MOLECULAR DYNAMICS SIMULATION OF ENTROPY AND SURFACE TENSION FOR GRAIN BOUNDARY OF α-Fe

The grain boundary is an interface and the surface tension is one of its important thermodynamic properties. In this paper, the surface tension of the Σ9 grain boundary for α-Fe at various temperatures and pressures is calculated by means of Computer Molecular Dynamics (CMD). The results agree satisfactorily with the experimental data. It. is shown that the contribution of entropy to surface tension of grain boundary can be ignored.

A new method for the determination of surface tension from molecular dynamics simulations applied to liquid droplets

For the determination of surface tension of liquid droplets by molecular dynamics simulations, the most timeconsuming part is the calculation of pressure tensor in the transition layer, which makes it difficult to enhance the precision of the computation. A new method for the calculation of surface tension of liquid droplets to reduce the calculation quantity of pressure tensor in transition layer to the minimum is proposed in this paper. Two thousand particles are taken as example to show how to carry out our scheme.

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.

A molecular dynamics study on surface properties of supercooled water

Molecular dynamics simulations were performed to study the surface properties of water in a temperature range from 228 to 293 K by using the extended simple point charge (SPC/E) and four-site TIP4P potentials. The calculated surface tension increases with the decrease of temperature, and moreover the slopes of the surface tension-temperature curves show a weak rise below 273 K, whereas no obvious anomalies appear near 228 K, which accords with the previous experiments. Compared with the measured values, the SPC/E potential shows a good agreement, and the TIP4P potential underestimates the surface tension. The main reason for that may be the reasonable description of the surface structure of supercooled water for the SPC/E. When simulating the orientational distributions of water molecules near the surface, the SPC/E potential produces higher ordering and larger surface potentials than the TIP4P potential.

Droplet Condensation and Transport Properties on Multiple Composite Surface:A Molecular Dynamics Study

To investigate the microscopicmechanism underlying the influence of surface-chemical gradient on heat andmass recovery,a molecular dynamicsmodel including droplet condensation and transport process has been developed to examine heat and mass recovery performance.This work aimed at identify optimal conditions for enhancing heat and mass recovery through the combination of wettability gradient and nanopore transport.For comprehensive analysis,the structure in the simulation was categorized into three distinct groups:a homogeneous structure,a small wettability gradient,and a large wettability gradient.The homogeneous surface demonstrated low efficiency in heat and mass transfer,as evidenced by filmwise condensation.In contrast,the surface with a small wettability gradient experienced a transition from dropwise condensation to filmwise condensation,resulting in a gradual decrease in the efficiency of vapor heat and mass transfer.Only a large wettability gradient could achieve periodic and efficient dropwise condensation heat and mass transfer which was attributed to the rapid droplet coalescence and transport to the nanopore after condensing on the cold surface.

Molecular dynamics simulation of the interaction of ethanol-water mixture with a Pt surface

An analysis of the molecular dynamics of ethanol solvated by water molecules in the absence and presence of a Pt surface has been performed using DL_POLY_2.19 code. The structure and diffusion properties of an ethanol–water system have been studied at various temperatures from 250 to 600 K. We have measured the self-diffusion coefficients of the 50:50% ethanol–water solution;in the absence of a Pt surface our results show an excellent agreement–within an error of 7.4% – with the experimental data. An increase in the self-diffusion coefficients with the inclusion of a Pt surface has been observed. The estimation of the diffusion coefficients of both water and ethanol in the presence of a Pt surface shows that they obey the Arrhenius equation;the calculated activation energies of diffusion of ethanol and water are 2.47 and 2.98 Kcal/mole, respectively. The radial distribution function graphs and density profiles have been built;their correlations with the self-diffusion coefficients of both ethanol and water molecules are also illustrated.

Tolman length of simple droplet: Theoretical study and molecular dynamics simulation

In 1949, Tolman found the relation between the surface tension and Tolman length, which determines the dimensional effect of the surface tension. Tolman length is the difference between the equimolar surface and the surface of tension. In recent years, the magnitude, expression, and sign of the Tolman length remain an open question. An incompressible and homogeneous liquid droplet model is proposed and the approximate expression and sign for Tolman length are derived in this paper. We obtain the relation between Tolman length and the radius of the surface of tension(R_(s)) and found that they increase with the Rs decreasing. The Tolman length of plane surface tends to zero. Taking argon for example, molecular dynamics simulation is carried out by using the Lennard–Jones(LJ) potential between atoms at a temperature of 90 K. Five simulated systems are used, with numbers of argon atoms being 10140, 10935, 11760, 13500, and 15360, respectively. By methods of theoretical study and molecular dynamics simulation, we find that the calculated value of Tolman length is more than zero, and it decreases as the size is increased among the whole size range. The value of surface tension increases with the radius of the surface of tension increasing, which is consistent with Tolman’s theory. These conclusions are significant for studying the size dependence of the surface tension.

Approximate expression of Young's equation and molecular dynamics simulation for its applicability

In 1805, Thomas Young was the first to propose an equation(Young's equation) to predict the value of the equilibrium contact angle of a liquid on a solid. On the basis of our predecessors, we further clarify that the contact angle in Young's equation refers to the super-nano contact angle. Whether the equation is applicable to nanoscale systems remains an open question. Zhu et al. [College Phys. 4 7(1985)] obtained the most simple and convenient approximate formula, known as the Zhu–Qian approximate formula of Young's equation. Here, using molecular dynamics simulation, we test its applicability for nanodrops. Molecular dynamics simulations are performed on argon liquid cylinders placed on a solid surface under a temperature of 90 K, using Lennard–Jones potentials for the interaction between liquid molecules and between a liquid molecule and a solid molecule with the variable coefficient of strength a. Eight values of a between 0.650 and 0.825 are used. By comparison of the super-nano contact angles obtained from molecular dynamics simulation and the Zhu–Qian approximate formula of Young's equation, we find that it is qualitatively applicable for nanoscale systems.

Structure and dynamics of ordered water in a thick nanofilm on ionic surfaces

The structure and dynamics of water in a thick film on an ionic surface are studied by molecular dynamic simulations. We find that there is a dense monolayer of water molecules in the vicinity of the surface. Water molecules within this layer not only show an upright hydrogen-down orientation, but also an upright hydrogen-up orientation. Thus, water molecules in this layer can form hydrogen bonds with water molecules in the next layer. Therefore, the two-dimensional hydrogen bond network of the first layer is disrupted, mainly due to the 0 atoms in this layer, which are affected by the next layer and are unstable. Moreover, these water molecules exhibit delayed dynamic behavior with relatively long residence time compared with those bulk-like molecules in the other layers. Our study should be halpful to further understand the influence of water film thickness on the interfacial water at the solid-liquid interface.

Interactions of ferro-nanoparticles(hematite and magnetite) with reservoir sandstone: implications for surface adsorption and interfacial tension reduction

There are a few studies on the use of ferro-nanofluids for enhanced oil recovery,despite their magnetic properties;hence,it is needed to study the adsorption of iron oxide(Fe2 O3 and Fe3 O4) nanoparticles(NPs) on rock surfaces.This is important as the colloidal transport of NPs through the reservoir is subject to particle adsorption on the rock surface.Molecular dynamics simulation was used to determine the interfacial energy(strength) and adsorption of Fe2 O3 and Fe3 O4 nanofluids infused in reservoir sandstones.Fourier transform infrared spectroscopy and X-ray photon spectroscopy(XPS) were used to monitor interaction of silicate species with Fe2 O3 and Fe3 O4.The spectral changes show the variation of dominating silicate anions in the solution.Also,the XPS peaks for Si,C and Fe at 190,285 and 700 eV,respectively,are less distinct in the spectra of sandstone aged in the Fe3 O4 nanofluid,suggesting the intense adsorption of the Fe3 O4 with the crude oil.The measured IFT for brine/oil,Fe2 O3/oil and Fe3 O4/oil are 40,36.17 and 31 mN/m,respectively.Fe3 O4 infused with reservoir sandstone exhibits a higher silicate sorption capacity than Fe2 O3,due to their larger number of active surface sites and saturation magnetization,which accounts for the effectiveness of Fe3 O4 in reducing IFT.

The diffusion and concentration effects of formamide on a TiO₂surface in the presence of a water solvent

The formamide-titanium oxide interaction mechanism is a research target of great importance for understanding the elementary events of the origin of life: the synthesis of nucleoside bases and formation of biological molecules needed for life. Titanium oxide (TiO2) can act as a strongly adsorbing surface or a catalytic material. In the present study, a comparative molecular dynamics analysis performed to clarify the adsorbing and diffusion properties of liquid formamide on a TiO2 surface in the presence of water molecules. The structural features of the formamide concentration effect (the accumulation of molecules) on a TiO2 surface in the presence and absence of water solvent are cleared up. Modification of the formamide diffusion abilities mediated by a water solvent is observed to correlate with the formamide-water concentration distribution on the surface.

Fragile to strong crossover and Widom line in supercooled water： A comparative study

The aim of this paper is to discuss the relationship between the dynamics and thermodynamics of water in the supercooled region. Reviewed case studies comprehend bulk water simulated with the SPC/E, TIP4P and TIP4P/2005 potentials, water at protein interfaces, and water in solution with electrolytes. Upon supercooling, the fragile to strong crossover in the s-relaxation of water is found to occur when the Widom line emanating from the liquid-liquid critical point is crossed. This appears to be a general characteristic of supercooled water, not depending on the applied interaction potential and/or different local environments.

Peptide friction in water nanofilm on mica surface

Peptide frictions in water nanofilms of various thicknesses on a mica surface are studied via molecular dynamics simulations. We find that the forced lateral motion of the peptide exhibits stick-slip behaviour at low water coverage; in contrast, the smooth gliding motion is observed at higher water coverage. The adsorbed peptide can form direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds with the substrate. We propose that the stick-slip phenomenon is attributed to the overall effects of direct and indirect hydrogen bonds formed between the surface and the peptide.

琥珀酸衍生物类非离子型表面活性剂的合成及性能评价

利用N-十二烷基琥珀酸酐、L-亮氨酸和聚醚醇为原料合成了两种非离子型表面活性剂,并通过核磁共振氢谱验证了分子结构;通过表面张力仪分别测试了两种非离子型表面活性剂在不同浓度下的表面张力,并根据γ-lgc曲线确定了临界胶束浓度;通过析水率研究了表面活性剂的乳化性能;使用分子动力学模拟技术,研究了表面活性剂在油水界面处的聚集形态以及与油和水之间的作用。结果表明,与表面活性剂1相比,表面活性剂2含更多的非离子亲水组分,因此降低表面张力效果更好;表面活性剂1及表面活性剂2的临界表面张力(γ_(CMC))分别为25.54、24.46 mN/m;在低浓度下,两种非离子型表面活性剂均具有较高的析水率,乳化效果不佳,但在高浓度下乳化效果较好,采用两种非离子型表面活性剂制备的乳液均具有一定的稳定性;表面活性剂分子的静电势分布影响了其亲水和亲油的性质。

油基泡沫钻井流体稳定机理研究

采用表面张力、黏度、界面膜流变性能、分子模拟等方法研究了不同类型表面活性剂在空气/白油表面的吸附行为和界面层的性质,通过对泡沫表观性能、液膜流变性质、微观分子模拟等研究结果的关联分析,讨论了油基泡沫的稳定机理。研究结果表明,椰油二乙醇酰胺、司盘80、长链烷基苯磺酸(C20-24)以反胶束的形式吸附在界面下层,很难有效降低表面张力,只能在外力的作用下形成瞬态泡沫;氟表面活性剂和自制的SH-OF-1在泡沫形成时可以吸附到表面定向排列,降低表面张力,形成相对稳定的泡沫;氟表面活性剂泡沫液膜呈现出较高的弹性性质,表面黏性不够,使得其抗扰动能力稍差,而SH-OF-1由于同时具有黏弹性能,因而表现出更好的泡沫稳定性。

纳米水滴在纳米粗糙壁面上润湿行为的分子动力学模拟

接触角是固体表面润湿性能的重要参数之一,主要取决于固体的表面化学性质和表面几何构型。今采用分子动力学模拟技术,探讨纳米水滴在纳米粗糙壁面上的润湿行为。模拟结果表明,当壁面粗糙度因子相同时,纳米水滴在纳米栏栅形或纳米方柱矩阵形粗糙壁面上的接触角相差不大。对于疏水粗糙壁面,随着粗糙度因子增加,接触角增加到某一值,然后基本保持不变;随着相面积分数的增加,接触角基本不变,当相面积分数达到0.7,接触角逐渐减小。对于中性粗糙壁面,随着粗糙度因子或相面积分数的增加,纳米水滴的接触角变化不大,纳米水滴为Wenzel接触模式。对于亲水粗糙壁面,纳米水滴的接触角随着粗糙度因子或相面积分数的增加而减小,纳米水滴为Wenzel接触模式。

用MD方法计算磁场条件下水的表面张力系数

本文采用分子动力学模拟(Molecular Dynamics simulation简称MD)方法对不同磁场和温度条件下水体系的表面张力系数进行了计算。结果表明水的表面张力系数和磁感应强度之间具有多极值关系,特别在常温下以及磁感应强度为0.25T时,磁场对水体系表面张力系数的影响最为明显。通过对径向分布函数的计算发现,磁场可以影响g(r)的分布,使水的结构发生变化,从而导致水的表面张力系数发生改变。

汽液界面表面张力模拟中的影响因素

用分子动力学模拟方法研究汽液界面性质时 ,适当选取模拟分子数目和势函数截断半径 ,既能满足合理的计算时间要求 ,又减少分子数目效应和势函数截断半径对计算结果的影响。在NVT系综中 ,以氩原子为对象 ,对长方形模拟盒中粒子数目为 5 0 0、86 4、1372和 2 0 4 8个的汽液平衡系统进行模拟计算。在计算时发现 ,汽液界面的表面张力值与模拟分子数目有关 ,模拟分子数目在 10 0 0个以上 ,表面张力值趋于恒定。势函数截断半径Rc=4 .5σ ,计算结果与实验值吻合较好。

α铁晶界熵及表面张力的分子动力学模拟

晶界也是一种界面。表面张力是晶界的一个重要的热力学量。本文采用计算机分子动力学模拟(CMD)方法计算 α-Fe,∑=9 的晶界在不同温度和压力下的表面张力,结果与实验值的比较是满意的。发现熵对晶界的表面张力的贡献是很小的,通常可以忽略不计。

纳米水滴在光滑壁面上润湿行为的分子动力学模拟

采用分子动力学模拟技术,研究了纳米水滴在光滑壁面上的润湿行为规律。模拟结果表明,壁面宽度、厚度以及水分子数对接触角及汽—液界面厚度的影响不大。随着壁面作用势能的减小,接触角线性增大;当壁面作用势能为1.674 k J/mol时,接触角约为90°。随着温度的提高,汽—液界面厚度逐渐增大;疏水壁面的接触角随温度的提高而逐渐增大;对于中性壁面,温度对接触角影响不大;亲水壁面的接触角随温度的提高而逐渐减小。