Orientations of special water dipoles that accelerate water molecules exiting from carbon nanotube

One-dimensional ordered water molecules entering and exiting from a carbon nanotube with an appropriate radius are studied with molecular dynamics simulations. It can be found that a water molecule near the nanotube end is more likely to be expelled from the nanotube if its dipole is almost perpendicular to the nanotube axis. The key to this observation is that those water molecules are closer to the wall of the nanotube away from the equilibrium position of the Lennar-Jones (LJ) potential. Thus, the interaction energy for those water molecules is relatively high. There are two particular structures of the perpendicular water molecule depending on the dipole direction of the adjacent water molecule in the nanotube. Although the probabilities of these structures are quite small, their contributions to the net flux across the nanotube end are approximately equal to the predominant structures. The present findings further show the possibility of controlling the water flow by regulating the dipole directions of the water molecules inside the nanochannels.     

Water structures inside and outside single-walled carbon nanotubes under perpendicular electric field

The structures of water inside and outside(6,6),(8,8), and(10,10) singlewalled carbon nanotubes(SWCNTs) under an electric field perpendicular to the tube axis are investigated by molecular dynamics simulations. The results show that dipole reorientation induced by electric field plays a significant role on the structures of confined water inside and outside SWCNTs. Inside SWCNTs, the average water occupancy and the average number of hydrogen bonds(H-bonds) per water molecule decrease as the electric intensity increases. Because the field intensity is sufciently strong, the initial water structures inside the SWCNTs are destroyed, and the isolated water clusters are found. Outside SWCNTs, the azimuthal distributions of the density and the average number of H-bonds per water molecule around the solid walls become more and more asymmetric as the electric intensity increases. The percentages of water molecules involved in 0–5 H-bonds for all the three types of SWCNTs under diferent field intensities are displayed. The results show that those water molecules involved with most H-bonds are the most important to hold the original structures. When the electric field direction is parallel with the original preferred orientation, the density and the H-bond connections in water will be increased; when the electric field direction is perpendicular to the original preferred orientation, the density and the H-bond connections in water will be decreased.     

Detailed investigation on single water molecule entering carbon nanotubes

The behavior of a water molecule entering carbon nanotubes (CNTs) is studied. The Lennard-Jones potential function together with the continuum approximation is used to obtain the van der Waals interaction between a single-walled CNT (SWCNT) and a single water molecule. Three orientations are chosen for the water molecule as the center of mass is on the axis of nanotube. Extensive studies on the variations of force, energy, and velocity distributions are performed by varying the nanotube radius and the orientations of the water molecule. The force and energy distributions are validated by those obtained from molecular dynamics (MD) simulations. The acceptance radius of the nanotube for sucking the water molecule inside is derived, in which the limit of the radius is specified so that the nanotube is favorable to absorb the water molecule. The velocities of a single water molecule entering CNTs are calculated and the maximum entrance and the interior velocity for different orientations are assigned and compared.     

Guided motion of short carbon nanotube driven by non-uniform electric field

The molecular dynamics simulations are performed to show that in aque- ous environments, a short single-walled carbon nanotube （SWCNT） guided by a long SWCNT, either inside or outside the longer tube, is capable of moving along the nanotube axis unidirectionally in an electric field perpendicular to the carbon nanotube （CNT） axis with the linear gradient. The design suggests a new way of molecule transportation or mass delivery. To reveal the mechanism behind this phenomenon, the free energy profiles of the system are calculated by the method of the potential of mean force （PMF）.     

SPH方法研究空气在平板入水中的影响

飞机水上迫降以及船体在高速行驶时,均会发生结构物砰击水面的现象,当结构物底面较为平坦,砰击水面时会捕获空气,在板底形成空气垫。研究空气垫的形成及影响,有助于对结构物进行更准确的载荷分析。该问题的研究涉及到气固液三相的相互作用,在数值模拟中仍是一个挑战,本文使用光滑粒子流体动力学(SPH)方法对该问题进行模拟。首先,与实验结果进行对比;其次,系统地研究了平板形成空气垫以及砰击水面的过程,考查了空气、水和平板的动能变化。对比了在SPH模拟中考虑空气与不考虑空气得到的水压力场和平板所受压力,说明了空气对结果的影响;最后,研究了平板宽度对空气垫形成的影响,从而进一步影响加速度。     

Localized dipoles: from 2D to rotating shallow water

The progress made in the theory of localized dipoles over the course of the past century is overviewed. The dependence between the dipole shape, on the one hand, and the vorticity–streamfunction relation in the frame of reference co-moving with the dipole, on the other hand, is discussed. We show that, in 2D non-divergent and quasi-geostrophic dipoles, circularity of the trapped-fluid region and linearity of the vorticity–streamfunction relation in this region are equivalent. The existence of elliptical dipoles of high smoothness is demonstrated. A generalization of the dipole theory to the rotating shallow water model is offered. This includes the construction of localized f-plane dipole solutions (modons) and demonstration of their soliton nature, and derivation of a necessary condition for an eastward-traveling β-plane modon to exist. General properties of such ageostrophic modons are discussed, and the fundamental dissimilarity of fast and/or large dipoles in the rotating shallow water model from quasi-geostrophic dipoles is demonstrated and explained.     

Yarn spun from carbon nanotube forests: Production,structure, properties and applications

The discovery of drawable carbon nanotube forests opened up the possibility of constructing a wide range of pure carbon nanotube macrostructures and sparked interests in developing applications from these structures, especially pure carbon nanotube yarns. This review examines the various facets of the drawable carbon nanotube forests, synthesis and drawability, and their resulting yarns, structure, production, properties and applications. The structure, formation and properties of carbon nanotube yarns are compared with those of conventional textile yarns in order to obtain a better understanding of the science, structural mechanics and processing technology involved in carbon nanotube yarns.     

Elastic interaction between force dipoles on a stretchable substrate

The objective of this paper is to study elastic interaction between force dipoles on the surface of a semi-infinite stretchable substrate. The substrate undergoes a uniform, finite pre-stretch, while the additional deformation induced by a force dipole is assumed infinitesimal. By adopting a neo-Hookean constitutive law, the surface Green function for the pre-stretched substrate is obtained. The result is then used to derive the energy of dipolar interaction on the surface. As an application, mutual interaction between two physisorbed molecules is discussed in detail. The dipole moment of a molecule is found from the elementary intermolecular potential, and its dependence on the pre-stretch is established explicitly. Numerical results indicate that pre-stretches can substantially alter the interaction, and thus provide a controllable way to guide the self-assembly of adsorbed molecules on the substrate surface.     

Forces acting on water droplets falling in oil under the influence of an electric field: numerical predictions versus experimental observations

The combination of an electric field and a moderate turbulent flow is a promising technique for separating stable water–oil emulsions. Field-induced charges on the water droplets will cause adjacent droplets to align with the field and attract each other. The present work describes the forces that influence the kinematics of droplets falling in oil when exposed to an electric field. Mathematical models for these forces are presented and discussed with respect to a possible implementation in a multi-droplet Lagrangian framework. The droplet motion is mainly due to buoyancy, drag, film-drainage, and dipole–dipole forces. Attention is paid to internal circulations, non-ideal dipoles, and the effects of surface tension gradients.Experiments are performed to observe the behavior of a droplet falling onto a stationary one. The droplet is exposed to an electric field parallel to the direction of the droplet motion. The behavior of two falling water droplets exposed to an electric field perpendicular to the direction of their motion is also investigated until droplet coalescence. The droplet motion is recorded with a high-speed CMOS camera. The optical observations are compared with the results from numerical simulations where the governing equations for the droplet motion are solved by the RK45 (Runge Kutta) Fehlberg method with step-size control and low tolerances. Results, using different models, are compared and discussed in detail. A framework is otlined to describe the kinematics of both a falling rigid spherical particle and a fluid droplet under the influence of an electric field.     

基于SPH/FEM方法的浅水冲击喷溅及其抑制结构研究

物体浅水冲击会造成结构响应和自由液面的剧烈喷溅,以往的研究多集中在物体受水的冲击响应方面,而对浅水层受物体冲击引起的剧烈喷溅问题及其喷溅抑制结构的相关研究很少。本文基于SPH/FEM耦合方法对浅水冲击的液体喷溅特性开展研究。首先,计算了刚性长方体结构的冲击喷溅,与文献中准二维溅水试验结果进行了比较,将浅水喷溅分为初始喷溅、二次喷溅和底部射流三部分。分别研究了各部分喷溅的形成机理,指出二次喷溅由结构对水层的挤压和底部射流的能量传递共同引起,并分析了水深对二次喷溅的影响规律。其次,对喷溅抑制结构的机理做了数值研究,发现拱形翻边构型可以抑制初始喷溅,但不能有效降低二次喷溅。最后,提出了一种带凹槽和阻水边条的新型翻边构型,对二次喷溅具有更好的抑制效果。