Molecular dynamics simulation of ion transportation through graphene nanochannels

The model of ion transportation through graphene nanochannels is established by the molecular dynamics simulation method. Statistics of the electric potential and charge distribution are made, respectively, on both sides of graphene nanopore with various diameters. Then, their changing relationship with respect to the nanopore diameter is determined. When applying a uniform electric field, polar water molecules are rearranged so that the corresponding relationship between the polarized degree of these molecules and the nanopore diameter can be created. Based on the theoretical model of ion transportation through nanochannels,the changing relationship between the concentration of anions/cations in nanochannels and bulk solution concentration is quantitatively analyzed. The results show that the increase of potential drop and charge accumulation, as well as a more obvious water polarization, will occur with the decrease of nanopore diameter. In addition, hydrogen ion concentration has a large proportion in nanochannels with a sodium chloride（NaCl） solution at a relative low concentration. As the NaCl concentration increases, the concentration appreciation of sodium ions tends to be far greater than the concentration drop of chloride ions. Therefore, sodium ion concentration makes more contribution to ionic conductance.

High density gas state at water/graphite interface studied by molecular dynamics simulation

In this paper molecular dynamics simulations are performed to study the accumulation behaviour of N2 and H2 at water/graphite interface under ambient temperature and pressure. It finds that both N2 and H2 molecules can accumulate at the interface and form one of two states according to the ratio of gas molecules number to square of graphite surface from our simulation results： gas films （pancake-like） for a larger ratio and nanobubbles for a smaller ratio. In addition, we discuss the stabilities of nanobubbles at different environment temperatures. Surprisingly, it is found that the density of both kinds of gas states can be greatly increased, even comparable with that of the liquid N2 and liquid H2. The present results are expected to be helpful for the understanding of the stable existence of gas film （pancake-like） and nanobubbles.

Molecular dynamics simulation studies on some topics of water molecules on hydrophobic surfaces

Molecular dynamics simulations have been used to study two topics of water molecules on hydrophobic surfaces. Some properties of the nanobubbles with different ingredients and behavior of single water chains in sin- gle-walled carbon nanochannels are exploited. Molecular simulations show that the density of the N2 and H2 are quite high, which is critical for the stability of the nanobubbles and may have potential applications, such as hydrogen storage, incorporated with recent experimental method to controllably produce hydrogen nanobubbles. The water molecules inside the nanochannel show an unexpected directed motion with long time period, which is indispensable in the future study of the dynamics of biological channels.

Molecular dynamics simulation of ion transport in a nanochannel

A molecular dynamics (MD) model of the fluidic electrokinetic transport in a nano-scale channel with two bulk sinks was presented, and the process of ion transport in the nanochannel was simulated in this paper. The model consists of two water sinks at the two ends and a pump in the middle, which is different from a single pump model in previous MD simulations. Simulation results show that the charged surfaces of the nanochannel result in the depletion of co-ions and the enrichment of counterions in the nanochannel. A stable current is induced because of the motion of ions when an external electric field is applied across the nanochannel, and the current in the pump region is mainly induced by the motion of counterions. In addition, the ion number in the pump region rapidly decreases as the external electric field is applied. In the equilibrated system, the electrically neutral character in the pump region is destroyed and this region displays a certain electrical character, which depends on the surface charge. The ion distribution is greatly different from the results predicted by the continuum theory, e.g. a smaller peak value of Na+ concentration appears near the wall. The transport efficiency of counterions (co-ions) can be effectively increased (decreased) by increasing the surface charge density. The simulation results demonstrate that the ion distribution in the electric double layer (EDL) of a nanochannel cannot be exactly described by the classical Gouy-Chapman-Stern (GCS) theory model. The mechanism of some special experimental phenomena in a nanochannel and the effect of the surface charge density on the ion-transport efficiency were also explored to provide some theoretical insights for the design and application of nano-scale fluidic pumps.

Size-dependent surface tension of a cylindrical nanobubble in liquid Ar

In view of the continued disputes on the fundamental question of whether the surface tension of a vapour bubble in liquid argon increases, or decreases, or remains unchanged with the increase of curvature radius, a cylindrical vapour bubble of argon is studied by molecular dynamics simulation in this paper instead of spherical vapour bubble so as to reduce the statistical error. So far, the surface tension of the cylindrical vapour bubble has not been studied by molecular dynamics simulation in the literature. Our results show that the surface tension decreases with radius increasing. By fitting the Tolman equation with our data, the Tolman length σ = -0.6225 sigma is given under cut-off radius 2.5σ, where σ = 0.3405 nm is the diameter of an argon atom. The Tolman length of Ar being negative is affirmed and the Tolman length of Ar being approximately zero given in the literature is negated, and it is pointed out that this error is attributed to the application of the inapplicable empirical equation of state and the neglect of the difference between surface tension and an equimolar surface.

On flow characteristics of liquid-solid mixed-phase nanofluid inside nanochannels

The atomic behavior of liquid-solid mixed-phase nanofluid flows inside nanochannels is investigated by a molecular dynamics simulation （MDS）. The results of visual observation and statistic analysis show that when the nanoparticles reach near each other, the strong interatomic force will make them attach together. This aggrega- tion continues until all nanoparticles make a continuous cluster. The effect of altering the external force magnitude causes changes in the agglomeration rate and system enthalpy. The density and velocity profiles are shown for two systems, i.e., argon （Ar）-copper （Cu） nanofluid and simple Ar fluid between two Cu walls. The results show that using nanopar- ticles changes the base fluid particles ordering along the nanochannel and increases the velocity. Moreover, using nanoparticles in simple fluids can increase the slip length and push the near-wall fluid particles into the main flow in the middle of the nanochannel.

Molecular dynamics study of viscosity of aqueous NaCl solution confined in nanoscale channels

A physical model of bulk-nanochannel-bulk with buffer baths is built up using nonequilibrium molecular dynamics （MD） simulation to study the effects of vibrating silicon atoms on the viscosity of aqueous NaCl solutions confined in the nanochannel. The simulation is performed under different moving speeds of the upper wall, different heights and different surface charge densities in the nanochannel. The simulation results indicate that with the increase in the surface charge density and the decrease in the nanochannel height and the shear rate, the vibration effect of silicon atoms on the shear viscosity of the confined fluid in the nanochannel cannot be ignored. Compared with still silicon atoms, the vibrating silicon atoms result in the decrease in the viscosity when the height of the nanochannel is less than 0.8 nm and the shear rate is less than 1.0 ×10^11 s^-1, and the effect of the vibrating silicon atoms on the shear viscosity is significant when the shear rate is small. This is due to the fact that the vibrating silicon atoms weaken the interactions between the counter-ions （Na^＋ ） and the charged surface.

Lattice Boltzmann Flow Simulation in a Combined Nanochannel

The widely used micro-flow wall-boundary conditions for lattice Boltzmann method(LBM)are evaluated in a force driven combined nanochannel flow.The flow field consists of a two-dimensional nanochannel(mother channel)of an infinite length having flat plates of a finite length inside.The flat plate is set above the bottom wall of the nanochannel with a narrow gap.The flow,thus,develops through this narrow gap(narrower channel)and the other side of the plate(wide gap).The Knudsen number based on the mother channel height is Kn=0.14 whereas the characteristic Knudsen number in the narrower channel is 1.1.To obtain the reference data,the molecular dynamics(MD)simulation is performed with a fully diffusive wall condition.The LBMs are based on the lattice BGK model and with the bounce-back/specular reflection(BSBC)and the diffuse scattering(DSBC)wall boundary conditions.The relaxation time is modified to include sensitivity to Kn.The DSBC shows generally satisfactory results in the test flow cases including fully developed force driven Poiseuille flows,where the BSBC performs worse at Kn>0.5 with a fixed bridge coefficient of b=0.7.This results in its overprediction of the flow rate in the narrower channel region since the characteristic Knudsen number there is 1.1.The MD simulation suggests that the flow develops gradually through the narrower channel region though all the LBM predictions show almost instant flow development.This fact suggests that the relaxation time model needs to have more sensitivity to the locally defined Kn.Further discussions of the BSBC with a different set of models suggest that the regularization process is required for predicting complex nanoscale flows.

纳米气泡浮选过程强化研究进展

浮选是低品质矿及煤分选提质的有效手段,其中微细粒浮选难题突出,而纳米气泡则是解决该难题的重要途径,但关于纳米气泡浮选过程强化的诸多基础科学问题仍未解决。为促进微细粒纳米气泡浮选过程强化技术的开发,重点围绕浮选过程中纳米气泡的界面选择性成核动力学、界面纳米气泡超常稳定性机理及纳米气泡强化颗粒-气泡捕获效率微观作用机制等三个关键科学问题,介绍了笔者团队在纳米气泡浮选过程强化方向的最新研究进展。研究结果表明:纳米气泡的选择性成核是纳米气泡浮选过程强化的关键,而纳米气泡的稳定性则是纳米气泡浮选过程强化的前提;纳米气泡浮选过程强化的机制主要包括促进颗粒絮团与缩短诱导时间,其内在作用机制来自纳米气泡长程疏水引力与边界滑移的协同作用。笔者团队的研究阐明了纳米气泡固-液界面选择性成核的能量作用机制,提出了基于界面高密度气层自动补偿的界面纳米气泡稳定性机理,建立了微纳力学-边界滑移协同驱动的纳米气泡强化浮选界面作用机制,进一步丰富发展了现代浮选基础理论,为开发微细粒纳米气泡浮选过程强化技术提供了一定的指导。

Ion specificity in NaCl solution confined in silicon nanochannels

Ion specificity of Na＋ and C1- ions for NaCI solution confined in silicon nanochannels is investigated with molecular dynamics （MD） simulations. The MD simulation results demonstrate that ion specificity for Na＋ and C1- ions exhibits clearly in na- nochannels with high surface charge density. The two types of ions show different density distributions perpendicular to the channel surface due to the ion specificity when they act as countefions near negatively and positively charged surfaces, respec- tively. Both the two counterion distributions cannot be predicted by Poisson-Boltzmann equation within 0.75 nm near the sur- face. In addition, the ion specificity is also demonstrated through affecting the water density distributions. In the nanochannel with negatively charged surfaces, the presence of the Na＋ ions reduces the number of water peaks in water density distribution profile. In comparison, when the C1- ions act as counterions near positively charged surfaces, they do not affect the number of the water peaks. Besides the influence on the water density distribution, ion specificity also exhibits through affecting the wa- ter molecule orientation in the adsorbed layer. It is found that C1- ions make the water molecules in the adsorbed layer align more orderly than Na~ ions do when the two types of ions act as the counterions near the positively and negatively charged surfaces with the same surface charge density.

Computational study of laser fragmentation in liquid:Phase explosion,inverse Leidenfrost effect at the nanoscale,and evaporation in a nanobubble

Laser fragmentation in liquid is an effective and environment-friendly processing technique capable of yielding colloidal nanoparticles and atomic clusters with a narrow size distribution. The advancement of this technique can be facilitated by an improved understanding of processes that control the sizes, shapes, and structures of the produced nanoparticles. In this work, the dependence of the fragmentation mechanisms on the energy density deposited by the laser pulse is investigated in atomistic simulations performed for 20 nm Au nanoparticles irradiated in water by 10 ps laser pulses. The simulations reveal that the decrease in the absorbed laser energy leads to sequential transitions from the regime of “strong” phase explosion, when all products of an explosive phase decomposition of the irradiated nanoparticle are promptly injected into the water surrounding a nanobubble formed around the nanoparticle, to two distinct regimes of nanoparticle fragmentation leading to the formation of a large central nanoparticle surrounded by smaller satellite fragments. First, in the regime of “mild” phase explosion, the central nanoparticle is produced by the reflection of some of the hot metal droplets generated by the explosive decomposition of the nanoparticle from the boundary of the nanobubble. This reflection is attributed to the inverse Leidenfrost effect acting at the nanoscale. The reflected droplets converge in the center of the nanobubble and coalesce into a single droplet that solidifies shortly after the collapse of the nanobubble. Further decrease in the absorbed laser energy brings the irradiation conditions below the threshold for the phase explosion and results in the formation of a core-satellite structure of the fragmentation products through an interplay of the intense evaporation from the surface of the irradiated nanoparticle, evolution of the nanobubble, and condensation of the metal vapor into clusters and small satellite nanoparticles. The computational predictions are related to the experimental observations, and the connections between the fragmentation mechanisms, the nanoparticle size distribution, and the generation of internal crystal defects are discussed.

棱形石墨烯纳米孔道内离子传输性能的分子动力学模拟

纳米通道的尺寸、结构和表面化学对其内部溶液的分布结构和输运性质有着重大影响.本文研究了一种全新的菱形石墨烯纳米通道.这种理想的通道与最近被广泛研究的金属有机框架材料(MOF)的内部结构类似,有着与传统的碳纳米管截然不同的内部结构.本文使用分子动力学模拟的方法研究在不同尺寸的菱形石墨烯纳米通道内的KCl溶液的性质,并将其与同尺寸的单壁碳纳米管进行了比较.研究结果表明在小孔道内(<20)其内部的溶液结构呈现若干个密度极高的聚集区域,即出现了结晶化的趋势.这一研究结果,将为MOF的结构设计提供思路,从而有望实现类似于生物离子通道的高选择性和高传输能力.

Shape effect of nanochannels on water mobility

Confinement can induce unusual behaviors of water. Inspired by the fabrication of carbon nanotubes with noncircular cross sections, we performed molecular dynamics simulations to investigate the mobilities of water confined in carbon nanochannels with circular, square, and equilateral triangular cross sections over a variety of dimensions. We find that water exhibits disparate mobilities across different types of channels below 0.796 nm(2). Notably, compared with the other two channels, water in equilateral triangular channels displays the greatest mobilities. Moreover, at 0.425 nm(2), different ordered structures are found in the three channels, and water inside the square channel exhibits an extremely low mobility. It is also found that above 0.796 nm(2), the mobilities along the tube axis of water converge to that of the bulk. These phenomena are understood by analyzing the structure, dynamics, and hydrogen bonding of water. Our work explores the mobilities of water across noncircular carbon nanochannels, which may expand the prospect of noncircular nanochannels in scientific studies and practical applications, such as desalination and drug delivery.

纳米通道内纳米流体流动特性的分子动力学模拟

利用分子动力学方法对铜-氩纳米流体和基础流体在不同剪切速度下的纳米尺度的Couette流进行模拟计算。结果表明:在纳米尺度通道内,纳米流体流动过程中颗粒存在旋转运动和平移运动,从而加强湍流效果,强化传热并影响整个流动区域内的流动速度分布,造成纳米流体速度呈非线性分布。壁面和纳米颗粒表面都会形成一层排布更为规则的液体原子吸附层,吸附层内液体分子在流体流动过程中一直伴随着壁面和纳米颗粒进行运动,且吸附层具有"类固"特性,可以增强纳米流体的传热能力。

纳米通道滑移流动的分子动力学研究

本文采用非平衡分子动力学方法对纳米通道内的Couette流动进行了研究,计算获得了不同壁面和流体间势能作用强度下通道内流体的速度分布和密度分布,反映出可能存在的速度滑移、表观无滑移和负滑移现象,并探讨了速度滑移程度和壁面作用强度之间的关系。

纳米通道内表面浸润性对气泡的作用

运用分子动力学模拟方法研究了在质量力驱动下不同浸润性壁面纳米通道中气泡的分布及其运动状况,提出了一种统计纳米通道中气泡运动速度的方法.结果显示,在亲水性壁面的纳米通道中,气泡位于通道中间,气泡的运动速度接近但小于通道中心流速,在势能强度较大时,壁面吸附的分子较多,气泡也较大,反之则气泡较小;对超疏水性壁面,气泡则位于固壁附近,两个壁面形成对称的一对气泡,气泡的运动速度接近但大于边缘速度.流体总的流动速度随着流体粒子与壁面粒子作用的减弱而增大,滑移速度则逐渐从负转变为正.

纳米通道内传热过程的分子动力学研究

对纳米尺度通道内的热量传递过程进行三维非平衡分子动力学模拟,引入嵌入式原子方法模拟固壁原子之间的相互作用势能,利用Green-Kubo公式计算导热系数和热流量。结果表明,由于固壁原子的强烈吸引作用使通道内的流体分子不再均匀变化,紧靠固壁表面的液体原子形成了相对稳定的"准固体"结构。纳米尺度下,粘性摩擦加热的效果相当明显。

微通道中流体流动的分子动力学模拟

本文建立了周期外力作用下流体在微通道间流动模型。以此模型为基础,模拟计算了氩在亲水性和憎水性平行平板壁面间的流动过程,并对速度分布、温度分布等进行了统计。模拟结果表明在憎水性壁面附近,易观察到速度滑移和温度阶跃现象,而亲水性壁面上则反之。与N-S方程和能量方程的解进行了比较,亲水性通道速度和温度的模拟结果与分析解吻合较好,憎水性通道的模拟结果与分析解相差较大。同时,对300℃下不同密度水的流动进行了模拟,随着密度的增大,在壁面附近流体流动特性发生了变化。

水在不均匀纳米通道内自扩散性质的分子动力学研究

本文用分子动力学(MD)方法模拟了受限水在不均匀润湿性微通道中的自扩散性质.通过考察不均匀润湿通道内水的自扩散行为,发现在微通道中水的扩散性质表现出明显的尺度效应,随着通道高度的增加水的扩散增强.更重要的是,由于通道高度的不同,通道内的不均匀润湿段对水的扩散与均匀通道相比有不同的影响.当通道高度为0.8 nm时,不均匀润湿通道内水的扩散增强;当通道高度为1.0 nm时,不均匀润湿通道内水的扩散减弱;而当通道高度达到1.2 nm时,不均匀润湿通道内水的扩散基本相同.

流体在纳米通道中流动分子动力学研究进展

分子动力学方法及分子动力学和计算流体力学混合的模拟方法是研究纳米通道中流体流动的重要手段。简要分析了纳米通道中流体流动的各种新特性及其出现的原因;针对不同流体模型的建立、不同流动过程的实现、不同模拟方法的使用和相近模拟过程结果的比较四个方面,介绍了应用分子动力学方法或混合方法模拟流体在纳米管道中流动的研究现状;最后从作用势函数、计算简化和模拟软件角度展望了分子动力学模拟技术的发展趋势,并从模拟方法、流体模型、热边界和势函数角度总结了分子动力学方法在模拟纳米通道中流体流动方面的改进趋势。