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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 initiomolecular 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 initiowater 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.

The adsorption phenomenon of refrigerant R-1233zd(E) molecules on a hematite Fe2O3(011̅2) 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= 500 MPa) and shearing (vs= 20 m/s).

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 significant change of the slip length. We finally discuss the consequences of the ultralow fluid friction that we observed on the macroscopic behavior of lubricated contacts.

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.

We report on a new method to characterize the hydrodynamic boundary condition (HBC) relying solely on the measurement of the thermal motion of confined colloids, using a home build fluorescence correlation spectroscopy (FCS) setup. With this method, we show that, for smooth surfaces, the HBC depends on the wetting properties of the wall : while the no-slip assumption is fulfilled on hydrophilic surfaces, a finite slippage is measured on hydrophobic surfaces, with a characteristic slip length of 18 ± 5 nm.

À l’aide de simulations de dynamique moléculaire, nous montrons que la dynamique de la double couche électrique dépend fortement des propriétés de mouillage du liquide sur les parois. Dans le cas d'une interaction mouillante, nous retrouvons la description classique des effets électrocinétiques avec la présence d'une couche immobile de Stern, et des effets électrocinétiques dont l’amplitude est contrôlée par les propriétés électrostatiques du système. Dans le cas d'une interaction non-mouillante en revanche, la couche immobile de Stern disparaît et les effets électrocinétiques sont considérablement amplifiés par l'apparition d'un glissement du liquide à la paroi.

We report on a new method to characterize the hydrodynamic boundary condition (HBC) relying solely on the measurement of the thermal motion of confined colloids, using a home build fluorescence correlation spectroscopy (FCS) setup. With this method, we show that, for smooth surfaces, the HBC depends on the wetting properties of the wall?: while the no-slip assumption is fulfilled on hydrophilic surfaces, a finite slippage is measured on hydrophobic surfaces, with a characteristic slip length of 18???5?nm.

We report on a new method to characterize the hydrodynamic boundary condition (HBC) relying solely on the measurement of the thermal motion of confined colloids, using a home build fluorescence correlation spectroscopy (FCS) setup. With this method, we show that, for smooth surfaces, the HBC depends on the wetting properties of the wall : while the no-slip assumption is fulfilled on hydrophilic surfaces, a finite slippage is measured on hydrophobic surfaces, with a characteristic slip length of 18 ± 5 nm.

The surface charge of a microfluidic system is a very important experimental factor, because the electroosmotic flow (and the separation efficiency) depends directly on it. So, the characterization of this parameter is very interesting. Some measurement methods of this surface charge already exist, but take generally a long time to be exploited. A more original method, by measurements of currents of flow is used here. The material which has here interested us is the PDMS, polymer largely used into microfluidic today. The surface charge of the PDMS was measured according to two important parameters: the relaxation time of the surface after plasma O2 oxidation, and the pH of the electrolyte filling the microchannels.

À l’aide de simulations de dynamique moléculaire, nous montrons que la dynamique de la double couche électrique dépend fortement des propriétés de mouillage du liquide sur les parois. Dans le cas d'une interaction mouillante, nous retrouvons la description classique des effets électrocinétiques avec la présence d'une couche immobile de Stern, et des effets électrocinétiques dont l’amplitude est contrôlée par les propriétés électrostatiques du système. Dans le cas d'une interaction non-mouillante en revanche, la couche immobile de Stern disparaît et les effets électrocinétiques sont considérablement amplifiés par l'apparition d'un glissement du liquide à la paroi.