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In this study, we carried out equilibrium molecular dynamics (EMD) simulations of the liquid-liquid interface between two different Lennard-Jones components with varying miscibility, where we examined the relation between the interfacial tension and isolation free energy using both a mechanical and thermodynamic approach. Using the mechanical approach, we obtained a stress distribution around a quasi-one-dimensional (1D) EMD systems with a flat LL interface. From the stress distribution, we calculated the liquid-liquid interfacial tension based on Bakker's equation, which uses the stress anisotropy around the interface, and measures how it varies with miscibility. The second approach uses thermodynamic integration by enforcing quasi-static isolation of the two liquids to calculate the free energy. This uses the same EMD systems as the mechanical approach, with both extended dry-surface and phantom-wall (PW) schemes applied. When the two components were immiscible, the interfacial tension and isolation free energy were in good agreement, provided all kinetic and interaction contributions were included in the stress. When the components were miscible, the values were significantly different. From the result of PW for the case of completely mixed liquids, the difference was attributed to the additional free energy required to separate the binary mixture into single components against the osmotic pressure prior to the complete detachment of the two components, i.e., the free energy of mixing., Comment: 5 figures

Thermo-osmotic flows, generated by applying a thermal gradient along a liquid-solid interface, could be harnessed to convert waste heat into electricity. While this phenomenon has been known for almost a century, there is a crucial need to gain a better understanding of the molecular origins of thermo-osmosis. In this paper, we start by detailing the multiple contributions to thermo-osmosis. We then showcase three approaches to compute the thermo-osmotic coefficient using molecular dynamics; a first method based on the computation of the interfacial enthalpy excess and Derjaguin's theoretical framework, a second approach based on the computation of the interfacial entropy excess using the so-called dry-surface method, and a novel non-equilibrium method to compute the thermo-osmotic coefficient in a periodic channel. We show that the three methods align with each other, in particular for smooth surfaces. In addition, for a polarized graphene-water interface, we observe large variations of thermo-osmotic responses, and multiple changes in flow direction with increasing surface charge. Overall, this study showcases the versatility of osmotic flows and calls for experimental investigation of thermo-osmotic behavior in the vicinity of charged surfaces.

When a contact line (CL) -- where a liquid-vapor interface meets a substrate -- is put into motion, it is well known that the contact angle differs between advancing and receding CLs. Using non-equilibrium molecular dynamics simulations, we reveal another intriguing distinction between advancing and receding CLs: while temperature increases at an advancing CL -- as expected from viscous dissipation, we show that temperature can drop at a receding CL. Detailed quantitative analysis based on the macroscopic energy balance around the dynamic CL showed that the internal energy change of the fluid along the pathline induced a remarkable temperature drop around the receding CL, in a manner similar to latent heat upon phase changes. This result provides new insights for modeling the dynamic CL, and the framework for heat transport analysis introduced here can be applied to a wide range of nanofluidic systems.

Heat transfer through the interface between a metallic nanoparticle and an electrolyte solution, has great importance in a number of applications, ranging from nanoparticle-based cancer treatments to nanofluids and solar energy conversion devices. However, the impact of surface charge and the dissolved ions on heat transfer has been scarcely explored so far. In this study, we compute the interface thermal conductance between hydrophilic and hydrophobic charged gold nanoparticles immersed in an electrolyte using equilibrium molecular dynamics simulations. Compared with an uncharged nanoparticle, we report a threefold increase of the Kapitza conductance for a nanoparticle surface charge +2 e/nm2. This enhancement is shown to be approximately independent of surface wettability, charge spatial distribution, and salt concentration. This allows us to express the Kapitza conductance enhancement in terms of surface charge density on a master curve. Finally, we interpret the increase of the Kapitza conductance as a combined result of a shift in the water density distribution toward the charged nanoparticle and an accumulation of the counter-ions around the nanoparticle surface which increase the Coulombic interaction between the liquid and the charged nanoparticle.

Solid-liquid friction plays a key role in nanofluidic systems. Yet, despite decades of method development to quantify solid-liquid friction using molecular dynamics (MD) simulations, an accurate and widely applicable method is still missing. Here, we propose a method to quantify the solid-liquid friction coefficient (FC) from equilibrium MD simulations of a liquid confined between parallel solid walls. In this method, the FC is evaluated by fitting the Green-Kubo (GK) integral of the S-L shear force autocorrelation for the range of time scales where the GK integral slowly decays with time. The fitting function was derived based on the analytical solution considering the hydrodynamic equations in our previous work [H. Oga et al., Phys. Rev. Research 3, L032019 (2021)], assuming that the timescales related to the friction kernel and to the bulk viscous dissipation can be separated. By comparing the results with those of other equilibrium MD-based methods and those of non-equilibrium MD for a Lennard-Jones liquid between flat crystalline walls with different wettability, we show that the FC is extracted with excellent accuracy by the present method, even in wettability regimes where other methods become innacurate. We then show that the method is also applicable to grooved solid walls, for which the GK integral displays a complex behavior at short times. Overall, the present method extracts efficiently the FC for various systems, with easy implementation and low computational cost., Comment: 22 pages, 6 Figures

Thermo-osmotic flows, generated at liquid-solid interfaces by thermal gradients, can be used to produce electric currents from waste heat on charged surfaces. The two key parameters controlling the thermo-osmotic current are the surface charge and the interfacial enthalpy excess due to liquid-solid interactions. While it has been shown that the contribution from water to the enthalpy excess can be crucial, how this contribution is affected by surface charge remained to be understood. Here, we start by discussing how thermo-osmotic flows and induced electric currents are related to the interfacial enthalpy excess. We then use molecular dynamics simulations to investigate the impact of surface charge on the interfacial enthalpy excess, for different distributions of the surface charge, and two different wetting conditions. We observe that surface charge has a strong impact on enthalpy excess, and that the dependence of enthalpy excess on surface charge depends largely on its distribution. In contrast, wetting has a very small impact on the charge-enthalpy coupling. We rationalize the results with simple analytical models, and explore their consequences for thermo-osmotic phenomena. Overall, this work provides guidelines to search for systems providing optimal waste heat recovery performance., Comment: 11 pages, 8 figures

The effect of temperature on friction and slip at the liquid-solid interface has attracted attention over the last twenty years, both numerically and experimentally. However, the role of temperature on slip close to the glass transition has been less explored. Here, we use molecular dynamics to simulate a bi-disperse atomic fluid, which can remain liquid below its melting point (supercooled state), to study the effect of temperature on friction and slip length between the liquid and a smooth apolar wall, in a broad range of temperatures. At high temperatures, an Arrhenius law fits well the temperature dependence of viscosity, friction and slip length. In contrast, when the fluid is supercooled, the viscosity becomes super-Arrhenian, while interfacial friction can remain Arrhenian or even drastically decrease when lowering the temperature, resulting in a massive increase of the slip length. We rationalize the observed superlubricity by the surface crystallization of the fluid, and the incommensurability between the structures of the fluid interfacial layer and of the wall. This study calls for experimental investigation of the slip length of supercooled liquids on low surface energy solids.

Our Society is in high need of alternatives to fossil fuels. Nanoporous systems filled with aqueous electrolytes show great promises for harvesting the osmotic energy of sea water or waste heat. At the core of energy conversion in such nanofluidic systems lie the so-called electrokinetic effects, coupling thermodynamic gradients and fluxes of different types (hydrodynamical, electrical, chemical, thermal) at electrified water-solid interfaces. This chapter starts by introducing the framework of linear irreversible thermodynamics, and how the latter can be used to describe the direct and coupled responses of a fluidic system, providing general relations between the different response coefficients. The chapter then focuses on the so-called osmotic flows, generated by non-hydrodynamic actuation at liquid-solid interfaces, and illustrate how the induced fluxes can be related to the microscopic properties of the water-solid interface. Finally, the chapter moves to electricity production from non-electric actuation, and discusses in particular the performance of nanofluidic systems for the harvesting of osmotic energy and waste heat., Comment: 52 pages, 4 figures

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

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

Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and nanofluidic systems. While EO has been studied extensively at liquid-solid interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and bubbles, remains to be explored. Here we perform molecular dynamics (MD) simulations of EO in a film of aqueous electrolyte covered with fluid layers of ionic surfactants and surrounded by gas. Following the experimental procedure, we compute the zeta potential from the EO velocity, defined as the velocity difference between the middle of the liquid film and the surrounding gas. We show that the zeta potential can be smaller or larger than existing predictions depending on the surfactant coverage. We explain the failure of previous descriptions by the fact that surfactants and bound ions move as rigid bodies and do not transmit the electric driving force to the liquid locally. Considering the reciprocal streaming current effect, we then develop an extended model, which can be used to predict the experimental zeta potential of surfactant-laden liquid-gas interfaces., Comment: 11 pages, 10 figures; accepted version

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

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 thickness 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 hydrodynamic slip.

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

The Poisson-Boltzmann (PB) equation provides a mean-field theory of electrolyte solutions at interfaces and in confinement, describing how ions reorganize close to charged surfaces to form the so-called electrical double layer (EDL), with numerous applications ranging from colloid science to biology. This formulary focuses on situations of interest for micro and nanofluidics, and gathers important formulas for the PB description of a Z:Z electrolyte solution inside slit and cylindrical channels. Different approximated solutions (thin EDLs, no co-ion, Debye-H\"uckel, and homogeneous/parabolic potential limits) and their range of validity are discussed, together with the full solution for the slit channel. Common boundary conditions are presented, the thermodynamics of the EDL is introduced, and an overview of the application of the PB framework to the description of electrokinetic effects is given. Finally, the limits of the PB framework are briefly discussed, and Python scripts to solve the PB equation numerically are provided., Comment: 24 pages, 6 figures

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., Comment: 7 pages, 3 figures

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

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

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

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

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., Comment: Current Opinion in Colloid & Interface Science (2018)

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

Thermo-osmotic and related thermo-phoretic phenomena can be found in many situations from biology to colloid science, but the underlying molecular mechanisms remain largely unexplored. Using molecular dynamics simulations, we measured the thermo-osmosis coefficient by both mechano-caloric and thermo-osmotic routes, for different solid-liquid interfacial energies. The simulations reveal in particular the crucial role of nanoscale interfacial hydrodynamics. For non-wetting surfaces , thermo-osmotic transport is largely amplified by hydrodynamic slip at the interface. For wetting surfaces, the position of the hydrodynamic shear plane plays a key role in determining the amplitude and sign of the thermo-osmosis coefficient. Finally, we measure a giant thermo-osmotic response of the water-graphene interface, which we relate to the very low interfacial friction displayed by this system. These results open new perspectives for the design of efficient functional interfaces for, e.g., waste heat harvesting., Comment: Physical Review Letters, American Physical Society, A Para{\^i}tre

Non-bonded potentials are included in most force fields and therefore widely used in classical molecular dynamics simulations of materials and interfacial phenomena. It is commonplace to truncate these potentials for computational efficiency based on the assumption that errors are negligible for reasonable cutoffs or compensated for by adjusting other interaction parameters. Arising from a metadynamics study of the wetting transition of water on a solid substrate, we find that the influence of the cutoff is unexpectedly strong and can change the character of the wetting transition from continuous to first order by creating artificial metastable wetting states. Common cutoff corrections such as the use of a force switching function, a shifted potential, or a shifted force do not avoid this. Such a qualitative difference urges caution and suggests that using truncated non-bonded potentials can induce unphysical behavior that cannot be fully accounted for by adjusting other interaction parameters.

Glass formers exhibit a viscoelastic behavior: at the laboratory timescale, they behave like (glassy) solids at low temperatures, and like liquids at high temperatures. Based on this observation, elastic models relate the long time supercooled dynamics to short time elastic properties of the supercooled liquid. In the present work, we assess the validity of elastic models for the shear viscosity and the $\alpha$-relaxation time of supercooled water, using molecular dynamics simulations with the TIP4P/2005f force field over a wide range of temperatures. We show that elastic models provide a good description of supercooled water dynamics. For the viscosity, two different regimes are observed and the crossover temperature is found to be close to the one where the Stokes-Einstein relation starts to be violated. Our simulations show that only shear properties are important to characterize the effective flow activation energy. This study calls for experimental determination of the high frequency elastic properties of water at low temperatures.

We have performed large-eddy simulations of turbulent separation control via impedance boundary conditions (IBCs) on a \nacafft airfoil in near-stalled conditions. The uncontrolled baseline flow is obtained for freestream Mach numbers of $M_\infty=0.3$, chord-Reynolds numbers $Re_c = 1.5\times10^6$ and angle of attack, $\alpha=14^{\circ{}}$. Flow control is applied via imposition of complex IBCs using the time-domain implementation developed by Scalo, Bodart, and Lele, $\textit{Phys. Fluids} $(2015). Separation is delayed due to the enhanced mixing associated with convectively amplified spanwise-oriented Kelvin-Helmholtz (KH) rollers, generated via hydro-acoustic instabilities. The latter are the result of the interaction of the wall-normal transpiration through the impedance panel and the overlying mean background shear. The result is an alteration of the coupled instability between the separating shear layer and the vortex shedding in the wake (already present in the uncontrolled baseline flow) yielding unique wake topologies associated with different intensities for the passively generated KH vortical structures. Specifically, enhancements up to +13\% in the lift coefficients have been obtained. Results show that tuning of the resonant cavities below the natural shedding frequency is required to generate KH rollers structures with a sufficiently large entrainment diameter to encompass the full extent of the separated region, thereby enhancing mixing and promoting reattachment. Overall, the results presented in this work show that the adoption of hydro-acoustically tuned resonant panels is a promising passive control technique for boundary layer separation control., Comment: 54th AIAA Aerospace Sciences Meeting, AIAA SciTech, 2017

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., Comment: Accepted for publication in the Journal of Chemical Physics

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.

Friction is one of the main sources of dissipation at liquid water/solid interfaces. Despite recent progress, a detailed understanding of water/solid friction in connection with the structure and energetics of the solid surface is lacking. Here we show for the first time that \textit{ab initio} molecular dynamics can be used to unravel the connection between the structure of nanoscale water and friction for liquid water in contact with graphene and with hexagonal boron nitride. We find that whilst the interface presents a very similar structure between the two sheets, the friction coefficient on boron nitride is $\approx 3$ times larger than that on graphene. This comes about because of the greater corrugation of the energy landscape on boron nitride arising from specific electronic structure effects. We discuss how a subtle dependence of the friction on the atomistic details of a surface, that is not related to its wetting properties, may have a significant impact on the transport of water at the nanoscale, with implications for the development of membranes for desalination and for osmotic power harvesting.

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{\"o}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.

Electrokinetic effects offer a method of choice to control flows in micro and nanofluidic systems. While a rather clear picture of these phenomena exists now for the liquid-solid interfaces, the case of liquid-air interfaces remains largely unexplored. Here we investigate at the molecular level electrokinetic transport in a liquid film covered with ionic surfactants. We find that the zeta potential, quantifying the amplitude of electrokinetic effects, depends on the surfactant coverage in an unexpected way. First, it increases upon lowering surfactant coverage from saturation. Second, it does not vanish in the limit of low coverage, but instead approaches a finite value. This behavior is rationalized by taking into account the key role of interfacial hydrodynamics, together with an ion-binding mechanism. We point out implications of these results for the strongly debated measurements of zeta potential at free interfaces, and for electrokinetic transport in liquid foams.

Liquid-gas-liquid interfaces stabilized by hydrophobic beads behave as ball bearings under shear and exhibit a giant slip. Using a scaling analysis and molecular dynamics simulations we predict that, when the contact angle theta between the beads and the liquid is large, the slip length diverges as R rho(-1) (pi -theta)(-3) where R is the bead radius, and theta is the bead density. DOI: 10.1103/PhysRevLett.110.104504

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., Comment: 6 pages

This article discusses the way the standard description of capillary filling dynamics has to be modified to account for liquid/solid slip in nanometric pores. It focuses in particular on the case of a large slip length compared to the pore size. It is shown that the liquid viscosity does not play a role, and that the flow is only controlled by the friction coefficient of the liquid at the wall. Moreover in the Washburn regime, the filling velocity does not depend on the tube radius. Finally, molecular dynamics simulations suggest that this standard description fails to describe the early stage of capillary filling of carbon nanotubes by water, since viscous dissipation at the tube entrance must be taken into account.

We investigate various possible definitions of an effective temperature for a particularly simple nonequilibrium stationary system, namely a heated Brownian particle suspended in a fluid. The effective temperature based on the fluctuation dissipation ratio depends on the time scale under consideration, so that a simple Langevin description of the heated particle is impossible. The short and long time limits of this effective temperature are shown to be consistent with the temperatures estimated from the kinetic energy and Einstein relation, respectively. The fluctuation theorem provides still another definition of the temperature, which is shown to coincide with the short time value of the fluctuation dissipation ratio.

The cooling dynamics of glass-embedded noble metal nanoparticles with diameters ranging from 4 to 26 nm were studied using ultrafast pump-probe spectroscopy. Measurements were performed probing away from the surface plasmon resonance of the nanoparticles to avoid spurious effects due to glass heating around the particle. In these conditions, the time-domain data reflect the cooling kinetics of the nanoparticle. Cooling dynamics are shown to be controlled by both thermal resistance at the nanoparticule?glass interface, and heat diffusion in the glass matrix. Moreover, the interface conductances are deduced from the experiments and found to be correlated to the acoustic impedance mismatch at the metal/glass interface.

We present in this Letter a free-energy approach to the dynamics of a fluid near a nanostructured surface. The model accounts both for the static phase equilibrium in the vicinity of the surface (wetting angles, Cassie-Wenzel transition) and the dynamical properties like liquid slippage at the boundary. This method bridges the gap between phenomenological phase-field approaches and more macroscopic lattice-Boltzmann models.

In this contribution, we study situations in which nanoparticles in a fluid are strongly heated, generating high heat fluxes. This situation is relevant to experiments in which a fluid is locally heated using selective absorption of radiation by solid particles. We first study this situation for different types of molecular interactions, using models for gold particles suspended in octane and in water. As already reported in experiments, very high heat fluxes and temperature elevations (leading eventually to particle destruction) can be observed in such situations. We show that a very simple modeling based on Lennard-Jones interactions captures the essential features of such experiments, and that the results for various liquids can be mapped onto the Lennard-Jones case, provided a physically justified (corresponding state) choice of parameters is made. Physically, the possibility of sustaining very high heat fluxes is related to the strong curvature of the interface that inhibits the formation of an insulating vapor film.