Drag on a nanotube in uniform liquid argon flow

In this work, nonequilibrium molecular dynamics (MD) simulations were performed to investigate uniform liquid argon flow past a carbon nanotube. In the simulation, nanotubes were modeled as rigid cylinders of carbon atoms. Both argon-argon and argon-carbon interactions were calculated based on Lennard-Jones potential. Simulated drag coefficients were compared with (i) published empirical equation which was based on experiments conducted with macroscale cylinders and (ii) finite element (FE) analyses based on Navier-Stokes equation for flow past a circular cylinder using the same dimensionless parameters used in MD simulations. Results show that classical continuum mechanics cannot be used to calculate drag on a nanotube. In slow flows, the drag coefficients on a single-walled nanotube calculated from MD simulations were larger than those from the empirical equation or FE analysis. The difference increased as the flow velocity decreased. For higher velocity flows, slippage on the surface of the nanotube was identified which resulted in lower drag coefficient from MD simulation. This explains why the drag coefficient from MD dropped faster than those from the empirical equation or FE simulation as the flow velocity increased. It was also found that the drag forces are almost equal for single- and double-walled nanotubes with the same outer diameter, implying that inner tubes do not interact with fluid molecules.     

Effect of chirality and length on the penetrability of single-walled carbon nanotubes into lipid bilayer cell membranes

The ability of carbon nanotubes to enter the cell membrane acting as drug-delivery vehicles has yielded a plethora of experimental investigations, mostly with inconclusive results because of the wide spectra of carbon nanotube structures. Because of the virtual impossibility of synthesizing CNTs with distinct chirality, we report a parametric study on the use of molecular dynamics to provide better insight into the effect of the carbon nanotube chirality and the aspect ratio on the interaction with a lipid bilayer membrane. The simulation results indicated that a single-walled carbon nanotube utilizes different time-evolving mechanisms to facilitate their internalization within the membrane. These mechanisms comprise both penetration and endocytosis. It was observed that carbon nanotubes with higher aspect ratios penetrate the membrane faster whereas shorter nanotubes undergo significant rotation during the final stages of endocytosis. Furthermore, nanotubes with lower chiral indices developed significant adhesion with the membrane. This adhesion is hypothesized to consume some of the carbon nanotube energy, thus resulting in longer times for the nanotube to translocate through the membrane.     

Investigation on rheological properties of carbon nanotube nanofluids

The effective viscosity of carbon nanotube nanofluids is strongly dependent on the temperature and concentration. The aggregation behaviour that carbon nanotubes exhibit in solution and the orientation variation of single carbon nanotube make rheological properties of nanofluids more complex. With the increase of shear rate, the degree of dispersion and orientation of carbon nanotubes will be improved. Based on previous studies and the fact mentioned above, a reasonable expression for viscosity of carbon nanotube nanofluids has been given, which is associated with the shear rate and aspect ratios of carbon nanotubes. The expression has been validated comparing with previous experimental data.     

Effects of size constraint on water filling process in nanotube

Molecular dynamics (MD) simulation and the potential of mean force (PMF) analysis are used to investigate the structural properties of water molecules near the end of nanotube for the whole process from the initial water filling up to the configuration stabilization inside the carbon nanotubes (CNTs). Numerical simulations showed that when a small-sized nanotube is immersed into the water bath, the size constraint will induce a prevailing orientation for the water molecule to diffuse into the tube and this effect can persist approximately 3.3 angstroms from the end of CNT. As the structure within the CNTs stabilizes, the ambient structural properties can indirectly reflect their corresponding properties inside the nanotube. Our results also showed that there exists a close correlation between the PMF analysis and the results of MD simulations, and the properties at nanometer scale are closely related to the size-constraint effect.     

An analytical bond‐order potential for carbon

Carbon is the most widely studied material today because it exhibits special properties not seen in any other materials when in nano dimensions such as nanotube and graphene. Reduction of material defects created during synthesis has become critical to realize the full potential of carbon structures. Molecular dynamics (MD) simulations, in principle, allow defect formation mechanisms to be studied with high fidelity, and can, therefore, help guide experiments for defect reduction. Such MD simulations must satisfy a set of stringent requirements. First, they must employ an interatomic potential formalism that is transferable to a variety of carbon structures. Second, the potential needs to be appropriately parameterized to capture the property trends of important carbon structures, in particular, diamond, graphite, graphene, and nanotubes. Most importantly, the potential must predict the crystalline growth of the correct phases during direct MD simulations of synthesis to achieve a predictive simulation of defect formation. Because an unlimited number of structures not included in the potential parameterization are encountered, the literature carbon potentials are often not sufficient for growth simulations. We have developed an analytical bond order potential for carbon, and have made it available through the public MD simulation package LAMMPS. We demonstrate that our potential reasonably captures the property trends of important carbon phases. Stringent MD simulations convincingly show that our potential accounts not only for the crystalline growth of graphene, graphite, and carbon nanotubes but also for the transformation of graphite to diamond at high pressure. © 2015 Wiley Periodicals, Inc.     

Strong correlations and Fickian water diffusion in narrow carbon nanotubes

The authors have used atomistic molecular dynamics (MD) simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solidlike ordering at room temperature, which they quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not subdiffusive. They also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. They find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal mode diffusion inside the nanotube. They attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length.     

DPH-环糊精纳米管状聚集体的计算机模拟

1,6-二苯基－1,3,5-己三烯（1,6-diphenyl-1,3,5-hexatriene）（简称DPH）不能与α-环糊精，但可以分别与β-、γ-环糊精通过超分子自组装作用形成纳米管状结构的聚集体.该文采用分子力学和分子动力学模拟对这些聚集体在中性和碱性下条件的理论模型进行了预测，并分析了其主客体间的非键相互作用以及氢键的形成情况.计算结果表明，在碱性条件下，环糊精分子间氢键的消失导致了纳米管状结构的解离；而空腔过小是α-环糊精和DPH之间不能形成纳米管状结构的原因.     

Air separation by single wall carbon nanotubes: thermodynamics and adsorptive selectivity

Separation of a nitrogen-oxygen mixture (air) by single wall carbon nanotubes has been studied using grand canonical Monte Carlo simulations at a range of nanotube diameters, temperatures, and pressures. It is demonstrated that depending on these operating parameters, the extent of adsorptive selectivity can vary significantly. Detailed calculations are also presented for the adsorption isotherms, energies, and isosteric heats of pure nitrogen, oxygen, and their mixture at 100 K in a carbon nanotube of 12.53-A diameter. In single-component simulations, it is found that near saturation loading nitrogen forms only an annular layer close to the nanotube wall, while smaller-sized oxygen also occupies the region near the center of the nanotube. In mixture adsorption, the energetically favored nitrogen is preferentially adsorbed at low loadings. However, at high loadings oxygen replaces nitrogen due to the dominant entropic effects, and therefore a high selectivity towards oxygen is observed close to the saturation loading. The effect of the entropic change on mixture adsorption is evident from the calculated isosteric heats of adsorption. The mixture isotherms obtained from simulations are found to be in good agreement with the predictions based only on the pure component simulation results.     

Monte Carlo simulations of hydrogen adsorption in alkali-doped single-walled carbon nanotubes

Monte Carlo simulations and Widom's test particle insertion method have been used to calculate the solubility coefficients (S) and the adsorption equilibrium constants (K) in single-walled (10,10) armchair carbon nanotubes including single nanotubes, and nanotube bundles with various configurations with and without alkali dopants. The hydrogen adsorption isotherms at room temperature were predicted by following the Langmuir adsorption model using the calculated constants S and K. The simulation results were in good agreement with experimental data as well as the grand canonical Monte Carlo simulation results reported in the literature. The simulations of nanotube bundle configurations suggest that the gravimetric hydrogen adsorption increases with internanotube gap size. It may be attributed to favorable hydrogen-nanotube interactions outside the nanotubes. The effect of alkali doping on hydrogen adsorption was studied by incorporating K+ or Li+ ions into nanotube arrays using a Monte Carlo simulation. The results on hydrogen adsorption isotherms indicate hydrogen adsorption of 3.95 wt% for K-doping, and 4.21 wt% for Li-doping, in reasonable agreement with the experimental results obtained at 100 atm and room temperature.     

Phase behavior of Ar and Kr films on carbon nanotubes

Recent experiments (Wang et al., 2010) have found evidence of phase transitions of gases adsorbed on a single carbon nanotube. In order to understand the observations, we have carried out classical grand canonical Monte Carlo simulations of this system, for the cases of Ar and Kr on zigzag and armchair nanotubes with radius R > 0.7 nm. The calculated behavior resembles the experimental results in the case of Ar. However, the prominent, ordered phase found for Kr in both simulations and (classical) energy minimization calculations differs from that deduced from the experimental data. A tentative explanation of the apparent discrepancy is that the experiments involve a nanotube of rather large radius (>1.5 nm).     

Dynamic Behavior of Rotation Transmission Nano-System in Helium Environment: A Molecular Dynamics Study

The molecular dynamics (MD) method is used to investigate the influence of the shielding gas on the dynamic behavior of the heterogeneous rotation transmission nano-system (RTS) built on carbon nanotubes (CNTs) and boron nitride nanotube (BNNT) in a helium environment. In the heterogeneous RTS, the inner CNT acts as a rotor, the middle BNNT serves as a motor, and the outer CNT functions as a stator. The rotor will be actuated to rotate by the motor due to the interlayer van der Waals effects and the end effects. The MD simulation results show that, when the gas density is lower than a critical range, a stable signal of the rotor will arise on the output and the rotation transmission ratio (RRT) of RTS can reach 1.0, but as the gas density is higher than the critical range, the output signal of the rotor cannot be stable due to the sharp drop of the RRT caused by the large friction between helium and the RTS. The greater the motor input signal of RTS, the lower the critical working helium density range. The results also show that the system temperature and gas density are the two main factors affecting the RTS transmission behavior regardless of the size of the simulation box. Our MD results clearly indicate that in the working temperature range of the RTS from 100 K to 600 K, the higher the temperature and the lower the motor input rotation frequency, the higher the critical working helium density range allows.     

Analysis of stability of nanotube dispersions using surface tension isotherms

In this paper, we present the analyses of surface tension of surfactant-stabilized dispersions of carbon nanotubes. This method allows one to study interactions of carbon nanotubes with surfactants at different levels of nanotube loading when optical methods fall short in quantifying the level of nanotube separation. Sodium dodecyl sulfate was used as a stabilizing agent to uniformly disperse single-walled carbon nanotubes in an aqueous media. We show that surface tension is very sensitive to small changes of nanotube and surfactant concentrations. The experimental data suggest that, at moderate concentrations, surfactant displaces carbon nanotubes from the air-water interface and the nanotubes are mostly moved into the bulk of the liquid. By analyzing the surface tension as a function of surfactant concentration, we obtained the dependence of critical micelle concentration on nanotube loading. We then constructed the adsorption isotherm for dodecyl sulfate on carbon nanotubes and bundles of carbon nanotubes. The results of these experiments enabled us to extend the phase diagram of the produced dispersions to a broader range of surfactant and nanotube concentrations.     

Effect of Hydrodynamic Forces on meso‐(4‐Sulfonatophenyl)‐Substituted Porphyrin J‐Aggregate Nanoparticles: Elasticity,Plasticity and Breaking

The J aggregates of 4‐sulfonatophenyl meso‐substituted porphyrins are non‐covalent polymers obtained by self‐assembly that form nanoparticles of different morphologies. In the case of high aspect‐ratio nanoparticles (bilayered ribbons and monolayered nanotubes), shear hydrodynamic forces may modify their shape and size, as observed by peak force microscopy, transmission electron microscopy of frozen solutions, small‐angle X‐ray scattering measurements in a disk‐plate rotational cell, and cone–plate rotational viscometry. These nanoparticles either show elastic or plastic behaviour: there is plasticity in the ribbons obtained upon nanotube collapse on solid/air interfaces and in viscous concentrated nanotube solutions, whereas elasticity occurs in the case of dilute nanotube solutions. Sonication and strong shear hydrodynamic forces lead to the breaking of the monolayered nanotubes into small particles, which then associate into large colloidal particles.     

异型碳纳米管储氢性能的分子动力学模拟研究

采用分子动力学(MD)方法对三种理想的Y型碳纳米管[记为Y(4,4), Y(6,6), Y(10,0)]和三种L型碳纳米管[记为L(9,0), L(6,6), L(10,0)]之储氢性能进行了模拟研究, 并与相应的直线型碳纳米管的储氢能力进行了比较, 同时考察了温度、碳纳米管的直径和螺旋性以及缺陷的位置和大小对异型碳纳米管储氢性能的影响. 结果表明, 在室温和低温条件下, 异型碳纳米管的储氢量高于直线型碳纳米管的储氢量, 且其储氢量大小随温度的降低和碳管直径的增大而增加, 椅式碳纳米管的储氢性能优于齿式碳纳米管, 而缺陷的位置和大小对异型碳管之储氢性能的影响则因碳管的形貌和直径的大小不同而存在差异.     

Dimensional changes as a function of charge injection in single-walled carbon nanotubes

Motivated by the central importance of charge-induced dimensional changes for carbon nanotube electromechanical actuators, we here predict changes in nanotube length and diameter as a function of charge injection for armchair and zigzag nanotubes having different diameters. Density functional theory with periodic boundary conditions is used, which we show provides results consistent with experimental observations for intercalated graphites. Strain-versus-charge relationships are predicted from dimensional changes calculated with a uniform background charge ("jellium") for representing the counterions. These jellium calculations are consistent with presented calculations that include specific counterions for intercalated graphite, showing that hybridization between the ions and the graphite sheets is unimportant. The charge-strain relationships calculated with the jellium approximation for graphite and isolated single-walled nanotubes are asymmetric with respect to the sign of charge transfer. The dependence of nanotube strain on charge approaches that for a graphite sheet for intermediate-sized metallic nanotubes and for larger diameter semiconducting nanotubes. However, the strain-charge curves strongly depend on nanotube type when the nanotube diameter is small. This reflects both the dependence of the frontier orbitals for the semiconducting nanotubes on the nanotube type and the pi-sigma mixing when the nanotube diameter is small.     

Simulation of the adhesion properties of the polyethylene/carbon nanotube interface

The properties of a polyethylene matrix in contact with carbon nanoinclusions, such as carbon nanotubes and graphene plates, are studied via the molecular-dynamics method. Ultimate shear stresses for the polyethylene/graphene interface are calculated, and the effect of the filler aspect ratio on the nanocomposite elastic modulus is considered.     

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.     

有限长碳纳米管中的分子输运

采用非平衡分子动力学方法研究了有限长度、开口单壁碳纳米管中氦分子的输运过程.结果表明氦分子在小管径碳纳米管中主要以超扩散方式运动.当碳纳米管直径大于某一阈值时发生从超扩散向弹道输运方式的转变,而随着管径的继续增大,分子输运重新以超扩散的方式进行.这种转变与纳米管端口效应有密切的联系.当碳纳米管内部分子通过弹道方式高速运动时,这种运动在端口处由于端部势垒的影响而被抑制,造成端部阻塞现象,其本质是受端部势垒和碳纳米管管径共同影响的结果.     

疏水蛋白吸附碳纳米管的分子动力学模拟

采用分子动力学方法模拟了水溶液中Ⅱ型疏水蛋白HFBI在单壁碳纳米管(SWNTs)表面的吸附过程, 考察了3种不同的HFBI初始取向, 并计算了结合自由能, 从累计240 ns的模拟轨迹中得到了不同的吸附结构. 结果表明, 当HFBI完全通过疏水面与SWNTs作用时, 其结合自由能最有利吸附, 且吸附最稳定. 另外, 由于HFBI含有4个二硫键, 因此吸附过程几乎不改变其二级结构.