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.  

改性对硅胶纳米通道润湿性的影响规律研究

研究了多孔硅胶在改性前后不同的壁面特征对纳米通道润湿性的影响规律。采用非平衡分子动力学方法研究了水分子在改性前羟基化及改性后不同链长硅烷化壁面的纳米通道内密度、速度的分布状况，并分析了氢键的径向分布函数。结果表明：羟基化壁面通道的固液界面处流体出现高密度层，几乎无速度滑移，流体密度最大值为1.24g/cm3，滑移速度仅为0.056?/ps；链长为C4、C8、C12的硅烷化壁面通道的固液界面处水的密度分别为1.18g/cm3、1.12g/cm3、1.01g/cm3，滑移速度分别为0.402?/ps、1.211?/ps、1.810?/ps；随烷基链长的增加，固液界面处水的密度接近纯水的密度1.0g/cm3，固液界面处出现了速度滑移；羟基化通道壁面由于较强的固液相互作用势呈强亲水性；改性后的硅烷化壁面呈疏水性，并随改性层烷基链长度的增加而增强，且固液界面处出现“滑移效应”；羟基化通道壁面与水分子存在较强的氢键作用，而改性后硅烷化通道壁面与水分子无氢键的形成，改性层烷基链长的增加减弱了两者的相互作用势，进一步揭示了其疏水机理。  

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.