纳米通道水输运与离子筛选的微观力学机理

发布时间：2018-11-28 10:23

【摘要】：纳米通道水输运和离子筛选被广泛应用于海水淡化、生物仿生、微纳器件制备等各个领域,对水输运和离子筛选微观机理的研究在物理、化学、力学、生命科学、材料科学等学科中具有重要意义。本文围绕边界滑移、接触角滞后和尺寸效应三个关键力学问题,系统研究了纳米通道水输运和离子筛选的微观力学机理。基于分子动理论提出了适用于复杂表面边界滑移的理论模型。引入等效势阱深度的概念用以表征固液作用机制对边界滑移的影响。采用分子动力学模拟研究了化学复杂表面和几何复杂表面上的纳米流动边界滑移行为。该扩展的分子动理论滑移模型能够预测大剪应力下复杂壁面上滑移速度的非线性变化,从微观尺度更直观地解释壁面构型对于界面流动的影响。基于接触角滞后机制分析研究了非平行光滑和锯齿壁面间液滴的定向输运现象。壁面的重复张开和闭合对液滴施加连续的拉伸和挤压,为液滴输运提供动力。不对称锯齿构型阻碍了液滴的后退运动,使得液滴在每个开合周期内都能获得净位移。借助于锯齿壁面的"类棘轮效应"不但抑制了疏水壁面上液滴远离尖端的反向运动,而且有效地提高了液滴向尖端的输运效率。证实了接触线钉扎并不是液滴输运过程中的必然阶段,液滴向尖端的输运可以通过完整开合周期内向尖端净位移的累积实现。基于螺旋纳米线和纳米管道的耦合效应设计形成了纳米螺旋泵,并将其用于水的定向输运。通过对不同几何参数纳米螺旋泵内水输运机制的研究发现,纳米螺旋泵中的水输运存在三种模式:簇状、伪连续和线性连续。研究了叶片转速、叶片润湿性以及螺旋泵管径等参数对于输运机制的影响。结果表明,水流率随螺旋线润湿性的降低而提高。疏水小管径螺旋泵中由于空间受限所导致的弱氢键作用加剧了水分子的非线性输运。将纳米螺旋泵扩展应用到脱盐领域。通过缩小叶片螺距创造受限空间,基于尺寸效应实现高效离子分离。螺旋泵内的水输运机制可以通过受限阻力和螺距决定的水输运模式的协同作用来解释。基于增大螺旋泵管径和减小螺旋叶片包含的螺距数目可以实现对水透过率的有效优化。盐离子的完全过滤和高水流率的同时实现证明了纳米螺旋泵不仅是海水脱盐的理想纳米器件,也可以被应用到其他更为普遍的基于尺寸效应的分离操作中。
[Abstract]:Nano-channel water transport and ion screening are widely used in various fields such as seawater desalination, bionics, fabrication of micro-nano devices and so on. The microscopic mechanism of water transport and ion screening has been studied in physics, chemistry, mechanics, life science, etc. Material science and other disciplines are of great significance. In this paper, the micromechanical mechanisms of water transport and ion screening in nanochannels are systematically studied, focusing on three key mechanical problems: boundary slip, contact angle hysteresis and size effect. Based on the theory of molecular dynamics, a theoretical model for boundary slip of complex surfaces is proposed. The concept of equivalent potential well depth is introduced to characterize the effect of solid-liquid interaction mechanism on boundary slip. The boundary slip behavior of nanoscale flow on a chemically complicated surface and a geometric complex surface was studied by molecular dynamics simulation. The extended molecular kinetic slip model can predict the nonlinear variation of slip velocity on complex wall under large shear stress, and explain the effect of wall configuration on the interface flow more intuitively from the microscopic scale. Based on the contact angle hysteresis mechanism, the directional transport of droplets between non-parallel smooth and sawtooth walls is studied. The repeated opening and closing of the wall exerts continuous stretching and extrusion on the droplets to provide power for droplet transport. The asymmetric serrated configuration hinders the droplet's backward movement and makes the droplet gain a net displacement during each opening and closing period. With the help of the ratchet effect on the sawtooth wall, not only the reverse motion of the droplets away from the tip on the hydrophobic wall is restrained, but also the transport efficiency of the droplets to the tip is improved effectively. It is proved that the pinning of contact line is not an inevitable stage in the process of droplet transport. The transport of droplets to the tip can be realized by the accumulation of net displacement to the tip during the complete opening and closing period. Based on the coupling effect of helical nanowires and nanotubes, a nano-screw pump was designed and used for the directional transport of water. By studying the mechanism of water transport in nano-screw pump with different geometric parameters, it is found that there are three modes of water transport in nano-screw pump: cluster, pseudo-continuous and linear continuity. The effects of blade speed, vane wettability and diameter of screw pump on the transport mechanism were studied. The results show that the flow rate increases with the decrease of helix wettability. The weak hydrogen bond in the hydrophobic small diameter helical pump due to space limitation intensifies the nonlinear transport of water molecules. The nanometer screw pump is extended to desalination field. High efficiency ion separation is realized based on size effect by reducing blade pitch to create limited space. The mechanism of water transport in the screw pump can be explained by the synergistic effect of the limited resistance and the helical pitch determined by the water transport mode. The water transmittance can be effectively optimized by increasing the diameter of the screw pump pipe and reducing the number of pitch contained in the screw blade. The complete filtration of salt ions and the realization of high flow rate at the same time prove that the nano-screw pump is not only an ideal nano-device for desalination of seawater, but also can be applied to other more general separation operations based on size effect.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O363.2

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