The receding contact line cools down during dynamic wetting.

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 due to the change of the potential field along the pathline out of the solid-liquid interface 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.

Enhanced Heat Flow between Charged Nanoparticles and an Aqueous Electrolyte.

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 the surface charge and 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 3-fold increase of the Kapitza conductance for a nanoparticle surface charge of +320 mC/m2. This enhancement is shown to be approximately independent of the surface wettability, charge spatial distribution, and salt concentration. This allows us to express the Kapitza conductance enhancement in terms of the surface charge density on a master curve. Finally, we interpret the increase of the Kapitza conductance as a combined result of the shift of the water density distribution toward the charged nanoparticle and an accumulation of the counterions around the nanoparticle surface which increase the Coulombic interaction between the liquid and the charged nanoparticle. These considerations help us to apprehend the role of ions in heat transfer close to electrified surfaces.

Complex coupling between surface charge and thermo-osmotic phenomena.

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 spatial 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.

Equilibrium molecular dynamics evaluation of the solid-liquid friction coefficient: Role of timescales.

Solid-liquid friction plays a key role in nanofluidic systems. Following the pioneering work of Bocquet and Barrat, who proposed to extract the friction coefficient (FC) from the plateau of the Green-Kubo (GK) integral of the solid-liquid shear force autocorrelation, the so-called plateau problem has been identified when applying the method to finite-sized molecular dynamics simulations, e.g., with a liquid confined between parallel solid walls. A variety of approaches have been developed to overcome this problem. Here, we propose another method that is easy to implement, makes no assumptions about the time dependence of the friction kernel, does not require the hydrodynamic system width as an input, and is applicable to a wide range of interfaces. In this method, the FC is evaluated by fitting the GK integral for the timescale range where it slowly decays with time. The fitting function was derived based on an analytical solution of the hydrodynamics equations [Oga et al., Phys. Rev. Res. 3, L032019 (2021)], assuming that the timescales related to the friction kernel and the bulk viscous dissipation can be separated. By comparing the results with those of other GK-based methods and non-equilibrium molecular dynamics, we show that the FC is extracted with excellent accuracy by the present method, even in wettability regimes where other GK-based methods suffer from the plateau problem. Finally, the method is also applicable to grooved solid walls, where the GK integral displays complex behavior at short times.

Anomalous ionic transport in tunable angstrom-size water films on silica.

Liquid and ionic transport through nanometric structures is central to many phenomena, ranging from cellular exchanges to water resource management or green energy conversion. While pushing down toward molecular scales progressively unveils novel transport behaviors, reaching ultimate confinement in controlled systems remains challenging and has often involved 2D Van der Waals materials. Here, we propose an alternative route which circumvents demanding nanofabrication steps, partially releases material constraints, and offers continuously tunable molecular confinement. This soft-matter-inspired approach is based on the spontaneous formation of a molecularly thin liquid film onto fully wettable substrates in contact with the vapor phase of the liquid. Using silicon dioxide substrates, water films ranging from angstrom to nanometric thicknesses are formed in this manner, and ionic transport within the film can then be measured. Performing conductance measurements as a function of confinement in these ultimate regimes reveals a one-molecule thick layer of fully hindered transport nearby the silica, above which continuum, bulk-like approaches account for experimental results. Overall, this work paves the way for future investigations of molecular scale nanofluidics and provides insights into ionic transport nearby high surface energy materials such as natural rocks and clays, building concretes, or nanoscale silica membranes used for separation and filtering.

Giant slip length at a supercooled liquid-solid interface.

The effect of temperature on friction and slip at the liquid-solid interface has attracted attention over the last 20 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 bidisperse 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.

Microscopic Origins of the Viscosity of a Lennard-Jones Liquid.

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.

Connection between water's dynamical and structural properties: Insights from ab initio simulations.

SignificanceFirst-principles calculations, which explicitly account for the electronic structure of matter, can shed light on the molecular structure and dynamics of water in its supercooled state. In this work, we use density functional theory, which relies on a functional to describe electronic exchange and correlations, to evaluate which functional best describes the temperature evolution of bulk water transport coefficients. We also assess the validity of the Stokes-Einstein relation for all the functionals in the temperature range studied, and explore the link between structure and dynamics. Based on these results, we show how transport coefficients can be computed from structural descriptors, which require shorter simulation times to converge, and we point toward strategies to develop better functionals.

Fast and versatile thermo-osmotic flows with a pinch of salt.

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. Thus, there is 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 effects 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.

Enhanced local viscosity around colloidal nanoparticles probed by equilibrium molecular dynamics simulations.

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.

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description.

Osmotic transport in nanoconfined aqueous electrolytes provides alternative 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 fundamental insights into the structure and osmotic transport of aqueous electrolytes on 2D materials and explores alternative pathways for efficient water desalination and osmotic energy conversion.

Electrokinetic sweeping of colloids at a reactive magnesium oxide interface.

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.

Fast increase of nanofluidic slip in supercooled water: the key role of dynamics.

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 ∼ 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 dependence of the enhancement in terms of interfacial density relaxation dynamics. While 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, could help explain the subtle phase and dynamical behaviors of supercooled confined water, and paves the way to explore new behaviors in supercooled nanofluidic systems.

Liquid-Solid Slip on Charged Walls: The Dramatic Impact of Charge Distribution.

Nanofluidic systems show great promise 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 counterions 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 into 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.

Molecular modeling of aqueous electrolytes at interfaces: Effects of long-range dispersion forces and of ionic charge rescaling.

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 [ECC 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 accurately describes image charge repulsion at the liquid-vapor interface, and the rescaling of LJ parameters in ECCR models-aimed at capturing local ion-ion and ion-water interactions in bulk- 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.

Ab initio nanofluidics: disentangling the role of the energy landscape and of density correlations on liquid/solid friction.

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.

Giant Thermoelectric Response of Nanofluidic Systems Driven by Water Excess Enthalpy.

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, 2 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 electro-osmotic 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.

Large effect of lateral box size in molecular dynamics simulations of liquid-solid friction.

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.

Shear force measurement of the hydrodynamic wall position in molecular dynamics.

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.