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Thermo-osmotic flows - flows generated in micro and nanofluidic systems by thermal gradients - could provide an alternative approach to harvest waste heat. However, such use would require massive thermo-osmotic flows, which are up to now only predicted for special and expensive materials. There is thus an urgent need to design affordable nanofluidic systems displaying large thermo-osmotic coefficients. In this paper we propose a general model for thermo-osmosis of aqueous electrolytes in charged nanofluidic channels, taking into account hydrodynamic slip, together with the different solvent and solute contributions to the thermo-osmotic response. We apply this model to a wide range of systems, by studying the effect of wetting, salt type and concentration, and surface charge. We show that intense thermo-osmotic flows can be generated using slipping charged surfaces. We also predict for intermediate wettings a transition from a thermophobic to a thermophilic behavior depending on the surface charge and salt concentration. Overall, this theoretical framework opens an avenue for controlling and manipulating thermally induced flows with common charged surfaces and a pinch of salt., 7 pages, 3 figures

International audience; Investigating the electrokinetic (EK) response in the vicinity of interfaces has regained interest due to the development of new membrane based processes for energy harvesting or soil depollution. However, the case of reactive interfaces, ubiquitous in these processes, remains scarcely explored. Here we experimentally investigate the EK response of a model interface between an aqueous electrolyte and a bulk MgO crystal surface (100), for different pH. For that purpose, we use a lab-scale non invasive method to monitor the zeta potential of the interface versus time, by confocal fluorescent particle tracking. An unexpected motion of the particles, repelled and then attracted again by the interface is observed. We attributed this motion to the surface reactivity, inducing ion concentration gradients perpendicular to the interface and subsequent diffusiophoresis of the charged particle. Accordingly, we could describe at a semi-quantitative level the particle dynamics by solving numerically the Poisson–Nernst–Planck equations to establish concentration profile in the system and subsequent diffusiophoretic motion. These experiments open the way to the characterization of both the EK response and the reaction rate in the vicinity of reactive interfaces.

Among all fluids, water has always been of special concern for scientists from a broad variety of research fields due to its rich behavior. In particular, some questions remain unanswered nowadays concerning the temperature dependence of bulk and interfacial transport properties of supercooled and liquid water, e.g. regarding the fundamentals of the violation of the Stokes-Einstein relation in the supercooled regime or the subtle relation between structure and dynamical properties. Here we investigated the temperature dependence of the bulk transport properties from ab initio molecular dynamics based on density functional theory, down to the supercooled regime. We determined from a selection of functionals, that SCAN better describes the experimental viscosity and self-diffusion coefficient, although we found disagreements at the lowest temperatures. For a limited set of temperatures, we also explored the role of nuclear quantum effects on water dynamics using ab initio molecular dynamics that has been accelerated via a recently introduced machine learning approach. We then investigated the molecular mechanisms underlying the different functionals performance and assessed the validity of the Stokes-Einstein relation. We also explored the connection between structural properties and the transport coefficients, verifying the validity of the excess entropy scaling relations for all the functionals. These results pave the way to predict the transport coefficients from the radial distribution function, helping to develop better functionals. On this line, they indicate the importance of describing the long-range features of the radial distribution function., Comment: 10 pages, 5 figures

Secondary instabilities growing over a time-evolving variable-density round jet subject to the primary Kelvin–Helmholtz (KH) instability at Atwood number $\vert \textit {At} \vert = 0.25$ are investigated with a non-modal linear stability analysis. Despite local modifications of the base flow vorticity induced by the baroclinic torque, these disturbances experience a short-term universal growth due to a combination of the Orr and lift-up mechanisms, whatever the azimuthal wavenumber $m$ . At $\textit {Re}=1000$ , the secondary energy growth stems from the development of elliptical and hyperbolic instabilities, with an E-type-to-H-type transition as $m$ and $\textit {Re}$ increase, as in the homogeneous case (Nastro et al., J. Fluid Mech., vol. 900, 2020, A13). In the light jet at $\textit {Re} = 1000$ , after the KH mode saturation, the high- $m$ H-type instability is replaced by a perturbation organised as counter-rotating streamwise vortices located in the base flow region of promoted strain rate. Increasing the Reynolds number up to $\textit {Re} = 10\,000$ yields larger energy growths and a strong anisotropy among energy and enstrophy components with a preferential increase of axial velocity and azimuthal vorticity. Both come from the linearised baroclinic source that drives the optimal response towards folded sheets of axial velocity that differ from those observed in the variable-density plane shear layers. When the perturbation is injected around the KH saturation time for $\textit {Re}=10\,000$ , the response to optimal perturbation takes the form of fast growing secondary KH instabilities whatever $m$ . We find these three-dimensional secondary KH instabilities to be good candidates for the transition to turbulence in variable-density jet flows.

International audience; With stratospheric flight ormartian exploration in perspective, the compressibility influence on thewake dynamics ofa NACA0012 profile is investigated. The unsteady flow past the airfoil at Re = 1000 is characterized using directnumerical simulations for various angles of attack α ∈ [0°; 20°] and Mach numbers up to M∞ = 0.5. Steady flowsobtained using the selective frequency damping (SFD) technique are used as base states for a global linear stabilityanalysis. The influence of both the angle of attack and the Reynolds number on the most amplifiedmode characteristicsis first investigated in the incompressible regime. Then, the compressibility effects in the subcritical regime areconsidered. A stabilizing or a destabilizing effect of compressibility is observed depending on the angle of attack andtheReynolds number. For α < 20°, compressibility has a destabilizing effect close to the critical threshold,which resultsin anearlierHopfbifurcation,whereas increasing theMach number always results in a decrease of the growth rate of themode well above the critical threshold. Finally, the mode frequency decreases with the Mach number.

Unlike crystalline solids or ideal gases, transport properties remain difficult to describe from a microscopic point of view in liquids, whose dynamics result from complex energetic and entropic contributions at the atomic scale. Two scenarios are generally proposed: one represents the dynamics in a fluid as a series of energy barrier crossings, leading to Arrhenius-like laws, while the other assumes that atoms rearrange themselves by collisions, as exemplified by the free volume model. To assess the validity of these two views, we computed, using molecular dynamics simulations, the transport properties of the Lennard-Jones fluid and tested to what extent the Arrhenius equation and the free volume model describe the temperature dependence of the viscosity and of the diffusion coefficient at fixed pressure. Although both models reproduce the simulation results over a wide range of pressure and temperature covering the liquid and supercritical states of the Lennard-Jones fluid, we found that the parameters of the free volume model can be estimated directly from local structural parameters, also obtained in the simulations. This consistency of the results gives more credibility to the free volume description of transport properties in liquids., Comment: 6 pages, 4 figures

Osmotic transport in nanoconfined aqueous electrolytes provides new venues for water desalination and "blue energy" harvesting; the osmotic response of nanofluidic systems is controlled by the interfacial structure of water and electrolyte solutions in the so-called electrical double layer (EDL), but a molecular-level picture of the EDL is to a large extent still lacking. Particularly, the role of the electronic structure has not been considered in the description of electrolyte/surface interactions. Here, we report enhanced sampling simulations based on ab initio molecular dynamics, aiming at unravelling the free energy of prototypical ions adsorbed at the aqueous graphene and hBN interfaces, and its consequences on nanofluidic osmotic transport. Specifically, we predicted the zeta potential, the diffusio-osmotic mobility and the diffusio-osmotic conductivity for a wide range of salt concentrations from the ab initio water and ion spatial distributions through an analytical framework based on Stokes equation and a modified Poisson-Boltzmann equation. We observed concentration-dependent scaling laws, together with dramatic differences in osmotic transport between the two interfaces, including diffusio-osmotic flow and current reversal on hBN, but not on graphene. We could rationalize the results for the three osmotic responses with a simple model based on characteristic length scales for ion and water adsorption at the surface, which are quite different on graphene and on hBN. Our work provides first principles insights into the structure and osmotic transport of aqueous electrolytes on two-dimensional materials and explores new pathways for efficient water desalination and osmotic energy conversion., Comment: 19 pages, 3 figures

Nanofluidics shows great promise for energy conversion and desalination applications. The performance of nanofluidic devices is controlled by liquid-solid friction, quantified by the Navier friction coefficient (FC). Despite decades of research, there is no well-established generic framework to determine the frequency dependent Navier FC from atomistic simulations. Here, we have derived analytical expressions to connect the Navier FC to the random force autocorrelation on the confining wall, from the observation that the random force autocorrelation can be related to the hydrodynamic boundary condition, where the Navier FC appears. The analytical framework is generic in the sense that it explicitly includes the system size dependence and also the frequency dependence of the FC, which enabled us to address (i) the long-standing plateau issue in the evaluation of the FC and (ii) the non-Markovian behavior of liquid-solid friction of a Lennard-Jones liquid and of water on various walls and at various temperatures, including the supercooled regime. This new framework opens the way to explore the frequency dependent FC for a wide range of complex liquids., Comment: 5 pages, 3 figures

Slightly more than half of the 74,150 Jews deported from France between 1942 and 1944 were arrested in Paris and its close suburbs. For the large majority of these 38,500 men, women, and children, their arrest was carried out by ordinary policemen belonging to the Paris Police Prefecture. The objective of this article is to propose a complete and synthetic analysis of the role of this institution and its agents in the Holocaust. In Paris, unlike anywhere else in Europe, the implementation of the ‘final solution’ was entrusted to the traditional administration. These police officers were competent and knew perfectly the environment of the persecution. But, generally speaking, they were not anti-Semite activists, they did not like the Germans, and, more importantly, they acted according to their own institutional logic. So, the French's repressive system did not automatically feed the Nazi machine of destruction. It is this complexity of the machine of persecution in occupied France which explains, in many respects, the toll of the Holocaust in France, and, more specifically, in the Paris region.

The border: whether the term evokes the reality of a customs office or is instead used more metaphorically, its polysemous nature allows us to grasp both the boundaries between social, political, or geographical spaces and the uses made of those boundaries. Such polysemy offers the opportunity to take the notions of borders and boundaries as reference points for reflection upon several themes: the identities of constituted groups; potential or condemnable transgressions; the leveraging action...

Under the Occupation, the French Jewish Statute made no provisions for an intermediary status for individuals of mixed heritage (who the administration called “half-Jews”), to the contrary of Nazi legislation in Germany. Consequently, there were no first- or second-degree mixed-heritage people in France; one was either Jewish or not. But the criteria for defining whether or not “half-Jews” were of “the Jewish race” varied between German and French texts, case law, and the practices of specialized bureaucrats. The boundary between “Jewish” and “non-Jewish” was tenuous, often arbitrary, and shaped by religious affiliation, a divorce, a marriage, or other fact of life. Being classified on the “good” or “bad” side of the “barricade” (according to the formulation of George Montandon, the ethno-racial expert of the Paris Prefecture of Police and the Commissariat-General for Jewish Affairs [CGQJ]) was very often a matter of life or death. The administration (mainly the “Jewish Service” of the Prefecture of Police and the CGQJ) and the courts had to rule on thousands of “ambiguous cases.” The CGQJ delivered over 11,000 “Certificates of Non-Belonging to the Jewish Race” after painstaking investigations. This article considers the categories that agents of the State used to sort out these borderline cases, and tries to understand the root causes of this dogged quest to determine the legal and political boundary between “Jew” and “non-Jew.” Sous l’Occupation, le statut des juifs ne prévoit pas, contrairement à la législation nazie, un statut intermédiaire pour les individus ayant une ascendance « mixte » (nommés « demi-juifs » par l’administration). En France, pas de « métis » au premier ou au deuxième degré ; on est soit juif, soit non-juif. Or, les critères pour définir l’appartenance ou non à la « race juive » des « demi-juifs » varient selon les textes allemands et français, la jurisprudence des tribunaux et la pratique des fonctionnaires spécialisés. La frontière entre « juif » et « non-juif » est ténue, souvent arbitraire, conditionnée par l’appartenance religieuse, un divorce, un mariage... Être classé du « bon » ou du « mauvais » côté de la « barricade » (selon la formule de l’expert ethno-racial auprès de la Préfecture de police de Paris et du commissariat général aux Questions juives, George Montandon) est, bien souvent, un enjeu de vie ou de mort. L’administration (« service juif » de la PP et CGQJ pour l’essentiel) et les tribunaux ont eu à statuer sur des milliers de « cas douteux ». Plus de 11 000 « certificats de non-appartenance à la race juive » ont été délivrés par le CGQJ, au bout d’enquêtes minutieuses. L’objet de cette contribution est de réfléchir sur les catégories mobilisées par les agents de l’État pour trancher sur ces cas limites, et de comprendre les ressorts de cette quête obstinée afin de déterminer la frontière, juridique et politique, entre « juif » et « non-juif ».18 septembre 2020.

The search for materials with high thermal resistance has promising applications in thermoelectric devices and boiling crisis retardation. In this paper, we study the interfacial heat transfer between water and gold, nanostructuring the gold surface and coating it with graphene. By trapping air (or vacuum in our simulations) between graphene and the nanopatterned surface, we observe a considerable increase in the interfacial resistance compared to the planar gold situation, which is shown to scale with the effective graphene–gold contact surface for both monolayer and multilayer graphene. With the massive thermal resistances we predict (up to 200 nm in terms of Kapitza length), the system proposed here represents a robust alternative to superhydrophobic Cassie materials. Moreover, since the low thermal conductance is achieved primarily due to geometry (vacuum trapping), it is straightforward to extend our results to any material with a structure equivalent to that of the nanopatterned gold wall considered here.

Sous l’Occupation, le statut des juifs ne prévoit pas, contrairement à la législation nazie, un statut intermédiaire pour les individus ayant une ascendance « mixte » (nommés « demi-juifs » par l’administration). En France, pas de « métis » au premier ou au deuxième degré ; on est soit juif, soit non-juif. Or, les critères pour définir l’appartenance ou non à la « race juive » des « demi-juifs » varient selon les textes allemands et français, la jurisprudence des tribunaux et la pratique des fonctionnaires spécialisés. La frontière entre « juif » et « non-juif » est ténue, souvent arbitraire, conditionnée par l’appartenance religieuse, un divorce, un mariage... Être classé du « bon » ou du « mauvais » côté de la « barricade » (selon la formule de l’expert ethno-racial auprès de la Préfecture de police de Paris et du commissariat général aux Questions juives, George Montandon) est, bien souvent, un enjeu de vie ou de mort. L’administration (« service juif » de la PP et CGQJ pour l’essentiel) et les tribunaux ont eu à statuer sur des milliers de « cas douteux ». Plus de 11 000 « certificats de non-appartenance à la race juive » ont été délivrés par le CGQJ, au bout d’enquêtes minutieuses. L’objet de cette contribution est de réfléchir sur les catégories mobilisées par les agents de l’État pour trancher sur ces cas limites, et de comprendre les ressorts de cette quête obstinée afin de déterminer la frontière, juridique et politique, entre « juif » et « non-juif ».18 septembre 2020. Under the Occupation, the French Jewish Statute made no provisions for an intermediary status for individuals of mixed heritage (who the administration called “half-Jews”), to the contrary of Nazi legislation in Germany. Consequently, there were no first- or second-degree mixed-heritage people in France; one was either Jewish or not. But the criteria for defining whether or not “half-Jews” were of “the Jewish race” varied between German and French texts, case law, and the practices of specialized bureaucrats. The boundary between “Jewish” and “non-Jewish” was tenuous, often arbitrary, and shaped by religious affiliation, a divorce, a marriage, or other fact of life. Being classified on the “good” or “bad” side of the “barricade” (according to the formulation of George Montandon, the ethno-racial expert of the Paris Prefecture of Police and the Commissariat-General for Jewish Affairs [CGQJ]) was very often a matter of life or death. The administration (mainly the “Jewish Service” of the Prefecture of Police and the CGQJ) and the courts had to rule on thousands of “ambiguous cases.” The CGQJ delivered over 11,000 “Certificates of Non-Belonging to the Jewish Race” after painstaking investigations. This article considers the categories that agents of the State used to sort out these borderline cases, and tries to understand the root causes of this dogged quest to determine the legal and political boundary between “Jew” and “non-Jew.”

La frontiere : que le terme evoque la realite du poste de douane ou qu’il soit au contraire utilise comme metaphore, sa polysemie permet d’apprehender, a la fois, la limite entre des espaces geographiques, sociaux ou politiques, et les usages faits de cette limite. Cette polysemie offre l’occasion de prendre les notions de frontieres et de limites comme des points d’ancrage pour reflechir sur les identites de groupes constitues, sur les transgressions possibles ou condamnables, sur les levier...

International audience; We investigate the development of three-dimensional instabilities on a time-dependent round jet undergoing the axisymmetric Kelvin–Helmholtz (KH) instability. A non-modal linear stability analysis of the resulting unsteady roll-up into a vortex ring is performed based on a direct-adjoint approach. Varying the azimuthal wavenumber m, the Reynolds number Re and the aspect ratio of the jet base flow α, we explore the potential for secondary energy growth beyond the initial phase when the base flow is still quasi-parallel and universal shear-induced transient growth occurs. For Re=1000 and α=10, the helical m=1 and double-helix m=2 perturbations stand as global optimals with larger growth rates in the post roll-up phase. The secondary energy growth stems from the development of elliptical (E-type) and hyperbolic (H-type) instabilities. For m>2, the maximum of the kinetic energy of the optimal perturbation moves from the large scale vortex core towards the thin vorticity braid. With a Reynolds number one order of magnitude larger, the kinetic energy of the optimal perturbations exhibits sustained growth well after the saturation time of the base flow KH wave and the underlying length scale selection favours higher azimuthal wavenumbers associated with H-type instability in the less diffused vorticity braid. Doubling the jet aspect ratio yields initially thinner shear layers only slightly affected by axisymmetry. The resulting unsteady base flow loses scale selectivity and is prone to a common path of initial transient growth followed by the optimal secondary growth of a wide range of wavenumbers. Increasing both the aspect ratio and the Reynolds number thus yields an even larger secondary growth and a lower wavenumber selectivity. At a lower aspect ratio of α=5, the base flow is smooth and a genuine round jet affected by the axisymmetry condition. The axisymmetric modal perturbation of the base flow parallel jet only weakly affects the first common phase of transient growth and the optimal helical perturbation m=1 dominates with energy gains considerably larger than those of larger azimuthal wavenumbers whatever the horizon time.

Nanofluidic systems show great promises for applications in energy conversion, where their performance can be enhanced by nanoscale liquid-solid slip. However, efficiency is also controlled by surface charge, which is known to reduce slip. Combining molecular dynamics simulations and analytical developments, we show the dramatic impact of surface charge distribution on the slip-charge coupling. Homogeneously charged graphene exhibits a very favorable slip-charge relation (rationalized with a new theoretical model correcting some weaknesses of the existing ones), leading to giant electrokinetic energy conversion. In contrast, slip is strongly affected on heterogeneously charged surfaces, due to the viscous drag induced by counter-ions trapped on the surface. In that case slip should depend on the detailed physical chemistry of the interface controlling the fraction of bound ions. Our numerical results and theoretical models provide new fundamental insight on the molecular mechanisms of liquid-solid slip, and practical guidelines for searching new functional interfaces with optimal energy conversion properties, e.g. for blue energy or waste heat harvesting., Comment: Main text: 7 pages, 3 figures; supplemental material: 22 pages, 5 figures; to be published in Physical Review Letters

International audience; Acoustic liners are specific devices dedicated to reduce noise pollution of aircraft. They contained a large number of cavities which are submitted to a turbulent grazing boundary flow. In some cases, the shear layer can become unstable, and couples with the acoustic waves propagating in the cavity, leading to a strong vortex-noise interaction. Whereas this vortex-noise coupling is well known in single-cavity configurations where the boundary layer can be described by its upstream characteristics, the interaction between a turbulent grazing flow and multiple cavities is less understood, mainly because the boundary layer may evolve along the liner. This study investigates numerically the coupling between acoustic waves and hydrodynamic fluctuations in an academic liner configuration containing multiple deep cavities. First a single 3D cavity is simulated using LES, revealing a strong vortex-noise coupling at the frequency of the quarter-wave mode at 750Hz, due to a shear layer instability with a wavelength shorter than the cavity width. Then, both a 2D and 3D configuration containing 101 cavities are computed and analyzed. When the acoustic feedback loop is cut off, no vortex-noise coupling occurs, however large flow structures can be observed in the boundary layer. When switching on the acoustic feedback loop, the vortex-noise coupling at 750Hz is again present, yet a more complex noise spectrum is obtained. In particular, lower frequencies are observed, associated to intermittency due to the desynchronization between cavities. Moreover, it is shown that the boundary layer evolves along the liner. One key phenomenon is the massive instability of the turbulent boundary layer, generating large coherent structures with a wavelength larger than the cavity width in the downstream part of the liner. A scenario is then proposed to explain this competition between these two phenomena: in the upstream part of the liner, the flow is dominated by the vortex-noise coupling at 750Hz. Further downstream, however, the shear layer becomes thicker, and is therefore more robust to transverse acoustic excitation, leaving the place for the large turbulent flow structures in the boundary layer to dominate the flowfield.

International audience; The dynamics of perturbed aircraft wake models is investigated in the two-dimensional limit by means of a linear stability analysis. The base flow, computed by a direct numerical simulation of the incompressible Navier-Stokes equations, is a viscous counter-rotating vortex dipole obtained from an initial condition which is either a superposition of two Lamb-Oseen vortices or a vorticity sheet with elliptical vorticity distribution. The former, referred to as the Lamb-Oseen dipole (LOD), is a model for the far field of the wake and gives rise to a family of quasisteady dipoles parametrized by their aspect ratio only. The later approaches the near-field wake of a wing during the rolling phase which eventually converges towards the LOD model at later times. First, a modal stability analysis of the LOD under the assumption of a frozen base flow is performed for aspect ratios a/b ranging from 0.05 to 0.36 at various Reynolds numbers. Several families of unstable antisymmetric and symmetric modes are observed. The maximal growth rates are reached at low Reynolds numbers. The results are consistent with those obtained by Brion et al. [Phys. Fluids 26, 064103 (2014)] for the higher aspect ratio inviscid Lamb-Chaplygin dipole (LCD). However, the a posteriori verification of the validity of the frozen base flow assumption shows that, except for a few modes occurring at the highest aspect ratios and large Reynolds numbers, these two-dimensional instabilities do not survive the base flow unsteadiness due to viscous diffusion. They are thus not likely to develop in the flow. Second, we focus on the transient dynamics of the dipoles by looking for the optimal perturbations through a nonmodal stability analysis based on a direct-adjoint approach. The observed energy gains are substantial and indicate the potential of transient mechanisms. In the short time dynamics, the optimal perturbation consists of intertwined vorticity layers located within each vortex core and leading to a m=2 deformation Kelvin wave excited by the Orr mechanism. For moderate to large horizon times, the optimal perturbation takes the form of vorticity layers localized outside the vortex core which eventually give rise to the two-dimensional unstable mode unveiled by the modal analysis through a combination of Orr mechanism and velocity induction. The robustness of these modes is examined by considering the initial stage of the development of aircraft wakes. The optimal perturbations developing on an elliptic and a double-elliptic vorticity sheet present a similar structure and rely on the same mechanisms as the ones observed for the LOD but come with lower energy gains. It is concluded that the rolling-up of the vorticity sheet in the near-field of the wake does not influence significantly the linear development of these two-dimensional perturbations.

Molecular dynamics simulations of aqueous electrolytes generally rely on empirical force fields, combining dispersion interactions - described by a truncated Lennard-Jones (LJ) potential - and electrostatic interactions - described by a Coulomb potential computed with a long-range solver. Recently, force fields using rescaled ionic charges (electronic continuum correction, ECC), possibly complemented with rescaling of LJ parameters (electronic continuum correction rescaled, ECCR), have shown promising results in bulk, but their performance at interfaces has been less explored. Here we started by exploring the impact of the LJ potential truncation on the surface tension of a sodium chloride aqueous solution. We show a discrepancy between the numerical predictions for truncated LJ interactions with a large cutoff and for untruncated LJ interactions computed with a long-range solver, which can bias comparison of force field predictions with experiments. Using a long-range solver for LJ interactions, we then show that an ionic charge rescaling factor chosen to correct long-range electrostatic interactions in bulk also describes accurately image charge repulsion at the liquid-vapor interface, and that the rescaling of LJ parameters in ECCR models - aimed at capturing local ion-ion and ion-water interactions in bulk - also describes well the formation of an ionic double layer at the liquid-vapor interface. Overall, these results suggest that the molecular modeling of aqueous electrolytes at interfaces would benefit from using long-range solvers for dispersion forces, and from using ECCR models, where the charge rescaling factor should be chosen to correct long-range electrostatic interactions., Comment: Main text: 6 pages, 3 figures; Supplemental material: 9 pages, 3 figures; to be published in J. Chem. Phys

International audience; Despite relevance to water purification and renewable energy conversion membranes, the molecular mechanisms underlying water slip are poorly understood. We disentangle the static and dynamical origin of water slippage on graphene, hBN and MoS2 by means of large-scale ab initio molecular dynamics. Accounting for the role of the electronic structure of the interface is essential to determine that water slips five and eleven times faster on graphene compared to hBN and to MoS2, respectively. Intricate changes in the water energy landscape as well as in the density correlations of the fluid provide, respectively, the main static and dynamical origin of water slippage. Surprisingly, the timescales of the density correlations are the same on graphene and hBN, whereas they are longer on MoS2 and yield a 100% slowdown in the flow of water on this material. Our results pave the way for an in silico first principles design of materials with enhanced water slip, through the modification of properties connected not only to the structure, but also to the dynamics of the interface.

Nanofluidics is an emerging field offering innovative solutions for energy harvesting and desalination. The efficiency of these applications depends strongly on liquid-solid slip, arising from a favorable ratio between viscosity and interfacial friction. Using molecular dynamics simulations, we show that wall slip increases strongly when water is cooled below its melting point. For water on graphene, the slip length is multiplied by up to a factor of five and reaches $230$nm at the lowest simulated temperature, $T \sim 225$K; experiments in nanopores can reach much lower temperatures and could reveal even more drastic changes. The predicted fast increase in water slip can also be detected at supercoolings reached experimentally in bulk water, as well as in droplets flowing on anti-icing surfaces. We explain the anomalous slip behavior in the supercooled regime by a decoupling between viscosity and bulk density relaxation dynamics, and we rationalize the wall-type dependency of the enhancement in terms of interfacial density relaxation dynamics. By providing fundamental insights on the molecular mechanisms of hydrodynamic transport in both interfacial and bulk water in the supercooled regime, this study is relevant to the design of anti-icing surfaces and it also paves the way to explore new behaviors in supercooled nanofluidic systems., Comment: 8 pages, 4 figures

Nanofluids-dispersions of nanometer-sized particles in a liquid medium-have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local viscosity in the immediate vicinity of a metallic nanoparticle for a wide range of solid-liquid interaction strength, where a nanolayer of thickness 1 nm is observed as a result of the interaction with the nanoparticle. The viscosity of the nanolayer, which is found to be higher than its corresponding bulk value, is directly dependent on the solid-liquid interaction strength. We discuss the origin of this viscosity enhancement and show that the liquid density increment alone cannot explain the values of the viscosity observed. Rather, we suggest that the solid-like structure of the distribution of the liquid atoms in the vicinity of the nanoparticle contributes to the nanolayer viscosity enhancement. Finally, we observe a failure of the Stokes-Einstein relation between viscosity and diffusion close to the wall, depending on the liquid-solid interaction strength, which we rationalize in terms of the hydrodynamic slip.

International audience; The classical paradigm for osmotic transport has long related the induced-flow direction to the solute membrane interactions, with the low-to-high concentration flow a direct consequence of the solute rejection from the semipermeable membrane. In principle, the same was thought to occur for the newly demonstrated membrane-free osmotic transport named diffusio-osmosis. Using a recently proposed nanofluidic setup, we revisit this cornerstone of osmotic transport by studying the diffusio-osmotic flows generated at silica surfaces by either poly(ethylene)glycol polymers or ethanol molecules in aqueous solutions. Strikingly, both neutral solutes yield osmotic flows in the usual low to high concentration direction, in contradiction with their propensity to adsorb on silica. Considering theoretically and numerically the intricate nature of the osmotic response that combines molecular-scale surface interaction and near-wall dynamics, these findings are rationalized within a generalized framework. These elements constitute a step forward toward a finer understanding of osmotically driven flows, at the core of rapidly growing fields ranging from energy harvesting to active matter.

Omori T., Inoue N., Joly L., et al. Full characterization of the hydrodynamic boundary condition at the atomic scale using an oscillating channel: Identification of the viscoelastic interfacial friction and the hydrodynamic boundary position. Physical Review Fluids, 4(11), 114201. https://doi.org/10.1103/physrevfluids.4.114201., Flows in nanofluidic systems are controlled by the hydrodynamic boundary condition (BC), involving the friction coefficient and the hydrodynamic wall position. Here we considered a liquid nanoslab confined between two walls, where we derived, from the Stokes equation and the Navier slip BC, analytical expressions for the liquid response to an oscillatory tangential motion of the walls in terms of the wall shear stress and mean fluid velocity. By fitting these expressions to molecular dynamics simulation results, we could extract both the viscoelastic friction coefficient and hydrodynamic wall position for walls with three different wettabilities, hence fully characterizing the frequency-dependent hydrodynamic boundary condition. The proposed method could be applied to a variety of liquid-solid interfaces of interest, e.g., for flows of complex fluids or fluids at a low temperature. It should also support methodological developments on the characterization of the hydrodynamic slip in general.

Nanofluidic systems could in principle be used to produce electricity from waste heat, but current theoretical descriptions predict a rather poor performance as compared to thermoelectric solid materials. Here we investigate the thermoelectric response of NaCl and NaI solutions confined between charged walls, using molecular dynamics simulations. We compute a giant thermoelectric response, two orders of magnitude larger than the predictions of standard models. We show that water excess enthalpy -- neglected in the standard picture -- plays a dominant role in combination with the electroosmotic mobility of the liquid-solid interface. Accordingly, the thermoelectric response can be boosted using surfaces with large hydrodynamic slip. Overall, the heat harvesting performance of the model systems considered here is comparable to that of the best thermoelectric materials, and the fundamental insight provided by molecular dynamics suggests guidelines to further optimize the performance, opening the way to recycle waste heat using nanofluidic devices., Comment: accepted in Physical Review Letters

The tribological performance of the R1233zd refrigerant in extreme confinement between two hematite $${\text{Fe}}_{ 2} {\text{O}}_{ 3} \left( {01\overline{1} 2} \right)$$ surfaces is studied thanks to large-scale molecular dynamics simulations based on a force field previously parametrized from ab initio calculations. With atomically smooth surfaces, and a refrigerant film thickness as small as 2 nm, adsorbed layers of R1233zd molecules on $${\text{Fe}}_{ 2} {\text{O}}_{ 3}$$ surfaces resist to high pressures and high sliding velocities. In ultra-confined systems, friction behaves non-monotonously, reaching a global maximum when a single saturated layer is formed. Moreover, sliding simulations with a rough surface reveal total film breakdown for a local pressure around 13 GPa. Interestingly, the addition of a sliding velocity enhances the performance through a hydrodynamic lift-like mechanism: the higher the sliding, the higher the chance for refrigerant molecules to be entrained into the asperity contact.

Oga H., Yamaguchi Y., Omori T., et al. Green-Kubo measurement of liquid-solid friction in finite-size systems. Journal of Chemical Physics, 151(5), 054502. https://doi.org/10.1063/1.5104335., To characterize liquid-solid friction using molecular dynamics simulations, Bocquet and Barrat (BB) [Phys. Rev. E 49, 3079-3092 (1994)] proposed to use the plateau value of a Green-Kubo (GK) integral of the friction force. The BB method is delicate to apply in finite-size simulations, where the GK integral vanishes at long times. Here, we derive an expression for the GK integral in finite-size systems, based on a Langevin description of a coarse-grained system effectively involving a certain thickness of liquid close to the wall. Fitting this expression to GK integrals obtained from simulations of a liquid slab provides the friction coefficient and the effective thickness of the coarse-grained system. We show that the coarse-grained system for a Lennard-Jones fluid between flat and smooth solid surfaces is 2-3 molecules thick, which provides a criterion for measuring the friction coefficient independently of confinement. As compared to nonequilibrium simulations, the new approach is more accurate and removes some ambiguities of nonequilibrium measurements. Overall, we hope that this new method can be used to characterize efficiently liquid-solid friction in a variety of systems of interest, e.g., for nanofluidic applications.

Cecilia Herrero, Takeshi Omori, Yasutaka Yamaguchi, and Laurent Joly, "Shear force measurement of the hydrodynamic wall position in molecular dynamics", The Journal of Chemical Physics 151, 041103 (2019) https://doi.org/10.1063/1.5111966., Flows in nanofluidic systems are strongly affected by liquid-solid slip, which is quantified by the slip length and by the position where the slip boundary condition applies. Here, we show that the viscosity, slip length, and hydrodynamic wall position (HWP) can be accurately determined from a single molecular dynamics (MD) simulation of a Poiseuille flow, after identifying a relation between the HWP and the wall shear stress in that configuration. From this relation, we deduce that in gravity-driven flows, the HWP identifies with the Gibbs dividing plane of the liquid-vacuum density profile. Simulations of a generic Lennard-Jones liquid confined between parallel frozen walls show that the HWP for a pressure-driven flow is also close to the Gibbs dividing plane (measured at equilibrium), which therefore provides an inexpensive estimate of the HWP, going beyond the common practice of assuming a given position for the hydrodynamic wall. For instance, we show that the HWP depends on the wettability of the surface, an effect usually neglected in MD studies of liquid-solid slip. Overall, the method introduced in this article is simple, fast, and accurate and could be applied to a large variety of systems of interest for nanofluidic applications.

International audience; In the past few decades great effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes and slit pores) due to the multiple technological applications of such systems, ranging from drug delivery to water desalination devices. To our knowledge, neither numerical/theoretical nor experimental approaches have so far reached a consensual understanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different nanoconfinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. It has been found that water is more structured near the less hydrophobic walls, independently of the confining geometries. Meanwhile, we observe an enhanced diffusion coefficient of water in both hydrophobic nanotubes. Finally, we propose a confined Stokes–Einstein relation to obtain the viscosity from diffusivity, whose result strongly differs from the Green–Kubo expression that has been used in previous works. While viscosity computed with the Green–Kubo formula (applied for anisotropic and confined systems) strongly differs from that of the bulk, viscosity computed with the confined Stokes–Einstein relation is not so much affected by the confinement, independently of its geometry. We discuss the shortcomings of both approaches, which could explain this discrepancy.

International audience; The present investigation targets the generation of airfoil trailing-edge broadband noise that arises from the interaction of turbulent boundary layer with the airfoil trailing edge. Large-eddy simulations, carried out using a massively parallel compressible solver CharLESX, are conducted for a Controlled-Diffusion (CD) airfoil with rounded trailing edge for seven configurations, characterized with a Reynolds number, angle of attack and Mach number. An analysis of the unsteady pressure signals in the boundary layer is proposed in regard to classical trailing edge noise modelling ingredients.

International audience; Molecular dynamics simulations are a powerful tool to characterize liquid-solid friction. A slab configuration with periodic boundary conditions in the lateral dimensions is commonly used, where the measured friction coefficient could be affected by the finite lateral size of the simulation box. Here we show that for a very wetting liquid close to its melting temperature, strong finite size effects can persist up to large box sizes along the flow direction, typically ∼30 particle diameters. We relate the observed decrease of friction in small boxes to changes in the structure of the first adsorbed layer, which becomes less commensurable with the wall structure. Although these effects disappear for lower wetting cases or at higher temperatures, we suggest that the possible effect of the finite lateral box size on the friction coefficient should not be automatically set aside when exploring unknown systems.

Defects are inevitably present in nanofluidic systems, yet the role they play in nanofluidic transport remains poorly understood. Here, we report ab initio molecular dynamics (AIMD) simulations of the friction of liquid water on defective graphene and boron nitride sheets. We show that water dissociates at certain defects and that these "reactive" defects lead to much larger friction than the "nonreactive" defects at which water molecules remain intact. Furthermore, we find that friction is extremely sensitive to the chemical structure of reactive defects and to the number of hydrogen bonds they can partake in with the liquid. Finally, we discuss how the insight obtained from AIMD can be used to quantify the influence of defects on friction in nanofluidic devices for water treatment and sustainable energy harvesting. Overall, we provide new insight into the role of interfacial chemistry on nanofluidic transport in real, defective systems.

Surface charge controls many static and dynamic properties of soft matter and micro/nanofluidic systems, but its unambiguous measurement forms a challenge. Standard characterization methods typically probe an effective surface charge, which provides limited insight into the distribution and dynamics of charge across the interface, and which cannot predict consistently all surface-charge-governed properties. New experimental approaches provide local information on both structure and transport, but models are typically required to interpret raw data. Conversely, molecular dynamics simulations have helped showing the limits of standard models and developing more accurate ones, but their reliability is limited by the empirical interaction potentials they are usually based on. This review highlights recent developments and limitations in both experimental and computational research focusing on the liquid-solid interface. Based on recent studies, we make the case that coupling of experiments and simulations is pivotal tomitigate methodological shortcomings and address open problems pertaining to charged interfaces., Current Opinion in Colloid & Interface Science (2018)

The streamwise breathing motion of the separation bubble, triggered by the shock wave/boundary layer interaction (SBLI) at large Mach number, is known to yield wall pressure and aerodynamic load fluctuations. Following the experiments by Wang et al. (2012), we aim to evaluate and understand how the introduction of microramp vortex generators (mVGs) upstream the interaction may reduce the amplitude of these fluctuations. We first perform a reference large-eddy simulation (LES) of the canonical situation when the interaction occurs between the turbulent boundary layer (TBL) over a flat plate at Mach number M=2.7 and Reynolds number Reθ=3600 and an incident oblique shock wave produced on an opposite wall. A high-resolution simulation is then performed including a rake of microramps protruding by 0.47δ in the TBL. The long time integration of the simulations allows to capture 52 and 32 low-frequency oscillations for the natural case and controlled SBLI, respectively. In the natural case, we retrieve the pressure fluctuations associated with the reflected shock foot motions at low-frequency characterized by StL=0.02−0.06. The controlled case reveals a complex interaction between the otherwise two-dimensional separation bubble and the array of hairpin vortices shed at a much higher frequency StL=2.4 by the mVGs rake. The effect on the map of averaged wall shear stress and on the pressure load fluctuations in the interaction zone is described, with a 20% and 9% reduction of the mean separated area and pressure load fluctuations, respectively. Furthermore, the controlled SBLI exhibits a new oscillating motion of the reflected shock foot, varying in the spanwise direction with a characteristic low-frequency of StL=0.1 in the wake of the mVGs and StL=0.05 in between.

International audience; Following our recent theoretical prediction of the giant thermo-osmotic response of the water–graphene interface, we explore the practical implementation of waste heat harvesting with carbon-based membranes, focusing on model membranes of carbon nanotubes (CNT). To that aim, we combine molecular dynamics simulations and an analytical model considering the details of hydrodynamics in the membrane and at the tube entrances. The analytical model and the simulation results match quantitatively, highlighting the need to take into account both thermodynamics and hydrodynamics to predict thermo-osmotic flows through membranes. We show that, despite viscous entrance effects and a thermal short-circuit mechanism, CNT membranes can generate very fast thermo-osmotic flows, which can overcome the osmotic pressure of seawater. We then show that in small tubes confinement has a complex effect on the flow and can even reverse the flow direction. Beyond CNT membranes, our analytical model can guide the search for other membranes to generate fast and robust thermo-osmotic flows.

The adsorption phenomenon of refrigerant R-1233zd(E) molecules on a hematite Fe2O3(0112) surface is studied at the quantum level thanks to density functional theory + U (DFT + U) calculations employing a van der Waals functional combined with a spin-polarized system. The results show different adsorption sites on the solid surface depending on orientations of the molecule, characterizing strong interactions between the refrigerant molecule and both iron and oxygen atoms. A range of binding energy values of −0.92 to −0.22 eV is observed. These ab initio results are used to parametrize a force field at the refrigerant–hematite interface for larger scale molecular dynamics simulations. Effects of these ab initio considerations on density and velocity profiles are studied, in the case of a confined fluid between two surfaces as in a lubricated contact. The high binding energy values induce a locking effect of the R-1233zd(E) molecules close to the hematite surface, showing a resistance to compression (Pz = 5...

The fast development of ionic liquids as new lubricants and carbon-based coatings as performant tribological surfaces calls for the characterization of their frictional and interfacial hydrodynamic behavior. Here we use molecular dynamics simulations to explore the response under shear of an ionic liquid confined between various carbon-based surfaces and an iron oxide surface for comparison. We show that extremely low fluid friction and giant hydrodynamic slippage can be obtained on graphite and to a lesser extent on diamond, but that friction on amorphous carbon surfaces is comparable to that on iron oxide. We relate these differences to the atom-scale roughness of the surfaces. In particular, although amorphous carbon surfaces are apolar, their nanometric roughness is enough to generate a fluid friction comparable to that of the extremely smooth but polar iron oxide surface. We also show that, at high shear rates, seemingly small differences in viscosity and interfacial friction can result in a signific...

Among the numerous anomalies of water, the acceleration of dynamics under pressure is particularly puzzling. Whereas the diffusivity anomaly observed in experiments has been reproduced in several computer studies, the parallel viscosity anomaly has received less attention. Here we simulate viscosity and self-diffusion coefficient of the TIP4P/2005 water model over a broad temperature and pressure range. We reproduce the experimental behavior, and find additional anomalies at negative pressure. The anomalous effect of pressure on dynamic properties becomes more pronounced upon cooling, reaching two orders of magnitude for viscosity at 220 K. We analyze our results with a dynamic extension of a thermodynamic two-state model, an approach which has proved successful in describing experimental data. Water is regarded as a mixture of interconverting species with contrasting dynamic behaviors, one being strong (Arrhenius), and the other fragile (non-Arrhenius). The dynamic parameters of the two-state models are remarkably close between experiment and simulations. The larger pressure range accessible to simulations suggests a modification of the dynamic two-state model, which in turn also improves the agreement with experimental data. Furthermore, our simulations demonstrate the decoupling between viscosity $\eta$ and self-diffusion coefficient $D$ as a function of temperature $T$. The Stokes-Einstein relation, which predicts a constant $D \eta/T$, is violated when $T$ is lowered, in connection with the Widom line defined by an equal fraction of the two interconverting species. These results provide a unifying picture of thermodynamics and dynamics in water, and call for experiments at negative pressure., Comment: 13 pages, 11 figures, 4 tables; results for one simulated state point replaced by results from a longer simulation, corresponding minor changes made to figures and tables, minor clarifications