Graphene Nanocapillaries Research Articles

Capillary Condensation of Water in Graphene Nanocapillaries.

Recent experiments have revealed that the macroscopic Kelvin equation remains surprisingly accurate even for nanoscale capillaries. This phenomenon was so far explained by the oscillatory behavior of the solid-liquid interfacial free energy. We here demonstrate thermodynamic and capillarity inconsistencies with this explanation. After revising the Kelvin equation, we ascribe its validity at nanoscale confinement to the effect of disjoining pressure. To substantiate our hypothesis, we employed molecular dynamics simulations to evaluate interfacial heat transfer and wetting properties. Our assessments unveil a breakdown in a previously established proportionality between the work of adhesion and the Kapitza conductance at capillary heights below 1.3 nm, where the dominance of the work of adhesion shifts primarily from energy to entropy. Alternatively, the peak density of the initial water layer can effectively probe the work of adhesion. Unlike under bulk conditions, high confinement renders the work of adhesion entropically unfavorable.

Puckered Zigzag Monolayer Ice: Does a Confined Flat Four-Coordinated Monolayer Ice Always Have a Corresponding Puckered Phase?

We note that a flat, four-coordinated monolayer ice under confinement always has a corresponding puckered phase. Recently, a monolayer ice consisting of an array of zigzag water chains (ZZMI) predicted by first-principles calculations of water under confinement is a flat four-coordinated monolayer ice. Herein, to investigate whether puckered ZZMI exists stably, we perform molecular dynamics simulations of two-dimensional (2D) ice formation for water constrained in graphene nanocapillaries. We find a novel monolayer ice structure that can be viewed as the ZZMI puckered along the direction perpendicular to the zigzag chain (pZZMI). Unlike ZZMI that does not satisfy the ice rule, each water molecule in pZZMI can form four hydrogen bonds (HBs) via forming two stable intersublayer HBs and two intrasublayer HBs. This work provides a fresh perspective on 2D confined ice, highlighting the intrinsic connections between 2D confined ices.

Variation of Critical Crystallization Pressure for the Formation of Square Ice in Graphene Nanocapillaries

Variation of Critical Crystallization Pressure for the Formation of Square Ice in Graphene Nanocapillaries

A Deep Neural Network Potential for Water Confined in Graphene Nanocapillaries

Water under nanoconfinement exhibits structural and dynamical properties remarkably different from those of bulk water, and empirical potential-based molecular dynamics has played an important role in furthering our understanding of the behavior of confined water. However, existing potentials for water were commonly parametrized based on the properties of the bulk water and may be unreliable for describing nanoconfined water. Here, we develop a machine learning potential for water confined in graphene nanocapillaries using deep neural networks trained on quantum mechanical density-functional theory (DFT) calculations. This deep neural network potential offers near-DFT accuracy in terms of potential energy and atomic forces but at a computational cost much lower than DFT-based ab initio molecular dynamics methods. In particular, this potential reproduces well the DFT reference for a wide range of properties, including O–H bond length distribution, density distribution, radial distribution functions, hydrogen bonding, etc. The developed deep neural network potential opens the door to simulations of nanoconfined water with large system sizes and time scales at near-DFT accuracy.

Anomalously low friction of confined monolayer water with a quadrilateral structure.

In this work, we explored how the structure of monolayer water confined between two graphene sheets is coupled to its dynamic behavior. Our molecular dynamics simulations show that there is a remarkable interrelation between the friction of confined water with two walls and its structure under extreme confinement. When the water molecules formed a regular quadrilateral structure, the friction coefficient is dramatically reduced. Such a low-friction coefficient can be attributed to the formation of long-range ordered hydrogen bond network, which not only decreases the structure corrugation in the direction perpendicular to the walls but also promotes the collective motion of the confined water. The regular quadrilateral structure can be formed only if the number density of confined water falls within a certain range. Higher number density results in larger structure corrugations, which increases the friction, while smaller number density leads to an irregular hydrogen bond network in which the collective motion cannot play the role. We demonstrated that there are four distinct stages in the diagram of the friction coefficient vs the number density of confined water. This research clearly established the connection between the dynamic characteristics of confined monolayer water and its structure, which is beneficial to further understand the mechanism of the high-speed water flow through graphene nanocapillaries observed in recent experiments.

Microscopic Kinetics Pathway of Salt Crystallization in Graphene Nanocapillaries.

The fundamental understanding of crystallization, in terms of microscopic kinetic and thermodynamic details, remains a key challenge in the physical sciences. Here, by using insitu graphene liquid cell transmission electron microscopy, we reveal the atomistic mechanism of NaCl crystallization from solutions confined within graphene cells. We find that rock salt NaCl forms with a peculiar hexagonal morphology. We also see the emergence of a transitory graphitelike phase, which may act as an intermediate in a two-step pathway. With the aid of density functional theory calculations, we propose that these observations result from a delicate balance between the substrate-solute interaction and thermodynamics under confinement. Our results highlight the impact of confinement on both the kinetics and thermodynamics of crystallization, offering new insights into heterogeneous crystallization theory and a potential avenue for materials design.

Thermal performance enhancement and optimization of two-phase closed thermosyphon with graphene-nanoplatelets coatings

The enhancement of phase-change heat transfer in the two-phase closed thermosyphon (TPCT) coated with graphene-nanoplatelets (GNPs) of different wettability is investigated. The intrinsically hydrophobic GNPs can be modified to become superhydrophilic through a simple thermal curing process. The performance of a TPCT is governed by three major processes, namely evaporation, condensation and circulation of condensate. By benchmarking with the uncoated TPCT, we experimentally examine the thermal enhancement and comprehend the thermal characteristics of the GNPs-coated TPCTs with different wettability. Endowed with the unique rapid water permeation property, the superhydrophilic graphene nanocapillaries spread the water film to a larger surface area which is favourable to the evaporation; a maximum enhancement of 96.2% is achieved by employing the Jakob number as an indicator. The uncured hydrophobic GNPs-coating is well suited to the condensation process that leads to an enhancement of 20.6%. By evaluating the minima of the total thermal resistance of the TPCT, the thermal optimization can be achieved by determining the optimized length ratio of the evaporator length to the condenser length. Computationally, molecular dynamic simulations are performed to provide insights on the rapid water permeation process that leads to the filmwise evaporation in the cured superhydrophilic GNPs-coating. This study is particularly of great interest for the potential applications of the graphene functionalized coatings in the field of phase-change heat transfer.

Extraordinarily enhanced evaporation of water droplets on graphene-nanostructured coated surfaces

Functionalized graphene coating possessing the unique property of ultrafast water permeation manifests itself in enhancing the evaporation heat transfer. The intermolecular interactions between the intercalated water molecules with the hydrophobic carbon atoms and the oxygenated functional groups in the graphene nanocapillaries facilitate the magnificent spreadability and permeability of water on a heated surface. The fast water permeation property of graphene can be controlled through a thermal curing process at different curing temperatures. Here we experimentally investigate the application of graphene nanoplatelets (GNPs) coatings for the enhancement of evaporation of small sessile water droplets at sub-boiling temperatures. The ultrafast water permeation within the functionalized nanostructured GNPs facilitates the fast absorption of latent heat of evaporation, leading to the anomalously effective filmwise evaporation. Ultimately, the cooling effectiveness of a water droplet on the GNPs-coated surface is extraordinarily enhanced by an unprecedented maximum of 4866% at a surface temperature of 75 °C compared against the bare surface. These results envision the great potential of graphene-based coatings to address the issues of increased heat generated from the high power density electronics, leading to enhanced performance and extended component lifespans.

Performance enhancement of graphene-coated micro heat pipes for light-emitting diode cooling

The rate of water transport through graphene nanocapillaries is profoundly enhanced compared to that in microscale capillaries due to the prevalence of exceptionally high capillary pressures and large slip lengths. As an inaugural study, we integrate graphene nanocapillaries into a micro heat pipe (MHP) for enhanced light-emitting diode (LED) cooling. With the use of graphene nanocapillaries, the ultrafast water transport synergically enhances the water circulation and evaporation process in the microfluidic device. The graphene-coated MHP achieves more than 45% enhancement in the overall performance compared to the uncoated counterpart. In turn, the experiments demonstrate a drastic reduction of LED's operating temperature (more than 25 °C) which translates into a significantly prolonged lifespan of LED. The molecular dynamics simulations reveal that the oxygenated functional groups attached on graphene further increase the capillary pressure (~1000 bar) and effective velocity (~20 m/s) of the nanoconfined water, compared to those (~500 bar and ~10 m/s) in a pristine graphene nanochannel. The ultrafast water transport in graphene nanocapillary is justified. This study provides a holistic analysis and important insight into the phenomenon of ultrafast water transport in graphene nanocapillaries that exhibits an enormous potential in thermal energy management applications for LED cooling.

Structure and Phase Behavior of the Confined Water in Graphene Nanocapillaries Studied by ABEEMσπ Polarizable Force Field

As the two-dimensional square ice in graphene nanocapillaries was observed by transmission electron microscopy (TEM), a variety of theoretical methods have been applied to explore this phenomenon. ...

Replica exchange MD simulations of two-dimensional water in graphene nanocapillaries: rhombic versus square structures, proton ordering, and phase transitions.

The hydrogen bond patterns, proton ordering, and phase transitions of monolayer ice in two-dimensional hydrophobic confinement are fundamentally different from those found for bulk ice. To investigate the behavior of quasi-2D ice, we perform molecular dynamics simulations of water confined between fixed graphene plates at a distance of 0.65 nm. While experimental results are still limited and theoretical investigations are often based on a single, often empirically based force field model, this work presents a systematic study modeling the water-graphene interaction by effective Lennard-Jones potentials previously derived from high-level ab initio CCSD(T) calculations of water adsorbed on graphene [Phys. Chem. Chem. Phys., 2013, 15, 4995]. For the water-water interaction different water force fields, i.e. SPCE, TIP3P, TIP4P, TIP4P/ICE, and TIP5P, are used. The water occupancy of the graphene capillary at a pressure of 1000 MPa is determined to be between 13.5 and 13.9 water molecules per square nanometer, depending on the choice of the water force field. Based on these densities, we explore the structure and dynamics of quasi-2D water for temperatures ranging from 200 K to about 600 K for each of the five force fields. To ensure complete sampling of the configurational space and to overcome the barriers separating metastable structures, these simulations are based on the replica exchange molecular dynamics technique. We report different tetragonal hydrogen bond patterns, which are classified as nearly square or as rhombic. While many of these arrangements are essentially flat, in some cases puckered arrangements are found, too. Also the proton ordering of the quasi-2D water structures is considered, allowing us to identify them as ferroelectric, ferrielectric or antiferroelectric. For temperatures between 200 K and 400 K we find several second-order phase transitions from one ice structure to another, changing in many cases both the arrangements of the oxygen atoms and the proton ordering. For temperatures between 400 K and 600 K there are melting-like transitions from a monolayer of ice to a monolayer of liquid water. These first-order phase transitions have a latent heat between 3.4 and 4.0 kJ mol-1. Both the values of the transition temperatures and of the latent heats display considerable model dependence for the five different water models investigated here.

Dynamic behavior of nanoscale liquids in graphene liquid cells revealed by in situ transmission electron microscopy

Recent advances in graphene liquid cells for in situ transmission electron microscopy (TEM) have opened many opportunities for the study of materials transformations and chemical reactions in liquids with high spatial resolution. However, the behavior of thin liquids encapsulated in a graphene liquid cell has not been fully understood. Here, we report real time TEM imaging of the nanoscale dynamic behavior of liquids in graphene nanocapillaries. Our observations reveal that the interfaces between liquid and gas bubble can fluctuate, leading to the generation of liquid nanodroplets near the interfaces. Liquid nanodroplets often show irregular shape with dynamic changes of their configuration under the electron beam. We consider that the dynamic motion of liquid-gas interfaces might be introduced by the electrostatic energy from transiently charged interfaces. We find that improving the wettability of graphene liquid cells by ultraviolet-ozone treatment can significantly modify the dynamic motion of the encapsulated liquids. Our study provides valuable information of the interactions between liquid and graphene under the electron beam, and it also offers key insights on the nanoscale fluid dynamics in confined spaces.

Publisher's Note: “Fast water flow through graphene nanocapillaries: A continuum model approach involving the microscopic structure of confined water” [Appl. Phys. Lett. 113, 083101 (2018)

Publisher's Note: “Fast water flow through graphene nanocapillaries: A continuum model approach involving the microscopic structure of confined water” [Appl. Phys. Lett. <b>113</b>, 083101 (2018)

Fast water flow through graphene nanocapillaries: A continuum model approach involving the microscopic structure of confined water

Water inside a nanocapillary becomes ordered, resulting in unconventional behavior. A profound enhancement of water flow inside nanometer thin capillaries made of graphene has been observed [Radha et al., Nature (London) 538, 222 (2016)]. Here, we explain this enhancement as due to the large density and the extraordinary viscosity of water inside the graphene nanocapillaries. Using the Hagen-Poiseuille theory with slippage-boundary condition and incorporating disjoining pressure term in combination with results from molecular dynamics simulations, we present an analytical theory that elucidates the origin of the enhancement of water flow inside hydrophobic nanocapillaries. Our work reveals a distinctive dependence of water flow in a nanocapillary on the structural properties of nanoconfined water in agreement with experiment, which opens a new avenue in nanofluidics.

Structure and transport of confined liquid in nanochannels

In recent years, the mass transport through nanochannels has received widespread attention from the academic and engineering communities, as well as industry, not only due to its cutting-edge significance in physical mechanics of surfaces and interfaces, but also because of its crucial application prospects in the fields of energy, environment, and health. At the nanometer scale, the interface effect and intermolecular interactions are prominent. The free movement of liquid molecules is confined, and its structure also exhibits layering, ordering, and other characteristics. Due to this fact, the transport of confined liquid yields distinguishable results from those at the macroscale. Both the governing equations and boundary conditions for nanoflow are worth further investigation. The understanding on the structure and transport of confined liquid also plays an important role in the applications of microfluidic and nanofluidic technologies. As the length scale of nanodevices decreases continuously, the surface and interface effects become significant. The structure and dynamic behaviours of confined liquid are dramatically different from that of the bulk phase. At the nanoscale, several physical mechanisms are intercoupling; various intermolecular and surface forces compete with each other. There are many characteristic length scales which fall in the range from a few angstroms to several nanometers. This review article focuses on the structure and transport of confined liquids, and systematically reviews the recent research progress in this field. Starting from the ordered structures of the confined water, we discussed not only the layered structure perpendicular to the confining walls, but also various two dimensional ice phases parallel to the walls. Emphasis was placed on the hot topics of water transport and ion sieving through various kinds of nanochannels, especially for capillaries existing in graphene oxide membranes and nanocapillaries fabricated by van der Waals assembly using two-dimensional materials. The source of the complexity of the ordered structure and transport process of confined liquids was analyzed. We also discussed that slip boundary conditions and driving forces in modeling such process require extra attention and in-depth investigations. It was revealed that the mechanism of liquid flow through nanochannels is dominated by intermolecular forces. Although there are still many unknown questions to be further explored, we tried to point out the possible directions of the organic combination of theory, simulation and experimental research, as well as the interdisciplinary integration.

Phase transitions of ordered ice in graphene nanocapillaries and carbon nanotubes

New phase diagrams for water confined in graphene nanocapillaries and single-walled carbon nanotubes (CNTs) are proposed, identifying ice structures, their melting points and revealing the presence of a solid-liquid critical point. For quasi-2D water in nanocapillaries, we show through molecular-dynamics simulations that AA stacking in multilayer quasi-2D ice arises from interlayer hydrogen-bonding and is stable up to three layers, thereby explaining recent experimental observations. Detailed structural and energetic analyses show that quasi-2D water can freeze discontinuously through a first-order phase transition or continuously with a critical point. The first-order transition line extends to a continuous transition line, defined by a sharp transition in diffusivity between solid-like and liquid-like regimes. For quasi-1D water, confined in CNTs, we observe the existence of a similar critical point at intermediate densities. In addition, an end point is identified on the continuous-transition line, above which the solid and liquid phases deform continuously. The solid-liquid phase transition temperatures in CNTs are shown to be substantially higher than 273 K, confirming recent Raman spectroscopy measurements. We observe ultrafast proton and hydroxyl transport in quasi-1D and -2D ice at 300 K, exceeding those of bulk water up to a factor of five, thereby providing possible applications to fuel-cells and electrolyzers.

Water Confined in Nanocapillaries: Two-Dimensional Bilayer Squarelike Ice and Associated Solid–Liquid–Solid Transition

Despite recent experimental evidence of the two-dimensional (2D) square ice in graphene nanocapillaries, based on transmission electron microscopy (TEM) imaging, the AA-stacked bilayer square ice structure has not been observed in all previous classical molecular dynamics (MD) simulations nor found in recent unbiased first-principles structure searches. Herein, we report the MD simulations of 2D bilayer ice formation for water confined between two parallel hydrophobic walls (nanoslit). We find a bilayer ice whose simulated TEM imaging resembles that of bilayer squarelike ice. This bilayer ice also demonstrates dynamical stability in first-principles phonon computations. The realistic structure of this bilayer ice, however, consists of two hexagonal monolayers with the AB-stacking order, where the hexagonal rings are slightly elongated with two unequal inner angles, 107 and 146° (rather than 120°). The phase diagram of the nanoslit width versus temperature exhibits a solid–liquid–solid triple point, where ...

Structural and dynamic characteristics in monolayer square ice.

When water is constrained between two sheets of graphene, it becomes an intriguing monolayer solid with a square pattern due to the ultrahigh van der Waals pressure. However, the square ice phase has become a matter of debate due to the insufficient experimental interpretation and the slightly rhomboidal feature in simulated monolayer square-like structures. Here, we performed classical molecular dynamics simulations to reveal monolayer square ice in graphene nanocapillaries from the perspective of structure and dynamic characteristics. Monolayer square-like ice (instantaneous snapshot), assembled square-rhombic units with stacking faults, is a long-range ordered structure, in which the square and rhombic units are assembled in an order of alternative distribution, and the other rhombic unit forms stacking faults (polarized water chains). Spontaneous flipping of water molecules in monolayer square-like ice is intrinsic and induces transformations among different elementary units, resulting in the structural evolution of monolayer square ice in dynamics. The existence of stacking faults should be attributed to the spontaneous flipping behavior of water molecules under ambient temperature. Statistical averaging results (thermal average positions) demonstrate the inherent square characteristic of monolayer square ice. The simulated data and insight obtained here might be significant for understanding the topological structure and dynamic behavior of monolayer square ice.

Superheating of monolayer ice in graphene nanocapillaries.

The freezing and melting of low-dimensional materials, either via a first-order phase transition or without any discontinuity in thermodynamic, still remain a matter of debate. Melting (superheating) in two-dimensional (2D) ice is fundamentally different from that in bulk counterpart. Here, we perform comprehensive molecular dynamics simulations of the superheating of monolayer ice in graphene nanocapillaries to understand the nature of melting transition in 2D water/ice. We find four different superheating (melting) scenarios can happen in the superheating of monolayer square-like ice, which are closely related to the lateral pressure and the channel width. The anomalous two-stage melting transition with arisen coexistence phase is found, which reveals the unknown extraordinary characteristics of melting in 2D water/ice. Under ultrahigh lateral pressure, the intermediate monolayer triangular amorphous ice will be formed during the superheating of monolayer square-like ice with both continuous-like and first-order phase transitions. Whereas, under low lateral pressure, the melting in monolayer square-like ice manifests typical discontinuity with notable hysteresis-loop in potential energy during the heating/cooling process. Moreover, we also find that highly puckered monolayer square-like ice can transform into bilayer AB-stacked amorphous ice with square pattern in the superheating process. The superheating behavior under high lateral pressure can be partly regarded as the compression limit of superheated monolayer water. The intrinsic phenomena in our simulated superheating of monolayer ice may be significant for understanding the melting behavior in 2D water/ice.

Superfluidity of the confined water

Superfluidity means non-sticky and frictionless when two bodies are set contacting motion. It seems that a substance can be transported in a channel with high velocity and negligible resistance. As a typical example, helium-4 below 2.17 K will become a superfluid. In this case, the flow rate of helium is independent of the driving pressure, and it is impossible to calculate the significant viscosity coefficient from the experimental data. The research on superfluid and supersolid has attracted considerable attention, which mainly focused on extreme conditions (ultra-low temperature, high pressure, etc.). One may ask that can certain substances show superfluid and supersolid behaviors under room temperature. We already know that the flow rate of water in nanometer channel is several orders of magnitude higher than that predicted by the classical theory of fluid mechanics. The molecular dynamics simulation predicts that the water molecules spontaneously form a two-dimensional solid-like square structure in graphene nanocapillaries. This is considered to be the reason for the rapid passage of water molecules through such nano-channels. Water transport in carbon nanotubes exibit similar behaviours. The friction coefficient of water transport in carbon nanotubes is positively correlated with the curvature radius of the nanotubes. That is, the smaller the radius of carbon nanotubes, the smaller the friction coefficient, the greater the flow enhancement. However, for boron nitride nanotubes, although with a similar structure with the carbon nanotubes, it does not show significant flow enhancement because of the different electrical properties of walls. Based on the recent experimental and theoretical simulations, we discussed the superfluid of liquid water exhibits superfluidity in nanochannels, and pointed out that superfluidity of the confined water under room temperature originates from the hydrogen bond relaxation between the low coordination water molecules and the high elasticity and polarization at the interface. We also revealed that the superfluidity of water and liquid helium has both commonality and individuality. The commonality is the quantum pinning and polarization of the local strain induced by the low coordination of atoms or molecules; the individuality is that the superfluid of liquid helium at ultra-low temperature is driven by the spin-resolved polarization, while the superfluidity of the water in the nanochannels may still need some external forces. There are wide applications for the superfluidity of the confined water under room temperature. In recent years, the experimental facilities and the computer capacity have been improved continuously. It has a significant impact on the understanding and cognition of the water transport mechanism at the nanometer scale. Moreover, it can also provide important reference significance for the study of superfluidity of common substances under conventional conditions. Through this article, we hope to stimulate more scholars’ interest in the basic scientific problem of superfluidity, further enhance the understanding of this problem and combine it with macroscopic superfluidity, super-lubrication, super-hydrophobic and super-solidity.