arXiv subjects

Simon Gravelle

Publications and source records attributed to Simon Gravelle.

13 recordsLinked to original sources

arXiv2026

Molecular-to-polymeric crossover in ion diffusion in glyme-based electrolytes: from vehicular to hopping transport

Ion transport in glyme-based electrolytes arises from a complex interplay between solvation structure, ion correlations, and polymer chain length. Here, combining pulsed-field gradient nuclear magnetic resonance (PFG-NMR), ionic conductivity measurements, and molecular dynamics (MD) simulations, we investigate the diffusion of monovalent cations (Li$^+$, Na$^+$, Cs$^+$) and TFSI$^-$ anions across a wide molecular-weight range, from monoglyme to long poly(ethylene oxide) (PEO) chains up to 4000~g/mol, corresponding to $n$ up to 88, where $n$ is the number of ethylene oxide repeat units. We identify a crossover region at $n \approx 8$ separating two transport regimes. For short chains, ion motion is consistent with a vehicular mechanism, accompanied by pronounced ion correlations. For longer chains, ion transport decouples from polymer motion and proceeds via rapid coordination exchanges within a slowly relaxing matrix. This transition is accompanied by reduced ion clustering and enhanced anion mobility, leading to increasingly anion-dominated charge transport. Overall, our results provide a molecular picture of ion transport across the molecular-to-polymeric transition and highlight the central role of solvation shell dynamics and polymer relaxation in governing ion dynamics in glyme-based electrolytes.

cond-mat.soft↗

arXiv2025

A Set of Tutorials for the LAMMPS Simulation Package

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 MD 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.

physics.comp-ph↗

arXiv2025

Giant mobility of surface-trapped ionic charges following liquid tribocharging

The sliding motion of aqueous droplets on hydrohobic 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.

cond-mat.soft↗

arXiv2025

Negative intrinsic viscosity in graphene nanoparticle suspensions induced by hydrodynamic slip

The viscosity of nanoparticle suspensions is always expected to increase with particle concentration. However, a growing body of experiments on suspensions of atomically thin nanomaterials such as graphene contradicts this expectation. Some experiments indicate effective suspension viscosity values that fall below that of pure solvent at high shear rates and low solid concentrations, i.e., the intrinsic viscosity is negative. To explain this puzzling phenomenon, we combined molecular dynamics and boundary integral simulations to investigate the shear viscosity of few-nanometer graphene sheets in water at high Péclet numbers (Pe $> 100$). Our results, covering geometric aspect ratios from 4.5 to 12.0, show robustly that the intrinsic viscosity decreases with increasing aspect ratio and becomes negative beyond a threshold aspect ratio $\approx 5.5$. We demonstrate that this anomalous behavior originates from hydrodynamic slip at the liquid-solid interface, which suppresses particle rotation and promotes stable alignment with the flow direction, thereby reducing viscous dissipation relative to dissipation in pure solvent. This slip mechanism holds for both fully 3D disc-like and quasi-2D particle geometries explored in the molecular simulations. As the concentration of graphene particles increases in the dilute regime, the viscosity initially decreases, falling below that of pure water. At higher concentrations, however, particle aggregation becomes significant, leading to a rise in viscosity after a minimum is reached. These findings confirm the occurrence of a negative intrinsic viscosity in a graphene suspension due only to hydrodynamic effects. Our work has important implications for the design of lubricants, inks, and nanocomposites with tunable viscosity.

cond-mat.soft↗

arXiv2025

Intermittent molecular motion and first passage statistics for the NMR relaxation of confined water

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.

cond-mat.soft↗

arXiv2024

Effect of thermal fluctuations on the average shape of a graphene nanosheet suspended in a shear flow

Graphene nanosheets display large hydrodynamic slip lengths in most solvents, and because of this, adopt a stable orientation in a shear flow instead of rotating when thermal fluctuations are negligible [Kamal et al., Nature Comm., 11.1,2020]. In this paper, we combine molecular dynamics simulations and boundary integral simulations to demonstrate that the time-averaged 'S' shape adopted by a flexible graphene nanosheet subject to moderate thermal fluctuation is comparable to the shape predicted when neglecting thermal fluctuations. The stable 'S' shape adopted by the particle results primarily from the normal hydrodynamic traction, which is sensitive to the orientation of the particle with respect to the flow direction. Our results imply that thermally-induced shape fluctuations have a relatively minor effect on the time-averaged rheology of dilute suspensions of graphene nanosheets for relatively large but finite Péclet numbers.

physics.comp-ph↗

arXiv2024

In silico design and prediction of metastable quaternary phases in Cu-Ni-Si-Cr alloys

Quaternary phases formed in copper alloys are investigated through a combination of quantum-mechanical and classical computer simulations and active machine learning. Focus is given on nickel, silicon, and chromium impurities in a copper matrix. The analysis of the formation enthalpies of candidate quaternary structures leads to the prediction of two novel quaternary phases and the assessment of their stability. For the predicted two phases, machine learned atomistic potentials are developed using active learning with a quantum-mechanical accuracy. Use of these potentials in atomistic simulations further elucidates the structure, temperature-dependent dynamics, and elastic behavior of the predicted quaternary phases in copper alloys. The combined in silico approach is thus proven highly efficient in both designing materials and elucidating their properties and potential combining different spatiotemporal scales. In the case of alloys, this computational scheme significantly reduces the effort in searching the huge chemical space of possible phases, enhancing the efficiency in synthesizing novel alloys with pre-defined properties.

cond-mat.mtrl-sci↗

arXiv2019

Liquid exfoliation of multilayer graphene in sheared solvents: a molecular dynamics investigation

Liquid-phase exfoliation, the use of a sheared liquid to delaminate graphite into few-layer graphene, is a promising technique for the large-scale production of graphene. But the micro and nanoscale fluid-structure processes controlling the exfoliation are not fully understood. Here we perform non-equilibrium molecular dynamics simulations of a defect-free graphite nanoplatelet suspended in a shear flow and measure the critical shear rate $\dot γ_c$ needed for the exfoliation to occur. We compare $\dot γ_c$ for different solvents including water and NMP, and nanoplatelets of different lengths. Using a theoretical model based on a balance between the work done by viscous shearing forces and the change in interfacial energies upon layer sliding, we are able to predict the critical shear rates $\dot γ_c$ measured in simulations. We find that an accurate prediction of the exfoliation of short graphite nanoplatelets is possible only if both hydrodynamic slip and the fluid forces on the graphene edges are considered, and if an accurate value of the solid-liquid surface energy is used. The commonly used "geometric-mean" approximation for the solid-liquid energy leads to grossly incorrect predictions.

cond-mat.soft↗

arXiv2019

Flow-Induced Shift of the Donnan Equilibrium for Ultra-Sensitive Mass Transport Measurement Through a Single Nanochannel

Despite mass flow is arguably the most elementary transport associated to nanofluidics, its measurement still constitutes a significant bottleneck for the development of this promising field. Here, we investigate how a liquid flow perturbs the ubiquitous enrichment-or depletion-of a solute inside a single nanochannel. Using Fluorescence Correlation Spectroscopy to access the local solute concentration, we demonstrate that the initial enrichment-the so-called Donnan equilibrium-is depleted under flow thus revealing the underlying mass transport. Combining theoretical and numerical calculations beyond the classical 1D treatments of nanochannels, we rationalize quantitatively our observations and demonstrate unprecedented flow rate sensitivity. Because the present mass transport investigations are based on generic effects, we believe they can develop into a versatile approach for nanofluidics.

cond-mat.soft↗

arXiv2016

Carbon membranes for efficient water-ethanol separation

We demonstrate, on the basis of molecular dynamics simulations, the possibility of an efficient water-ethanol separation using nanoporous carbon membranes, namely carbon nanotube membranes, nanoporous graphene sheets, and multilayer graphene membranes. While these carbon membranes are in general permeable to both pure liquids, they exhibit a counter-intuitive "self-semi-permeability" to water in the presence of water-ethanol mixtures. This originates in a preferred ethanol adsorption in nanoconfinement that prevents water molecules from entering the carbon nanopores. An osmotic pressure is accordingly expressed across the carbon membranes for the water-ethanol mixture, which agrees with the classic van't Hoff type expression. This suggests a robust and versatile membrane-based separation, built on a pressure-driven reverse-osmosis process across these carbon-based membranes. In particular, the recent development of large-scale 'graphene-oxide' like membranes then opens an avenue for a versatile and efficient ethanol dehydration using this separation process, with possible application for bio-ethanol fabrication.

cond-mat.soft↗

arXiv2016

Anomalous capillary filling and wettability reversal in nanochannels

This work revisits capillary filling dynamics in the regime of nanometric to subnanometric channels. Using molecular dynamics simulations of water in carbon nanotubes, we show that for tube radii below one nanometer, both the filling velocity and the Jurin rise vary non-monotonically with the tube radius. Strikingly, with fixed chemical surface properties, this leads to confinement-induced reversal of the tube wettability from hydrophilic to hydrophobic for specific values of the radius. By comparing with a model liquid metal, we show that these effects are not specific to water. Using complementary data from slit channels, we then show that they can be described using the disjoin-ing pressure associated with the liquid structuring in confinement. This breakdown of the standard continuum framework is of main importance in the context of capillary effects in nanoporous media, with potential interests ranging from membrane selectivity to mechanical energy storage.

physics.flu-dyn↗

arXiv2015

Large permeabilities of hourglass nanopores: From hydrodynamics to single file transport

In fluid transport across nanopores, there is a fundamental dissipation that arises from the connection between the pore and the macroscopic reservoirs. This entrance effect can hinder the whole transport in certain situations, for short pores and/or highly slipping channels. In this paper, we explore the hydrodynamic permeability of hourglass shape nanopores using molecular dynamics (MD) simulations, with the central pore size ranging from several nanometers down to a few Angstr{ö}ms. Surprisingly, we find a very good agreement between MD results and continuum hydrodynamic predictions, even for the smallest systems undergoing single file transport of water. An optimum of permeability is found for an opening angle around 5 degree, in agreement with continuum predictions, yielding a permeability five times larger than for a straight nanotube. Moreover, we find that the permeability of hourglass shape nanopores is even larger than single nanopores pierced in a molecular thin graphene sheet. This suggests that designing the geometry of nanopores may help considerably increasing the macroscopic permeability of membranes.

cond-mat.soft↗

arXiv2013

Optimizing water permeability through the hourglass shape of aquaporins

The ubiquitous aquaporin channels are able to conduct water across cell membranes, combining the seemingly antagonist functions of a very high selectivity with a remarkable permeability. Whereas molecular details are obvious keys to perform these tasks, the overall efficiency of transport in such nanopores is also strongly limited by viscous dissipation arising at the connection between the nanoconstriction and the nearby bulk reservoirs. In this contribution, we focus on these so-called entrance effects and specifically examine whether the characteristic hourglass shape of aquaporins may arise from a geometrical optimum for such hydrodynamic dissipation. Using a combination of finite-element calculations and analytical modeling, we show that conical entrances with suitable opening angle can indeed provide a large increase of the overall channel permeability. Moreover, the optimal opening angles that maximize the permeability are found to compare well with the angles measured in a large variety of aquaporins. This suggests that the hourglass shape of aquaporins could be the result of a natural selection process toward optimal hydrodynamic transport. Finally, in a biomimetic perspective, these results provide guidelines to design artificial nanopores with optimal performances.

cond-mat.soft↗