分子动力学模拟剪切速度对纳米间隙中角鲨烷界面滑移的影响

doi:10.11949/j.issn.0438-1157.20150755

摘要/Abstract

摘要：

采用聚合物一致性力场(PCFF),分别在7种剪切速度V和3种油膜厚度h下对纳米间隙中润滑剂角鲨烷进行分子动力学模拟,分析固液界面的密度、分子和流速的分布,探究纳米薄膜润滑的润滑机理和剪切速度对界面滑移的影响。结果表明,纳米间隙中润滑剂存在分层现象,各层间距相近,并非越远离固体壁面层间距越大,层间距约为角鲨烷分子单个C-C键距离的3~4倍;随着油膜厚度的减小,纳米间隙中润滑剂层状分布越明显,固化层密度越大;当油膜厚度为3.44 nm时,固液界面滑移现象明显,滑移长度b值随着V先增大后减小,当V为22.8 m·s^-1时,b达到最大值4.35 nm;根据模拟和计算结果,给出滑移长度与剪切速度的关系公式。

关键词: 分子模拟, 纳米结构, 聚合物, 界面滑移, 薄膜润滑, 角鲨烷

Abstract:

Molecular dynamics (MD) simulations using the polymer consistent force field (PCFF) were adopted to investigate the density, molecular and velocity distributions of lubricant squalane in nanogap at 293 K, three different film thicknesses and a wide range of shear velocities. The lubrication mechanism and boundary slip were analyzed. The results showed that the lubricant atoms tended to form layers parallel to the confining wall. The distances between the layers of lubricant atoms were irregular rather than broadening far away from the walls and were about three to four times the length of C-C bond in the squalane. The tendency of lubricant atoms to form layers and the density of solid-like layer increased with decreasing film thickness. It was clearly to find the boundary slip at the solid-liquid interface from the velocity profile. The slip lengths increased with increasing velocity of substrates at the beginning, and then decreased. When the film thickness was 3.44 nm, the maximum slip length was 4.35 nm at the substrate velocity of 22.8 m·s^-1. According to the simulations, the relationship between the slip length and the shear velocity was given.

Key words: molecular simulation, nanostructure, polymers, boundary slip, thin film lubrication, squalane

中图分类号:

TH117.2

潘伶, 高诚辉. 分子动力学模拟剪切速度对纳米间隙中角鲨烷界面滑移的影响[J]. 化工学报, 2016, 67(4): 1440-1447.

PAN Ling, GAO Chenghui. Molecular dynamics simulation of boundary slip in nanogap: effect of shear velocity[J]. CIESC Journal, 2016, 67(4): 1440-1447.

参考文献 34

[1]	OLDSTEIN S. Modern Developments in Fluid Dynamics[M]. Vol Ⅱ. Oxford: Clarendon Press, 1957: 676-680.
[2]	BEAVERS G S, JOSEPH D D. Boundary conditions at a naturally permeable wall[J]. Journal of Fluid Mechanics, 1967, 30 (1): 197-207. DOI: 10.1017/S0022112067001375
[3]	MA G J, WU C W, ZHOU P. Wall slip and hydrodynamics of two-dimensional journal bearing[J]. Tribology International, 2007, 40: 1056-1066. DOI: 10.1016/j.triboint.2006.10.003.
[4]	THOMPSON P A, TROIAN S M. A general boundary condition for liquid flow at solid surfaces[J]. Nature, 1997, 389: 360-362. DOI: 10.1038/38686.
[5]	RIEZJEV N V, TROIAN S M. Molecular origin and dynamic behavior of slip in sheared polymer films[J]. Physical Review Letters, 2004, 92 (1): 018302. DOI: 10.1103/PhysRevLett.92.018302.
[6]	JING D, BHUSHAN B. Boundary slip of superoleophilic, oleophobic, and superoleophobic surfaces immersed in deionized water, hexadecane, and ethylene glycol[J]. Langmuir, 2013, 29 (47): 14691-14700. DOI: 10.1021/la4030876.
[7]	ESPINOSA-MARZAL R M, ARCIFA A, ROSSI A, et al. Microslips to "avalanches" in confined, molecular layers of ionic liquids[J]. The Journal of Physical Chemistry Letters, 2013, 5 (1): 179-184. DOI: 10.1021/jz402451v.
[8]	陈其乐, 孔宪, 卢滇楠, 等. 外壁电荷性质对双壁碳纳米管中水分子运动行为的影响[J]. 化工学报, 2014, 65 (1): 319-327. DOI: 10.3969/j.issn.0438-1157.2014.01.042. CHEN Q L, KONG X, LU D N, et al. Molecular simulation of outer surface charge on water transport through double-wall carbon nanotube[J]. CIESC Journal, 2014, 65 (1): 319-327. DOI: 10.3969/j.issn.0438-1157.2014.01.042.
[9]	张程宾, 赵沐雯, 陈永平, 等. 流体密度对纳通道内流动滑移的影响[J]. 化工学报, 2012, 63 (S1): 12-16. DOI: 10.3969/j.issn.0438-1157.2012.zl.003. ZHANG C B, ZHAO M W, CHEN Y P, et al. Effects of fluid density on velocity slip in nanochannels[J]. CIESC Journal, 2012, 63 (S1): 12-16. DOI: 10.3969/j.issn.0438-1157.2012.zl.003.
[10]	潘伶, 高诚辉. 纳米间隙润滑剂季戊四醇四酯的压缩性能分子动力学模拟[J]. 机械工程学报, 2015, 51 (5): 76-82. DOI: 10.3901/JME.2015.05.076. PAN L, GAO C H. Molecular dynamics simulation on the compressibility of pentaerythritol tetra in nanogap[J]. Journal of Mechanical Engineering, 2015, 51 (5): 76-82. DOI: 10.3901/JME.2015.05.076.
[11]	PAN L, GAO C H. Confined fluid density of a pentaerythritol tetraheptanoate lubricant investigated using molecular dynamics simulation[J]. Fluid Phase Equilibria, 2015, 385: 212-218. DOI: 10.1016/j.fluid.2014.11.014.
[12]	TSIGE M, PATNAIK S S. An all-atom simulation study of the ordering of liquid squalane near a solid surface[J]. Chemical Physics Letters, 2008, 457 (4/5/6): 357-361. DOI: 10.1016/j.cplett.2008.04.026.
[13]	KARNIADAKIS G, BESKOK A, ALURU N. Microflows and Nanoflows: Fundamentals and Simulation[M]. Berlin Heidelberg, New York: Springer, 2004: 1-23.
[14]	PERTSIN A J, GRUNZE M. Long-ranged solvation forces in a fluid with short-ranged interactions[J]. Journal of Chemical Physics, 2003, 118 (17): 8004-8009. DOI: 10.1063/1.1564051.
[15]	LOI S, SUN G, FRANZ V, et al. Rupture of molecular thin films observed in atomic force microscopy (II): Experiment[J]. Physical Review E, 2002, 66 (3): 031602. DOI: 10.1103/PhysRevE.66.031602.
[16]	FRANZ V, BUTT H-J. Confined liquids: solvation forces in liquid alcohols between solid surfaces[J]. The Journal of Physical Chemistry B, 2002, 106 (7): 1703-1708. DOI: 10.1021/jp012541w.
[17]	陈天星. 利用原子力显微镜对近壁面受限液体性质的研究[D]. 北京: 清华大学, 2011. CHEN T X. Study on properties of the confined liquids at solid surface with AFM[D]. Beijing: Tsinghua University, 2011.
[18]	SIVEBAEK I M, SAMOILOV V N, PERSSON B N J. Squeezing molecularly thin alkane lubrication films: layering transitions and wear[J]. Tribology Letters, 2004, 16 (3): 195-200. DOI: 10.1023/B:TRIL.0000009730.31175.82.
[19]	胡元中, 王慧, 郭炎. 超薄油膜润滑的分子动力学模拟[J]. 摩擦学学报, 1995, 15 (2): 138-144. DOI: 10.16078/j.tribology.1995.02.007. HU Y Z, WANG H, GUO Y. Molecular dynamics simulation of ultra thin film lubrication (I): Rigid molecule model[J]. Tribology, 1995, 15 (2): 138-144. DOI: 10.16078/j.tribology.1995.02.007.
[20]	王慧, 胡元中, 郭炎. 超薄润滑膜界面滑移现象的分子动力学研究[J]. 清华大学学报 (自然科学版), 2000, 40 (4): 107-110. WANG H, HU Y Z, GUO Y. Molecular dynamics study of interfacial slip behavior of ultrathin lubricating films[J]. J. Tsinghua Univ. (Sci. & Tech.), 2000, 40 (4): 107-110.
[21]	NAGAYAMA G, CHENG P. Effects of interface wettability on microscale flow by molecular dynamic simulation[J]. International Journal of Heat and Mass Transfer, 2004, 47: 501-513. DOI: 10.1016/j.ijheatmasstransfer.2003.07.013.
[22]	ASPROULIS N, DRIKAKIS D. Boundary slip dependency on surface stiffness[J]. Physical Review E, 2010, 81 (6): 061503. DOI: 10.1103/PhysRevE.81.061503.
[23]	ZHANG H, ZHANG Z, YE H. Molecular dynamics-based prediction of boundary slip of fluids in nanochannels[J]. Microfluidics and Nanofluidics, 2012, 12 (1-4): 107-115. DOI: 10.1007/s10404-011-0853-y.
[24]	NOORIAN H, TOGHRAIE D, AZIMIAN A R. Molecular dynamics simulation of poiseuille flow in a rough nano channel with checker surface roughnesses geometry[J]. Heat and Mass Transfer, 2014, 50 (1): 105-113. DOI: 10.1007/s00231-013-1232-x.
[25]	PLIMPTON S. LAMMPS molecular dynamics simulator[EB/OL].[2015-5-15]. http://lammps.sandia.gov.
[26]	PLIMPTON S. Fast parallel algorithms for short-range molecular dynamics[J]. Journal of Computational Physics, 1995, 117 (1): 1-19. DOI: 10.1006/jcph.1995.1039.
[27]	SUN H. Ab initio calculations and force field development for computer simulation of polysilanes[J]. Macromolecules, 1995, 28 (3): 701-712. DOI: 10.1021/ma00107a006.
[28]	SUN H. COMPASS: an ab initio force-field optimized for condensed-phase applications overview with details on alkane and benzene compounds[J]. Journal of Physical Chemistry B, 1998, 102 (38): 7338-7364. DOI: 10.1021/jp980939v.
[29]	NAVIER C L M H. Memoire sur les du movement des fluids[J]. Mem l'Acad Roy Sci l'Inst France, 1823, 6: 389-440.
[30]	VADAKKEPATT A, DONG Y, LICHTER S, et al. Effect of molecular structure on liquid slip[J]. Physical Review E, 2011, 84 (6): 066311. DOI: 10.1103/PhysRevE.84.066311.
[31]	MENDONCA A C F, FOMIN Y D, MALFREYT P, et al. Novel ionic lubricants for amorphous carbon surfaces: molecular modeling of the structure and friction[J]. Soft Matter, 2013, 9 (44): 10606-10616. DOI: 10.1039/C3SM51689J.
[32]	LIM R, O'SHEA S J. Solvation forces in branched molecular liquids[J]. Physical Review Letters, 2002, 88 (24): 246101. DOI: 10.1103/PhysRevLett.88.246101.
[33]	LIM R, LI S F Y, O'SHEA S J. Solvation forces using sample-modulation atomic force microscopy[J]. Langmuir, 2002, 18 (16): 6116-6124. DOI: 10.1021/la011789+.
[34]	ZHENG X, ZHU H, KOSASIH B, et al. A molecular dynamics simulation of boundary lubrication: the effect of n-alkanes chain length and normal load[J]. Wear, 2013, 301 (1/2): 62-69. DOI: 10.1016/j.wear.2013.01.052.