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

Relationship between Capillary Wettability, Mass, and Momentum Transfer in Nanoconfined Water: The Case of Water in Nanoslits of Graphite and Hexagonal Boron Nitride

The flow of water confined in nanosize capillaries is subject of intense research due to its relevance in the fabrication of nanofluidic devices and in the development of theories for fluid transport in porous media. Here, using molecular dynamics simulations carried out on 2D capillaries made up of graphite, hexagonal boron nitride (hBN) and a mix of the two, and of sizes from subnanometer to few nanometers, we investigate the relationship between the wettability of the wall capillary, the water diffusion, and its flow rate. We find that the water diffusion is decoupled from its flow properties as the former is not affected either by the height or chemistry of the capillary (except for the subnanometer slits), while the latter is dependent on both. The capillaries containing hBN show a reduced flow rate compared to those that are purely graphitic, likely due to the high friction coefficient between water and hBN. Such resistance to the flow is, however, at its maximum in the smallest capillary and lower for larger ones. Finally, we show that the flow rate values obtained from the Hagen-Poiseuille theory are almost always smaller than those obtained from simulations, indicating that either the slip length or the viscosity of nanoconfined water could be substantially different from the bulk values.

The role of the water contact layer on hydration and transport at solid/liquid interfaces

Understanding the structure in the nanoscopic region of water that is in direct contact with solid surfaces, so-called contact layer, is key to quantifying macroscopic properties that are of interest to e.g. catalysis, ice nucleation, nanofluidics, gas adsorption, and sensing. We explore the structure of the water contact layer on various technologically relevant solid surfaces, namely graphene, MoS[Formula: see text], Au(111), Au(100), Pt(111), and Pt(100), which have been previously hampered by time and length scale limitations of ab initio approaches or force field inaccuracies, by means of molecular dynamics simulations based on ab initio machine learning potentials built using an active learning scheme. Our results reveal that the in-plane intermolecular correlations of the water contact layer vary greatly among different systems: Whereas the contact layer on graphene and on Au(111) is predominantly homogeneous and isotropic, it is inhomogeneous and anisotropic on MoS[Formula: see text], on Au(100), and on the Pt surfaces, where it additionally forms two distinct sublayers. We apply hydrodynamics and the theory of the hydrophobic effect, to relate the energy corrugation and the characteristic length-scales of the contact layer with wetting, slippage, the hydration of small hydrophobic solutes and diffusio-osmotic transport. Thus, this work provides a microscopic picture of the water contact layer and links it to macroscopic properties of liquid/solid interfaces that are measured experimentally and that are relevant to wetting, hydrophobic solvation, nanofluidics, and osmotic transport.

Characterizing the Orderliness of Interfacial Water through Stretching Vibrations

Spatial orderliness, which is the orderly structure of molecules, differs significantly between interfacial water and bulk water. Understanding this property is essential for various applications in both natural and engineered environments. However, the subnanometer thickness of interfacial water presents challenges for direct and rapid characterization of its structural orderliness. Herein, through molecular dynamics simulations and infrared spectral analysis of interfacial water in a graphene slit pore, we reveal a hyperbolic tangent relationship between the water ordering and its O-H stretching information in the infrared spectrum. Specifically, O-H symmetric stretching dominated in the highly ordered water structure, while a transition to the asymmetric stretching corresponded to an increase in the degree of disorder. Thus, the O-H stretching behavior could serve as a useful and quick assessment of the orderliness of interfacial water. This work provided insights into interfacial water's unique molecular network and structural dynamics and identified the stretching vibrations' key role in its degree of order, providing insight for fields such as nanotechnology, biology, and material science.

Passive fractionating mechanism for oil spill using shear-wettability modulation

Oil spillage and organic solvent leakage have been a frequent occurrence in recent years, which pose a significant threat not only to the aquatic ecosystems but also result in substantial economic burdens. This has necessitated the search for materials capable of separating oil from water at enhanced efficiency with superior mechanical and thermal properties. In this study, we conduct a set of systematic molecular dynamics simulations to investigate the potential of two-dimensional graphene-like channels under extreme confinement to achieve efficient oil-water separation. Effective modulation of the wetting characteristics of graphene-like surfaces juxtaposed with unconventional flow behavior at the nanoscale unveils differential interaction of water and oil molecules towards the wall, thereby resulting in distinct separation zones for varying compositions of the oil-water mixture. Such separation zones have been observed to be highly correlated with mixture temperature, which provides effective separation pathways across diverse environmental conditions. Our study offers a paradigm shift in oil-water separation strategies, which not only provides deeper insights into the equilibrium and dynamic behavior of a two-phase mixture but also holds immense implications for the development of smart, wettability-based oil separation devices.

Anomalous friction of supercooled glycerol on mica

Although friction of liquids on solid surfaces is traditionally linked to wettability, recent works have unveiled the role of the solid's internal excitations on interfacial dissipation. In order to directly evidence such couplings, we take advantage of the considerable variation of the molecular timescales of supercooled glycerol under mild change of temperature to explore how friction depends on the liquid's molecular dynamics. Using a dedicated tuning-fork AFM, we measure the slippage of glycerol on mica. We report a 100 fold increase of slip length upon cooling, while liquid-solid friction exhibits a linear scaling with molecular relaxation rate at high temperature. This scaling can be explained by a contribution of mica's phonons which resonate with density fluctuations in the liquid, allowing efficient momentum transfer to mica. These results suggest that engineering phononic spectra of materials could enhance flow performance in nanofluidic channels and industrially relevant membranes.

Interfacial thermal resonance in an SiC-SiC nanogap with various atomic surface terminations

Quasi-Casimir coupling can induce phonon heat transfer across a sub-nanometer vacuum gap between monoatomic solid walls without electromagnetic fields. However, it remains unclear how the atomic surface terminations in diatomic molecules contribute to phonon transmission across a nanogap. Herein, we study the thermal energy transport across an SiC-SiC nanogap with four pairs of atomic surface terminations using classical nonequilibrium molecular dynamics simulations. In the case of identical atomic surface terminations, the net heat flux and thermal gap conductance are much greater than those in the nonidentical cases. Thermal resonance occurs between identical atomic terminated layers, whereas it vanishes between nonidentical ones. A notable heat transfer enhancement in the identical case of C-C is due to optical phonon transmission, with thermal resonance between the C-terminated layers. Our findings deepen the understanding of phonon heat transfer across a nanogap and provide insights into thermal management in nanoscale SiC power devices.

Temperature-dependent slip length for water and electrolyte solution

HYPOTHESIS

The temperature dependence of boundary slip at liquid-solid interface is critical both for the fundamental theory and applications of fluid mechanics on micro and nanoscale, such as sustainable cooling of electronic devices. However, there is a controversy on the temperature dependence of boundary slip which lacks experimental evidence, we aim to resolve it by hypothesizing that the temperature dependent slip length depends on the variation in the interfacial energy barrier.

EXPERIMENTS

Here, we measured l_s - T relation of water and NaCl solution on self-assembled FDTS (Perfluorodecyltrichlorosilane) surface using colloidal probe AFM. The transition of l_s - T monotonicity is found. For water and 0.1 M NaCl solution, l_s is negatively correlated with T, while for 1 M NaCl solution, l_s is positively correlated with T.

FINDINGS

Together with molecular dynamics simulations, such observation is quantitatively explained with an analytical model based on rate theory, where the l_s - T monotonicity depends on the difference between liquid-solid interfacial energy barrier and liquid internal energy barrier. Our results provide not only solid experimental evidence for the boundary slip being a rate process, but also a basis for the thermal-hydrodynamic design of microfluidic and nanofluidic devices.

Self-diffusion and shear viscosity for the TIP4P/Ice water model

With an ever-increasing interest in water properties, many intermolecular force fields have been proposed to describe the behavior of water. Unfortunately, good models for liquid water usually cannot provide simultaneously an accurate melting point for ice. For this reason, the TIP4P/Ice model was developed for targeting the melting point and has become the preferred choice for simulating ice at coexistence. Unfortunately, available data for its dynamic properties in the liquid state are scarce. Therefore, we demonstrate a series of simulations aimed at the calculation of transport coefficients for the TIP4P/Ice model over a large range of thermodynamic conditions, ranging from T = 245 K to T = 350 K, for the temperature, and from p = 0 to p = 500 MPa, for the pressure. We have found that the self-diffusion (shear viscosity) exhibits smaller (increased) values than TIP4P/2005 and experiments. However, rescaling the temperature with respect to the triple point temperature, as in a corresponding states plot, we find that TIP4P/Ice compares very well with TIP4P/2005 and experiment. Such observations allow us to infer that despite the different original purposes of these two models examined here, one can benefit from a vast number of reports regarding the behavior of transport coefficients for the TIP4P/2005 model and utilize them following the routine described in this paper.

Liquid-Liquid Flow at Nanoscale: Slip and Hydrodynamic Boundary Conditions

Nonequilibrium molecular dynamics (NEMD) simulations have been performed to describe the flow of a fluid nanolayer confined by another fluid. The results show that the behavior of liquids can still be described by the Navier-Stokes equation at the nanoscale, i.e., when only few molecular layers are involved. NEMD furthermore gives additional knowledge on flow. Indeed, while a very small slip is evidenced for a solid-liquid interface as, e.g., in lubrication, the slip lengths are significantly larger at the liquid-liquid interface, as encountered, e.g., in droplet coalescence. The slip lengths of the two fluids are linked. The increase in hydrodynamic slip for liquid-liquid interfaces is attributed to the enhancement of fluid diffusion, which reduces friction.

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.

Electrical Conductance of Charged Nanopores

A nanopore's response to an electrical potential drop is characterized by its electrical conductance, . For the last two decades, it has been thought that at low electrolyte concentrations, , the conductance is concentration-independent such that . It has been recently demonstrated that surface charge regulation changes the dependency to , whereby the slope typically takes the values α = 1/3 or 1/2. However, experiments have observed slopes of 2/3 and 1 suggesting that additional mechanisms, such as convection and slip-lengths, appear. Here, we elucidate the interplay between three mechanisms: surface charge regulation, convection, and slip lengths. We show that the inclusion of convection does not change the slope, and when the effects of hydrodynamic slip are included, the slope is doubled. We show that when all effects are accounted for, α can take any value between 0 and 1 where the exact value of the slope depends on the material properties. This result is of utmost importance in designing any electro-kinetically driven nanofluidic system characterized by its conductance.

Translucency and negative temperature-dependence for the slip length of water on graphene

Carbonous materials, such as graphene and carbon nanotubes, have attracted tremendous attention in the fields of nanofluidics due to the slip at the interface between solid and liquid. The dependence of slip length for water on the types of supporting substrates and thickness of the carbonous layer, which is critical for applications such as sustainable cooling of electronic devices, remains unknown. In this paper, using colloidal probe atomic force microscopy, we measured the slip length l_s of water on graphene supported by hydrophilic and hydrophobic substrates, i.e., SiO₂ and octadecyltrimethoxysilane (OTS). The l_s on single-layer graphene supported by SiO₂ is found to be 1.6 ± 1.9 nm, and that of OTS is 8.5 ± 0.9 nm. When the thickness of few-layer graphene increases to 3-4 layers, both l_s values gradually converge to the value of graphite (4.3 ± 3.5 nm). Such a thickness dependence is termed slip length translucency. Further, l_s is found to decrease by about 70% when temperature increases from 300 K to 350 K for 2-layer graphene supported by SiO₂. These observations are explained by analysis based on the Green-Kubo relation and McLachlan theory. Our results provide the first set of reference values for the slip length of water on supported few-layer graphene. They can not only serve as a direct experimental reference for solid-liquid interaction, but also provide a guideline for the design of nanofluidics-based devices, for example thermo-mechanical nanofluidic devices.

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.

How do interfaces alter the dynamics of supercooled water?

The structure of liquid water in the proximity of an interface can deviate significantly from that of bulk water, with surface-induced structural perturbations typically converging to bulk values at about ∼1 nm from the interface. While these structural changes are well established it is, in contrast, less clear how an interface perturbs the dynamics of water molecules within the liquid. Here, through an extensive set of molecular dynamics simulations of supercooled bulk and interfacial water films and nano-droplets, we observe the formation of persistent, spatially extended dynamical domains in which the average mobility varies as a function of the distance from the interface. This is in stark contrast with the dynamical heterogeneity observed in bulk water, where these domains average out spatially over time. We also find that the dynamical response of water to an interface depends critically on the nature of the interface and on the choice of interface definition. Overall these results reveal a richness in the dynamics of interfacial water that opens up the prospect of tuning the dynamical response of water through specific modifications of the interface structure or confining material.

Quasi-Casimir coupling can induce thermal resonance of adsorbed liquid layers in a nanogap

In a vacuum nanogap, phonon heat transfer can be induced by quasi-Casimir coupling in the absence of electromagnetic fields. However, it is unknown whether phonons can be transmitted across a...