Nanoslit Pores Research Articles

Comparisons between full molecular dynamics simulation and Zhang's multiscale scheme for nanochannel flows.

In a very small surface separation, the fluid flow is actually multiscale consisting of both the molecular scale non-continuum adsorbed layer flow and the intermediate macroscopic continuum fluid flow. Classical simulation of this flow often takes over large computational source and is not affordable owing to using molecular dynamics simulation (MDS) to model the adsorbed layer flow, if the flow field size is on the engineering size scale such as of 0.01-10mm or even bigger like occurring in micro or macro hydrodynamic bearings. The present paper uses full MDS to validate Zhang's multiscale flow model, which yields the closed-form explicit flow equations respectively for the adsorbed layer flow and the intermediate continuum fluid flow. Here, full MDS was carried out for the pressure-driven flow of methane in the nano slit pore made of silicon respectively with the channel heights 5.79nm, 11.57nm, and 17.36nm. According to the number density distribution, the flow areas were respectively discriminated as the adsorbed layer zone and the intermediate fluid zone. The values of the characteristic parameters for Zhang's multiscale scheme were extracted from full MDS and input to Zhang's multiscale flow equations respectively for calculating the flow velocity profile and the volume flow rates of the adsorbed layers and the intermediate fluid. It was found that for these three channel heights, the flow velocity profiles calculated from Zhang's model approximate those calculated from full MDS, while the total flow rates through the channel calculated from Zhang's model are close to those calculated from full MDS. The accuracy of Zhang's multiscale flow model is improved with the increase of the channel height. The recent modification of the optimized potential for liquid simulation (MOPLS) model was used to calculate the interaction force between two methane molecules. In order to calculate the interaction force between the wall atoms and the methane molecules accurately, our previous non-equilibrium multiscale MDS was used. The interaction forces between the methane molecule and the wall atom were obtained from the coupled potential function by the L-B mixing rule when the fluid molecules arrived at near wall. The methane molecule diameter was obtained from the radial distribution function by using equilibrium MDS under the same initial conditions. The local viscosities across the adsorbed layer were obtained from the local velocity profile by using the Poiseuille flow method. The motion equation of the methane molecule was solved by the leapfrog method. The temperature of the simulation system was checked by Bhadauria's method, i.e., the system temperature was rectified by the velocities in the y- and z-directions. The flow velocity distributions across the channel height and the volume flow rates through the channel were also calculated from Zhang's closed-form explicit flow equations respectively for the adsorbed layer flow and the intermediate fluid flow. The results respectively obtained from full MDS and Zhang's multiscale flow equations were then compared.

Competitive transport and adsorption of CO2/H2O in the graphene nano-slit pore: A molecular dynamics simulation study

The study of pore structure, surface sites, and the competitive adsorption and mass transfer of CO2, particularly in conjunction with H2O, in porous solid materials is crucial for developing CO2 capture materials with superior performance and achieving carbon emission reduction goals. This paper presents the development and investigation of amino-modified graphene lamellae pore models using standard number of particles, volume, and temperature system integrated molecular dynamics simulations in a non-equilibrium state. The findings reveal that CO2 adsorption on the pore surface of graphene lamellae occurs in a two-layered manner, influenced by van der Waals and electrostatic forces. The adsorption capacity of the initial layer at a constant temperature is contingent upon the material’s adsorption area, while the site density determines the second layer’s adsorption capacity. Increasing the site density from 0/Å2 to 0.008/Å2 enhances the number of CO2 molecules in the second layer by approximately 35.6 %, though it significantly reduces their diffusion rate. Additionally, changes in temperature and pore size predominantly affect the diffusion rate and CO2 adsorption capacity. The study also explores the competitive adsorption dynamics of H2O molecules with CO2 in the pore channels. It was observed that at low temperatures, H2O molecules predominantly undergo surface diffusion, occupying the pore channel surfaces and thus impacting CO2 adsorption. At a CO2:H2O ratio of 1:1, H2O molecules cover 54.1 % of the adsorption area and may form clusters that obstruct the pore channels, thereby hindering CO2 diffusion. Calculations of surface diffusion rates for H2O molecular clusters at various CO2/H2O ratios indicate that smaller clusters exhibit higher diffusion rates. The computational approach employed in this study demands minimal resources and enables swift calculations, offering a new theoretical model for optimizing adsorbent design, enhancing CO2 capture efficiency, and exploring novel adsorbent materials.

Finding Infinities in Nanoconfined Geothermal Electrolyte Static Dielectric Properties and Implications on Ion Adsorption/Pairing.

Infinities should naturally occur in the dielectric responses of ionic solutions relevant to many geochemical, energy storage, and electrochemical applications at a strictly zero frequency. Using molecular dynamics simulations cross-referenced with coarse-grained Monte Carlo models, using nanoslit pore models at hydrothermal conditions, and treating confined mobile charges as polarization, we demonstrate the far reaching consequences. The dielectric permittivity profile perpendicular to the slit (ϵ⊥(z)) increases, not decreases, with ionic concentration, unlike in the more widely studied megahertz-to-gigahertz frequency range. In confined electrolytes, the divergences in ϵ⊥(z) correctly describe crossovers between bulk- and surface-dominated dielectric behavior. Nanoconfinement at low ionic concentrations changes monovalent ion energetics by 1-2 kJ/mol, but no dielectric property studied so far is universally correlated to ion adsorption or ion-ion interactions. We caution that infinities signal violation of the "electrical insulator" dielectric assumption.

Occurrence Characteristics of Water in Nano-Slit Pores under Different Solution Conditions: A Case Study on Kaolinite.

The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl2 solution, and a H2O/CH4 mixed phase were created at varied temperatures (80-120 °C) and pressures (70-120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si-O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH4-H2O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas-solid interactions and solid-liquid-gas interactions. CH4 molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.

Does twist angle affect the properties of water confined inside twisted bilayer graphene?

Graphene nanoslit pores are used for nanofluidic devices, such as, in water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices, because a small misalignment leads to a significant alteration in various properties of the devices. Here, we focus on the rotational misalignment between two parallel graphene sheets. Using molecular dynamics simulation, we probe the structure and dynamics of monolayer water confined inside graphene nanochannels for a range of commensurate twist angles. With SPC/E and TIP4P/2005 water models, our simulations reveal the independence of the equilibrium number density- n ∼ 13nm-2 for SPC/E and n ∼ 11.5nm-2 for TIP4P/2005- across twists. Based on the respective densities of the water models, the structure and dielectric constant are invariant of twist angles. The confined water structure at this density shows square ice ordering for SPC/E water only. TIP4P/2005 shows ordering at the vicinity of a critical density (n ∼ 12.5nm-2). The average perpendicular dielectric constant of the confined water remains anomalously low (∼2 for SPC/E and ∼6 for TIP4P/2005) for the studied twist angles. We find that the friction coefficient of confined water molecules varies for small twist angles, while becoming independent for twists greater than 5.1°. Our results indicate that a small, angular misalignment will not impair the dielectric properties of monolayer water within a graphene slit-pore, but can significantly influence its dynamics.

Understanding gas-enhanced methane recovery in graphene nanoslits via molecular simulations

Shale gas and coalbed methane are energy sources that mainly consist of methane stored in an adsorbed state in the pores of the organic-rich rock and coal seams. In this study, the graphene nanoslit model is employed to model the nanometer slit pores in shale and coal. Grand canonical Monte Carlo and molecular dynamics modeling methods are used to investigate the mechanisms of adsorption and displacement of methane in graphene-based nanoslit pores. It is found that as the width of the slit pore increases, the adsorption amount of gas molecules increases, and the number density profile of adsorbed methane molecules alters from monolayer to multilayer adsorption. The minimum slit pore width at which methane molecules can penetrate the slit pore is determined to be 0.7 nm. Moreover, it is demonstrated that by lowering the temperature, the adsorption rate of the methane increases since the adsorption is an exothermic process. Enhancing methane recovery was investigated by the injection of gases such as CO2 and N2 to displace the adsorbed methane. The comparison of adsorption isotherms of gas molecules provides the following order in terms of the amount of adsorption, CO2 > CH4 > N2, for the same slit pore width and the same temperature and pressure conditions. Cited as: Bekeshov, D., Ashimov, S., Wang, Y., Wang, L. Understanding gas-enhanced methane recovery in graphene nanoslits via molecular simulations. Capillarity, 2023, 6(1): 1-12. https://doi.org/10.46690/capi.2023.01.01

Anisotropy of the Proton Kinetic Energy as a Tool for Capturing Structural Transition in Water Confined in a Graphene Nanoslit Pore.

The proton dynamics of a 2D water monolayer confined inside a graphene slit pore is studied in Cartesian and molecular frames of reference using molecular dynamics simulations. The vibrational density of states of the proton was calculated versus temperature and was further used to deduce the mean kinetic energy of the hydrogen atoms, Ke(H), in both frames of reference. The directional components of Ke(H) are in good agreement with experimental observations for bulk as well as nanoconfined water. Nonetheless, while in the molecular frame of reference the effect of temperature on the anisotropy ratios of Ke(H) (the ratio between its directional components) are practically invariant between the 2D and 3D cases, those in the Cartesian frame of reference reveal a rather notable reduction across 200 K, indicating the occurrence of an order-disorder transition. This result is further supported by the calculated entropy and enthalpy of the confined water molecules. Overall, it is shown that Ke(H) anisotropy ratios may serve as a valuable order parameter for detecting structural transformations in hydrogen bonds containing molecular systems.

Fabrication of Plasmonic Nano-Slit Pore Double Layer for Single Molecule Detection

We fabricated the plasmonic nano-slit pore platforms for single molecule detection and other various applications. The nano-apertures on the Au/insulator/Au/SiN flat membrane, in addition to the 200 nm thick Au film on ~ 20 nm thick SiN membrane were fabricated using a focused ion beam drilling technique. The insulating materials of SiO2 or Al2O3 were deposited using an atomic layer epitaxy technique. The nanopores with a few nanometer size were fabricated under the proper control of drilling depth and pore opening. Optical characteristics were carried out dependent upon the pore opening width and sample thickness. The broad emission spectra from the (7x 7) slit pore array are obtained via spp-mediated emission on the ~200 nm thick Au/~ 20 nm thick SiN flat samples. A sharp strong infrared emission peak is also obtained due to Au nanoparticle. The fabricated Au double layer platform will be investigated for a single molecule sensor device and a possible radiative thermal energy transfer device.

Toward Next Generation Plasmonic Nanopore Slit Platform with a ~10 nm Slit-Width

We fabricated various nanoaperture plasmonic platforms for single-molecule detection. We fabricated nanoapertures like nanopores on a pyramid and nanoslits on an Au flat membrane using a Ga ion focused ion beam drilling technique, followed by irradiating with a high energy electron beam, dependent on the electron beam current density to obtain nanoapertures with a few nanometer sizes (circular nanopore, nanoslit pores). We examined their optical characteristics with varying aperture sizes and sample thicknesses. We obtained broad emission spectra in the visible and infrared region from the (7 x 7) slit array and a sharp, strong infrared emission peak from the Au nanoparticle on the substrate. The fabricated Au platform with ~10 nm nanometer opening can be employed as a single-molecule sensor and an infrared thermal emission device.

Optical Characteristics of the Nano-Apertures on the Pyramid

Recently there have been tremendous interests for developing the single molecule analysis device. We have previously reported the fabrication of the nanopores and nano-slit pores on the various nanostructures such as the Au-coated pyramid and the flat membrane for optical detection detection device. In this report, we address the fabrication of the various nano-aperture optical platform, and optical characterization of the fabricated devices. For aperture opening less than 50 nm, the strong spp-mediated intraband transmission peak at 500 nm is presented, while for the aperture size greater than 200 nm, the surface plasmonic coupling peaks among the nano-apertures are observed. As an example, we present the optical intensities measured from the various size of the nanopores on pyramid as in figure. The fabricated spp-mediated optical intensites from the optical platform device can be utilized for the next generation optical detection device. Figure 1

Electrode surface modification of graphene-MnO2 supercapacitors using molecular dynamics simulations.

In this study, molecular dynamics (MD) simulations have been performed to explore the variation of ion density and electric potential due to electrode surface modification. Two different surface morphologies, having planer and slit pore with different conditions of surface charge, have been studied for graphene-MnO2 surface using LAMMPS. For different pore widths, the concentration of ions in the double layer is observed to be very low when the surface of the graphene-MnO2 electrode is charged. With a view to identify the optimal pore size for the simulation domain considered, three different widths for the nano-slit type pores and the corresponding ion-ion interactions are examined. Though this effect is negligible for pores with 9.23 and 3.55Å widths, a considerable increase in the ionic concentration within the 7.10Å pores is observed when the electrode is kept neutral. The edge region of these nano-slit pores leads to effective energy storage by promoting ion separation and a significantly higher charge accumulation is found to occur on the edges compared to the basal planes. For the simulation domain of the present study, partition coefficient is maximum for a pore size of 7.10Å, indicating that the ions' penetration and movement into nano-slit pores are most favorable for this optimum pore size for MnO2-graphene electrodes with aqueous NaCl electrolyte. Graphical Abstract The importance of understanding the commercial feasibility of supercapacitor material has made qualitatively predicting the optimized electrode structure one of the main targets of energy related researches. While great progress has been made in recent years, a coherent theoretical picture of the optimized electrode structure remains elusive. This article discusses the most favorable design of supercapacitor electrode for ion-electrode interaction.

Polyvinylbutyral-Polyaniline Nanocomposite for High Microwave Absorption Efficiency.

A coatable polyvinylbutyral (PVB)–polyaniline (PANI) nanocomposite was designed for high microwave absorption efficiency. The maximum absorption efficiency 88.2 dB GHz/mm was obtained for the PANI nanofiber-loaded PVB (PVBPN) nanocomposite with a large bandwidth, whereas a pristine PANI-containing composite shows 53.5 dB GHz/mm in the frequency range 8.2–18 GHz. The presence of nanoslit pores in PVBPN also helps to achieve a large bandwidth and hence high microwave absorption efficiency. Standard electromagnetic simulation also shows that power absorbed by the PVBPN nanocomposite is high and its ultrathin coating over the dielectric substrate (epoxy) is promising for broadband tuneable reflection loss.

Size effect on nanochannel flow explored by the flow factor approach model

The flow factor approach model (FFAM) was newly compared with molecular dynamics simulation (MDS) when simulating the Poiseuille flow in a nano slit pore where the effect of the solidified layer on the confining wall may be considerable. For direct comparisons, the MDS results were for no fluid-wall interfacial slippage under a relatively strong fluid-wall interaction; In this case, certain solidified layers may occur on both the confining walls. The comparisons show that these two approaches match well not only in the flow velocity profiles across the channel height but also in the volume flow rates through the channel. The study shows that when the total thickness of the solidified layers on the walls is comparable to the channel height, the effective channel height which is the nominal channel height minus the total thickness of the solidified layers on the walls, may need to be incorporated by FFAM for simulating a nanochannel flow; Otherwise, the effect of the solidified layer on the wall is negligible.

Effect of wall surface modification in the combined Couette and Poiseuille flows in a nano channel

The paper analytically investigates the influence of wall surface modification on the mass transfer in a nano slit pore when the Couette and Poiseuille flows simultaneously occur, by using the flow factor approach model. The coupled walls forming the nano channel are perfectly smooth and may be parallel to one another or form a convergent gap with a small tilting angle. One wall was stationary and the other wall was sliding against it. It was found that for homogenous surface properties of both the coupled walls, a stronger fluid-wall interaction gives a larger mass flow rate of the fluid through the channel because of the physical adsorption and non-continuum effects of the fluid. This effect is especially significant for small wall separations. However, inhomogeneous surface properties of the stationary wall giving different fluid-wall interactions respectively in the inlet and outlet zones of the channel were found to give the fluid mass flow rate considerably larger than homogeneous wall surface properties give, when the tilting angle between the walls is relatively large and the wall separation is small. In this circumstance, even a mixture of surface properties giving the medium-level and weak fluid-wall interactions respectively in the inlet and outlet zones of the channel can give a considerably larger fluid mass flow rate than a strong fluid-wall interaction gives in the case of homogeneous surface properties, when the relative slip amounts in the inlet or outlet zones of the channel are respectively positive and negative but with large magnitudes. For the designed inhomogeneous surface properties, the fluid mass flow rate is more sensitive to the interfacial slippage in the inlet zone than that for homogeneous surface properties, while it is much less sensitive to the interfacial slippage in the outlet zone than that for homogeneous surface properties.

Effect of wall surface roughness on mass transfer in a nano channel

The effect of wall surface roughness on the fluid mass flow rate through a nano slit pore in the Poiseuille flow was analytically studied by using the flow factor approach model. One wall surface was perfectly smooth and the other coupled wall surface was periodically distributed with rectangular protrusions or dents. Weak, medium-level and strong fluid–wall interactions were respectively considered. It was shown that even the wall surface protrusion with a small height significantly reduces the mass flow rate through the channel in spite of the fluid–wall interaction; this effect is especially significant for a strong fluid–wall interaction. The increase of the width of the wall surface protrusion also significantly reduces the mass flow rate through the channel. The effect of the wall surface protrusion is weakened with the increase of the channel height. The wall surface dent was found to increase the mass flow rate through the channel and the mass flow rate is enhanced with the increases of both the depth and the width of the dent. This effect is more significant for a stronger fluid–wall interaction. However, for deep dents, the effect of the wall surface dent on the mass flow rate is almost independent on both the depth of the dent and the fluid–wall interaction. While for narrow dents, the effect of the wall surface dent is negligible in spite of the fluid–wall interaction. The effect of the wall surface dent is also weakened with the increase of the channel height.

Calculating the maximum flowing velocity of the Poiseuille flow in a nanochannel by the flow factor approach model

The paper provides a calculation of the absolute value of the maximum flowing velocity of the Poiseuille flow across the wall separation in a nanoslit pore from the flow factor approach model, based on the input data known from the molecular dynamics simulation (MDS) results. It was found that this calculation matches well with the MDS result.

Investigation of some well known bulk fluid regularities for Lennard–Jones confined fluid in nanoslit pore

The modified fundamental measure theory has been employed to investigate some well known regularities of bulk fluid for the Lennard–Jones fluid confined in nanoslit pore. The regularities investigated include common compression point, common bulk modulus point, Tait–Murnaghan equation, and the linear regularity between pressure and temperature for each isochore. All these regularities have been investigated for two different components of pressure for confined fluid. Our results show that the common compression and common bulk modulus point remain valid for fluids confined in nanoslit pores of different sizes and with different wall–fluid potentials. The density of the common compression and common bulk modulus point are different from corresponding ones for the bulk fluid. Our observations also show that the Tait–Murnaghan equation and pressure–temperature linear regularity also hold for confined fluid. The sign of the intercept of pressure–temperature regularity is determined by the difference between attraction and repulsion terms in the compressibility factor.

On the adsorption and hydrodynamics behavior of H2S and CO2 molecules in organic liquids inside nanoslit pores in vicinity of calcite [formula omitted] surface

Confinement-induced structural anisotropy was studied in a variety of hydrocarbon fluid/H2S/H2O/CO2 mixtures in vicinity of calcite {101¯4} surfaces, using the molecular dynamics (MD) technique under canonical ensemble. The effect of ions presence was, in addition, analyzed by incorporating salt ions into the simulation box. Results suggest ordered arrangements for pure H2O/H2S molecules, inside the adsorption layer over calcite surface. Confinement was found to intricately affect the distribution of water molecules near calcite surface (in the mixture state), such that the bulk of water molecules were situated in the middle-region of the simulation box (far from the calcite surface). The presence of salt ions decreased the thickness of the water adsorption layer (in the mixture state), and had a decreasing effect on the self-diffusivity of water molecules.

The flow factor approach model for the fluid flow in a nano channel

The flow factor approach model was re-visited for the fluid flow in a nano slit pore. According to this model, the velocity profiles of the Couette and Poiseuille flows of a confined fluid were respectively calculated for different fluid–wall interactions and different wall separations. The calculation showed that the velocity profile across the film thickness of the Couette flow of a confined fluid is normally considerably distorted because of the fluid–wall interaction, compared with the conventional theory description. Such a distortion is more severe for a smaller wall separation. However, the calculated average velocity across the film thickness of the confined fluid in the Couette flow is still equal to that calculated from the conventional theory. For the same pressure gradient, the magnitude of the velocity of the Poiseuille flow calculated from this model is normally significantly smaller than that calculated from the conventional theory. Such a deviation is more severe for a stronger fluid–wall interaction. As a combined effect, the total volume flow rate through the wall gap of the confined fluid is greater than that calculated from the conventional theory when the Couette and Poiseuille flows are in contrary directions, while it is smaller than that calculated from the conventional theory when these two flows are in the same direction. Such differences are greater for stronger fluid–wall interactions. This may be one of the reasons why the generated hydrodynamic pressures and load-carrying capacity of a molecular-scale film lubricated contact both are greater than those calculated from conventional hydrodynamic lubrication theory. The results calculated from this model were compared with the molecular dynamics simulation results for the same operating conditions. Very good agreements between them were obtained.

PREDICTION OF FLUID SLIP AT GRAPHENE AND CARBON NANOTUBE INTERFACES

The hydrodynamic boundary condition is now a subject of greater interest than ever before, even though the problem of formulating the correct boundary condition has existed from the beginning of the 19th century. Since then, many researchers have attempted to formulate a general boundary condition for fluid-solid interfaces. The 21st century has seen revolutionary advancement in nanoscale science and technology, which in turn, poses many fundamental questions about the nature of fluid flow in nanometric pores such as carbon nanotubes (CNTs) and aquaporins. Among them, one of the most important is the boundary condition. In this work, based on a statistical mechanics approach, we present a method to calculate the intrinsic interfacial friction coefficient between a fluid and solid at a planar and cylindrical interface, which determines the slip and boundary condition. We apply the method in conjunction with equilibrium molecular dynamics (EMD) simulation technique to fluids such as argon, methane and water flowing in planar graphene nanoslit pores and CNTs. We compare our model predictions against direct non-equilibrium molecular dynamics (NEMD) simulations and find excellent agreement. We identify several limitations of generally used NEMD methods to predict the slip and boundary condition and show that great care needs to be taken in analyzing the results of NEMD slip data for high-slip systems. We suggest some procedures to increase the reliability of the slip estimates. We also study the shear rate and external field dependent behavior of slip in Couette and Hagen-Poiseuille type flows. The slip length remains constant (indicating a linear response of the fluid to the external perturbation) only for low shear rates/external fields and as the field increases, the slip length increases rapidly. At these high fields the Navier-slip model breaks down. We attempt to resolve the highly debated issue of flow rates of water in carbon nanotubes, the values of which are scattered over 1 to 5 orders of magnitude in literature. We accurately predict these flow rates using both the CNT diameter dependent interfacial friction coefficient between water-CNTs and NEMD simulations streaming velocity profiles. Very narrow tubes show higher flow rate enhancements and as the tube diameter increases, the flow rates approach classical Navier-Stokes predictions with the no slip boundary condition. As the diameter of the tube increases, the slip length decreases monotonically and asymptotically approaches a constant value, which is equal to the slip length on a planar graphene surface. Our model gives the linear regime slip length which corresponds to experimental condition flow rates, which is otherwise cumbersome to find using NEMD simulation techniques. The proposed method is robust, general and can be used to find the slip and boundary condition accurately at any fluid-solid interface.