arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1403.4615cond-mat.soft
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纳米尺度下的毛细管粘附

Capillary Adhesion at the Nanometer Scale

Shengfeng Cheng, Mark O. Robbins

摘要:

利用分子动力学模拟研究了非挥发性液体弯月面在球形针尖与平面基底之间的毛细粘附。针尖的原子结构、针尖半径、液体在两个表面上的接触角以及液体桥的体积是变化的。针尖和基底之间的毛细作用力被计算为它们之间的距离h的函数。力同意连续预测h下降到~ 5至10 nm。在较小的h,力往往比预测的吸引力小,并有很强的振荡。在连续介质理论中,毛细力的这种振荡分量完全被忽略,连续介质理论仅包括来自弯月面圆周周围的表面张力和弯月面截面上的压力差的贡献。振荡被发现是由于分子层的液体被限制在尖端和基板之间的狭窄间隙。这种效应对于较大的尖端半径和/或光滑表面最为明显。另外两个组成部分考虑的连续统理论也确定。的表面张力项,以及弯月面形状,准确地描述了连续预测h下降到~ 1 nm,但毛细管压力项总是比相应的连续结果更积极。毛细管压力的这种变化降低了平均粘附力的一个因素,从其连续值大至2,并被发现是由于在压力张量的各向异性。横截面分量与连续介质理论预测的毛细管压力一致(即,杨-拉普拉斯方程),但确定毛细力的法向压力总是比连续介质对应物更正。

英文摘要:

Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions for h down to ~ 5 to 10nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the continuum theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the continuum theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the continuum prediction for h down to ~ 1nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The cross-sectional component is consistent with the capillary pressure predicted by the continuum theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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