Self-diffusivity,hydrogen bonding and density of different water models in carbon nanotubes

In this paper, the density, hydrogen bonding and self-diffusivity of water confined in carbon nanotubes are investigated. Molecular dynamics is used to simulate a large variety of nanotubes with various water models. Our results produce, for the first time, the complete trend of these properties from narrow nanotubes, where water shows particularly anomalous behaviour, to large ones where its characteristics are similar to those of bulk.     

Water transport through carbon nanotubes with defects

Non-equilibrium molecular dynamics simulations are performed to investigate how changing the number of structural defects in the wall of a (7,7) single-walled carbon nanotube (CNT) affects water transport and internal fluid dynamics. Structural defects are modelled as vacancy sites (missing carbon atoms). We find that, while fluid flow rates exceed continuum expectations, increasing numbers of defects lead to significant reductions in fluid velocity and mass flow rate. The inclusion of such defects causes a reduction in the water density inside the nanotubes and disrupts the nearly frictionless water transport commonly attributed to CNTs.     

Brittle ductile transition in carbon nanotube bundles

The superior strength and stiffness of carbon nanotubes (CNTs) make them attractive for many structural applications. Although the strength and stiffness of CNTs are extremely high, fibres of aligned CNTs have been found to date to be far weaker than the constituent CNTs. The intermolecular interactions between the CNTs in the fibres are governed by weak van der Waals forces, resulting in slippage between CNTs which occurs at tensions well below the breaking strength of the CNTs. Both theoretical and experimental studies show that by introducing chemical bonds between the CNTs increases load transfer and prevents the CNTs from slipping.     

Atomistic simulations on interwall sliding behaviour of double-walled carbon nanotube: effects of structural defects

In this study, we investigated the interwall sliding behaviours of double-wall carbon nanotubes (DWCNTs) using molecular dynamics (MD) simulations, focusing on the effects of different structural defects including the vacancy, adsorbed atom (Adatom) and Stone-Wales (SW) defects. The simulation results showed that structural defects, especially the Adatom ones, caused large fluctuations and decreased the overall pull-out force. Stick-slip motions were observed in the interwall sliding processes of DWCNTs containing multiple structural defects. Among three types of structural defects, the Adatom defects most significantly weaken the interwall load transferring capability and degrade the interface shear strength (IFSS). This work provides useful information for promoting DWCNTs’ applications in Micro/Nano Electro-Mechanical Systems (M/NEMS).     

Gigahertz frequency tuner based on a telescoping double-walled carbon nanotube: molecular dynamics simulations

The schematics of a gigahertz-range tuner is addressed as an application of a telescoping multi-walled carbon nanotube (CNT) that can be used repeatedly, and its dynamic operation is investigated via classical molecular dynamics simulations based on a (5,5)(10,10) double-walled CNT. Fine control of the telescoped length of the double-walled CNT enables its resonance frequency to be matched to one of the signal frequencies, and the telescoped nanotube can be tuned to its resonance frequency for use as a component of a bandpass filter.     

Predicting doxorubicin drug delivery by single-walled carbon nanotube through cell membrane in the absence and presence of nicotine molecules: a molecular dynamics simulation study

Abstract

In order to study the interaction of the anticancer agent Doxorubicin with the single-walled carbon nanotubes with different diameters as drug delivery systems, the molecular dynamics (MD) simulations have been used. Also, for design and development of intracellular Doxorubicin drug delivery systems, a series of steered MD simulations are applied to explore the possibility of encapsulated Doxorubicin–carbon nanotube penetration through a lipid bilayer in presence and absence of Nicotine molecules at different pulling rates. Our simulation results showed that in spite of the adsorption of drug molecules on the outer sidewall of the nanotubes, the spontaneous localization of one Doxorubicin molecule into the cavity of the nanovectors with larger diameters is observed. It is found that the presence of Nicotine molecules in extracellular medium increases the required force for pulling nanotube-encapsulated drug as well as the required time for penetration process, especially at higher velocity. Also, the entering process of the Nicotine molecules into the carbon nanotube causes that the encapsulated drug molecule is fully released in the hydrophobic phase of the lipid bilayer.

Communicated by Ramaswamy H. Sarma     

Enhancement of water flow across a carbon nanotube

It is important to investigate how to enhance the flow rate of single-file water molecules across nanochannels. To our knowledge, all the existing methods are based on pressure gradients, external point charges or uniform/graded electric fields. Accordingly, these methods are all based on exogenous tools, and thus bring challenges for both energy-saving and miniaturisation. In contrast, here we manage to reveal an endogenously determined mechanism of flow enhancement. On the basis of molecular dynamics simulations, we investigate water permeation across a single-walled carbon nanotube (SWCNT A) in the presence of another SWCNT (SWCNT B). We find that the flow rate of the single-file water molecules across SWCNT A is enhanced for the case of unblocked SWCNT B compared with the rate for the case of blocked SWCNT B, and that this flow rate is determined by the separation between the two SWCNTs and the diameter of SWCNT B.     

Molecular dynamics simulation studies of mechanical properties of different carbon nanotube systems

Various mechanical properties of single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) are evaluated using molecular dynamics (MD) simulations. A tensioning process was first performed on a SWCNT whose interaction is based on the Brenner’s ‘second generation’ potential under varying length–diameter ratios and strain rates, in order to understand the SWCNT’s behaviour under axial tension. The results showed an increase in the SWCNT’s ultimate tensile strength and a decrease in critical strain given the conditions of increasing strain rate and a decreasing length–diameter ratio. Comparison was done with previous studies on axial tensioning of SWCNT to validate the results obtained from the set-up, based on the general stress–strain relationship and key mechanical properties such as the strain at failure and the Young’s modulus. A DWCNT was then constructed, and Lennard-Jones ‘12-6’ potential was used to describe the energy present between the nanotube layers. Extraction of the inner tube in a DWCNT was performed using two inner wall tubings of different diameters to draw comparison to the energies needed to separate fully the outer and inner tubing. Finally, a bending test was performed on two DWCNTs with different intertube separations. Insights into the entire bending process were obtained through analyses of the variations in the strain energy characteristic of the surface atoms near the bending site, as the DWCNT is gradually bent until failure.     

Effect of chirality,length and diameter of carbon nanotubes on the adsorption of 20 amino acids: a molecular dynamics simulation study

We carried out molecular dynamics simulations to study the adsorption of all the 20 amino acids (AAs; aromatic, polar and non-polar) on the surface of chiral, zigzag and armchair single-walled carbon nanotubes. The adsorption was occurring in all systems. In the aromatic AAs, the π–π stacking and the semi-hydrogen bond formation cause a strong interaction with the carbon nanotubes (CNTs). We also investigated the chirality, length and diameter dependencies on adsorption energies. We found that all AAs have more tendency to adsorption on the chiral and zigzag CNTs over the armchair. The results show that increasing both the diameter and the length causes the enhancement of the adsorption energy. But, the effect of the length is more than of the diameter. For example, the adsorption energy of Trp on the surface of CNT (4,1), with 2 nm length, is 20.4 kcal/mol. When the length of CNT becomes twice, the adsorption energy increases by 24 ± 0.3%. But by doubling the diameter, the adsorption energy increased only by 9.8 ± 0.25%.     

Dynamic ejection behaviour of water molecules passing through a nano-aperture nozzle

This study used molecular dynamics (MD) simulation to investigate the passage of water molecules through a composite graphene/Au nano-nozzle. Our focus was on the degree to which system temperature, extrusion speed, and nozzle diameter affect jet dynamics and the associated transient phenomena. Our findings show that high pressure and spatial confinement cause the nanojet from a small nozzle diameter (1.0?nm) to bend and twist, whereas the jets from a nozzle with a diameter of 1.5?nm present columns of greater stability. At 100?K, the H₂O nanojet froze at the outlet of the nozzle in the form of condensed icicles. At 500?K, the H₂O nanojet formed a loose spray and gaseous clusters. High extrusion speed of 55.824?m/s produced recirculating flow downstream from the nanojet with the appearance of an erupting volcano, which further prompted the jet column to thicken. Lower extrusion speeds produced jets with flow velocity insufficient to overcome the capillary force at the outlet of the nozzle, which subsequently manifests as unstable fluctuations in the flow rate.

• HIGHLIGHTS

• Water molecules through a composite graphene/Au nano-nozzle forming a nanojet is investigated.

• High pressure and spatial confinement cause the nanojet from a small nozzle diameter (≤1.0?nm) to bend and twist.

• High extrusion speed (≧55.824?m/s) produced recirculating flow downstream from the nanojet.

• Figure abstract: Schematic of the H₂O nano-jet through a nano-nozzle of graphene/Au

     

Molecular dynamics simulations of valveless pumping in a closed microfluidic tube-system

In this paper, we study the flow which is generated by a valveless pumping mechanism in a closed micro-fluidic tube-system consisting of two tubes with different radii. This system has been investigated by Ottesen [J. T. Ottesen, J. Math. Biol. 46, 309, (2003)] at a macroscopic level. We find that the results from the microscopic simulations qualitatively agree with the macroscopic results. Especially, it is shown that the direction of the flow changes with changing pumping frequency and the location of the pumping device. We study the local flow generated in the system and show that the stream velocities away from the pumping area can be modeled by a superposition of a plug flow and a zero-mean oscillatory flow.     

Resonance frequency distribution of cantilevered (5,5)(10,10) double-walled carbon nanotube with different intertube lengths

Analysis of ultrahigh frequency nanomechanical resonators, which are based on double-walled carbon nanotubes (DWCNTs) with various wall lengths, was carried out via classical molecular dynamics simulations. In the case of the inner wall entirely encapsulated inside the outer wall, the outer wall vibration has a significant effect on the vibration of the DWCNT; while in the case of the inner wall longer than the outer wall, the vibration of the extruded inner wall has a substantially stronger effect on the DWCNT vibration. It is shown that variations of the DWCNT resonance frequency with different wall lengths can be well fitted by Pearson VII and Gauss distribution functions. This result is potentially useful for developing design guidelines for making very fine tuners using DWCNT resonators of various wall lengths.     

Molecular dynamics simulations on hydrogen adsorption in finite single walled carbon nanotube bundles

Molecular dynamics simulations of the adsorption of hydrogen molecules in finite single-walled carbon nanotube bundles are presented using a curvature dependent force field. The heat of formation and the effective adsorption capacity are expressed as a function of H₂ distance from adsorbent. The heat of adsorption decreases rapidly with the distance and increasing H₂ loading results in weakening adsorption strength. The effects of nanotube packing and bundle thickness on hydrogen adsorption strength were investigated and the results show that the heat of adsorption can be improved slightly if hydrogen molecules are placed in thicker and inhomogeneously packed nanotube bundles. Only very small diameter nanotube bundles were found to hold promise for significant hydrogen storage for onboard applications.     

Interaction parameters between carbon nanotubes and water in Dissipative Particle Dynamics

Flow of water past an array of single-walled carbon nanotubes (SWNTs) is simulated in this work to determine the interaction parameters of carbon nanotubes (CNTs) and water using Dissipative Particle Dynamics (DPD). For this flow configuration, results from molecular dynamics simulations by Walther et al. are available and can be used for validation (Phys. Rev. E, 2004, 062201). The hydrodynamic properties for SWNT (32, 0) with diameter of 2.5 nm were determined in different Reynolds number flows. A set of appropriate DPD parameters was found so that the drag coefficients of the CNT agreed well with the Stokes–Oseen analytical solution and the fluid slip length on the CNT wall was comparable with the Walther et al. results. It was also found that it is feasible to apply these parameters in longer length and time scales by increasing the number of water molecules grouped into each DPD bead and still maintain the hydrodynamic properties of CNTs as well as their hydrophobic surface character.     

Effect of charge redistribution on the binding of DNA nucleotide to carbon nanotube in molecular dynamics

All-atom molecular dynamics (MD) simulations are performed to study the binding of DNA nucleotides with two carbon nanotubes (CNTs) with similar diameters but different chiralities. Two schemes for assigning partial atomic charges (PACs) are adopted: (I) using PACs obtained from isolated DNA nucleotide and CNT optimised in vacuum, and (II) using PACs obtained from optimising nucleotide-CNT hybrid in solution. The former approach is what most MD simulations have used in the study of DNA-CNT hybrids, while in the latter approach, a redistribution of the PACs has occurred upon the hybridisation. Our results show that the charge redistribution has a profound effect on the dynamics of binding. In particular, PACs obtained from (II) lead to more stable binding structures in the MD simulations. The findings suggest that care should be taken in simulating DNA-CNT interactions using the classical force field approach.     

The influence of nicotine on pioglitazone encapsulation into carbon nanotube: the investigation of molecular dynamic and density functional theory

In this work, molecular dynamics simulations of the insertion of pioglitazone into the nanotube with chirality (10, 10) at 400 K and 1 bar in the presence and absence of nicotine molecules and in different drug concentrations have been studied. The main aim is consideration of the effect of nicotine in the drug encapsulation process. The results indicate that encapsulation of pioglitazone could be attributed to the water flow via van der Waals and hydrophilic interactions. Because of the existence of the partial π–π interactions between aromatic rings of pioglitazone and the conjugated aromatic rings of nanotube, pioglitazone molecule can enter inside the nanotube. Some physical properties such as hydrogen bonding, number of contacts, also, the diffusion coefficient of the pioglitazone and water molecules, and variation of the center of mass have been calculated during the simulation. Furthermore, computing the electronic structure has also been done on model systems for quantitative determination of the adsorption energy (E_ads). The B3LYP/6-31G* level calculations on four different configurations of pioglitazone/carbon nanotube (CNT) and nicotine/CNT show that the interaction of drug with the inside of the nanotube is stronger than the other forms.     

基于稳定同位素的SPAC水碳拆分及耦合研究进展

土壤-植被-大气连续体(SPAC)是陆地水文学、生态学和全球变化领域的重要研究对象,其水碳循环过程及耦合机制是前沿性问题.稳定同位素技术示踪、整合和指示的特征有助于评估分析生态系统固碳和耗水情况.本文在简述稳定同位素应用原理和技术的基础上,重点阐释了基于稳定同位素光学技术的SPAC系统水碳交换研究进展,包括:在净碳通量中拆分光合与呼吸量,在蒸散通量中拆分蒸腾与蒸发量,以及在系统尺度上的水碳耦合研究.新兴的技术和方法实现了生态系统尺度上长期高频的同位素观测,但在测量精准度、生态系统呼吸拆分、非稳态模型适应性、尺度转换和水碳耦合机制等方面存在挑战.本文探讨了现有主要研究成果、局限性以及未来研究展望,以期对稳定同位素生态学领域的新研究和技术发展有所帮助.     

Neon atoms oscillating inside carbon and boron nitride nanotubes: a fully atomistic molecular dynamics investigation

In the present work, based on extensive fully atomistic molecular dynamics simulations, we discuss the dynamics of neon atoms oscillating inside (5,5) single-walled carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). Our results show that sustained high-frequency oscillatory regimes are possible for a large range of temperatures. Our results also show that the general features of the oscillations are quite similar to those observed in CNT and BNNT, in contrast with some speculations in previous works in the literature about the importance of broken symmetry and chirality exhibited by BNNTs.     

Interfacial layering and orientation ordering of ionic liquid around single-walled carbon nanotube: a molecular dynamics study

Single-walled carbon nanotubes (SWNTs) tend to aggregate to heavily tangled bundles due to the strong van der Waals attraction. Ionic liquids (ILs) are a kind of newly proposed solvents in which SWNT can be physically well dispersed. In this article, the cylindrical interface has been investigated by molecular dynamics simulation between IL of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and an infinite long armchair (6,6) SWNT. The highly ordered structure of the cations and anions is elucidated by the simulation results. Two evident dense layers are found for both the cations and anions along the surface normal direction of the SWNT. In addition, we have observed two different orientation patterns of the cations in the first layer. In sublayer 1A, which is the nearest to the surface, the imidazolium rings of the cations prefer to be parallel to the surface, with a slight tilt angle less than 15°. In sublayer 1B, they tend to be perpendicular to the surface, with their butyl chains appearing in sublayer 1A. The [BF₄]^?  anions are found to cling to the nanotube surface with three fluoride atoms, also indicating a highly ordered orientation. The simulation results in this work provide a clue to understand the stabilisation and dispersion of SWNT bundles in ILs.     

Molecular dynamics simulations of heat conduction in multi-walled carbon nanotubes

Heat conduction in multi-walled carbon nanotubes (MWNTs) was studied using non-equilibrium molecular dynamics simulations. This research focuses on the effects of the multi-wall structure of the MWNTs on the heat conduction. The results show that the thermal conductivity of a MWNT is almost the same as that of the corresponding single-walled carbon nanotubes (SWNTs) rather than much smaller as has been suggested. Thus, the multi-wall structure does not significantly affect the thermal conduction in the MWNTs. Analysis of the temperature profiles and the phonon density of states confirms that there is almost no heat transport between the MWNT layers and that each layer conducts heat nearly independently along parallel channels. This is physically reasonable since the weak inter-wall interactions and large interfacial thermal resistances make the MWNT layers behave like parallel thermal circuits.