation with varying surface chemistry or confinement strength.

The availability of open-source molecular simulation software packages allows scientists and engineers to focus on running and analyzing simulations without having to write, parallelize, and validate their own simulation software. While molecular simulations thus become accessible to a larger audience, the ‘‘black box’’ nature of such software packages and wide array of options and features can make it challenging to use them correctly, particularly for beginners in the topic of simulations. LAMMPS is one such versatile molecular simulation code, designed for modeling particle-based systems across a broad range of materials science and computational chemistry applications, including atomistic, coarse-grained, mesoscale, grid-free continuum, and discrete element models. LAMMPS is capable of efficiently running simulations of varying sizes from small desktop computers to large-scale supercomputing environments. Its flexibility and extensibility make it ideal for complex and extensive simulations of atomic and molecular systems, and beyond. This article introduces a suite of tutorials designed to make learning LAMMPS more accessible to new users. The first four tutorials cover the basics of running molecular simulations in LAMMPS with systems of varying complexities. The second four tutorials address more advanced molecular simulation techniques, specifically the use of a reactive force field, grand canonical Monte Carlo, enhanced sampling, and the REACTER protocol. In addition, we introduce LAMMPS–GUI, an enhanced cross-platform graphical text editor specifically designed for use with LAMMPS and able to run LAMMPS directly on the edited input. LAMMPS–GUI is used as the primary tool in the tutorials to edit inputs, run LAMMPS, extract data, and visualize the simulated systems.

Background The competition between hydrophilic and hydrophobic interactions in water-ethanol mixtures gives rise to abnormal properties, such as the formation of an ethanol-rich surface layer at non-polar interfaces like graphite. This phenomenon, resulting from the balance between solid-liquid and liquid-liquid interactions, could benefit the development of microporous membranes for separating water-alcohol mixtures. Methods We used molecular dynamics simulations of water-ethanol mixtures near planar organosilica surfaces, focussing on the role of surface properties in interfacial ethanol enrichment. Surface hydrophobicity was controlled by adjusting the ratio of methyl to hydroxyl groups. We analyzed the impact of surface composition, roughness, and the spatial distribution of functional groups on ethanol adsorption. Results Our simulations reveal that ethanol preferentially adsorbs onto hydrophobic organosilica surfaces functionalized with methyl groups, similar to previous findings for graphite. Ethanol enrichment is strongly influenced by mixture composition and surface properties, including roughness and functional group distribution. Hydrophobic surfaces consistently exhibit enhanced ethanol affinity, whereas hydroxylated surfaces show weaker adsorption. Conclusions These findings demonstrate the ability of organosilica materials to preferentially adsorb ethanol over water. Overall, this work identifies a cost-effective alternative to carbon-based materials for the fabrication of filtration membranes.

The structure and dynamics of fluids confined in nanoporous media differ from those in bulk, which can be probed using NMR relaxation measurements. We here show, using atomistic molecular dynamics simulations of water in a slit nanopore, that the behavior of the NMR relaxation rate, R1, with varying surface interaction and confinement strength can be estimated from the exchange statistics of fluid molecules between the adsorbed surface layer and the bulk region, where molecules undergo intermittent dynamics. We employ first return passage time calculations to quantify the molecular exchange statistics, thereby linking microscopic parameters of the confined fluid-such as adsorption time, pore size, and diffusion coefficient-to the NMR relaxation rate. This approach allows to predict and interpret the molecular relaxation of fluids at interfaces using merely concepts of statistical mechanics and can be generalized to closed and open geometries.

The competition between hydrophilic and hydrophobic interactions in water-ethanol mixtures gives rise to abnormal properties, such as the formation of an ethanol-rich surface layer at non-polar interfaces like graphite. This phenomenon, resulting from the balance between solid-liquid and liquid-liquid interactions, could benefit the development of microporous membranes for separating water-alcohol mixtures. In this study, we used molecular dynamics simulations of water-ethanol mixtures near planar organosilica surfaces, focussing on the role of surface properties in interfacial ethanol enrichment. Organosilica surfaces were chosen for their tunable hydrophobicity, achieved by varying the ratio of methyl to hydroxyl groups. Our findings reveal significant ethanol enrichment at hydrophobic organosilica surfaces decorated with methyl groups, similar to previous observations with graphite. We analyze ethanol enrichment in relation to mixture composition and surface properties, such as roughness and the spatial organization of surface groups. By demonstrating the ability of organosilica materials to preferentially adsorb ethanol over water, this work identifies a cost-effective alternative to carbon-based materials for the fabrication of filtration membranes.