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How to overcome the friction between the micro components has become a key point of the successful operation of the micro/nano-electric mechanical systems. The understanding of the friction mechanism of the alkane liquid film confined between two substrates is important when the friction law on a macro/nano scale is not applicable. In this work, the molecular dynamics simulations are used to study the effect of the chain length on the friction properties of the liquid films that are confined between two golden substrates. There are seven pure alkane liquid films that are composed of one molecule CnH2n + 2(n = 6, 8, 10, 12, 14, 16, 18), and six mixed alkane liquid films that are composed of two molecules C6H14/CnH2n + 2(n = 8, 10, 12, 14, 16, 18) with a ratio of 1∶1. The results show that the friction force and the coefficient of friction of pure alkane liquid films both increase as the chain length increases when the carbon atom number is less than 12, whereas the friction property keeps stable when the carbon atom number of the alkane molecule is greater than 10 and the pure hexadecane liquid film has the largest friction force. In the mixed films, the addition of short chain alkane molecules can strengthen the friction, and the hexane/dodecane mixed film has the maximum friction force. The short chain molecule dilutes the C8H18 film and C10H22 film which cause the friction force to decrease. During the sliding progress, the formation of solid-like high density-packet layers is the main reason for the friction reduction. When no solid-like layer or just one solid-like layer is formed at the interface of golden base, the liquid alkane film is liquid-like and its viscosity becomes much larger than that in the normal state, which leads to high friction force. The short chain molecules reduce the density of the solid-like layers, which causes the film to transform from solid-like state to liquid state, thus resulting in the increase of friction. The friction property mainly depends on the layered structure, and the interaction between the golden surface and liquid film contributes to the friction. This study helps to understand the friction mechanism of ultra-thin liquid films.
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Keywords:
- nanotribology /
- liquid film /
- chain length /
- structure
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图 1 液态烷烃C18H38润滑膜模型
Figure 1. Liquid alkane C18H38 lubricant film model
图 2 (a) C12H26液体膜在滑动过程中的摩擦力随滑动距离的变化; (b) 7种液体膜的平均摩擦力和平均摩擦系数
Figure 2. (a) Friction curve of C12H26 liquid film in sliding process with sliding distance; (b) the average friction force and average coefficient of friction (COF) of the seven liquid films
图 3 六种混合分子膜的平均摩擦力和平均摩擦系数
Figure 3. The average friction force and average COF of the six mixed films
图 4 (a)纯C12H26液体膜在滑动过程中的分层结构; (b)四种纯液体膜沿Z方向的密度分布
Figure 4. (a) Layered structure of C12H26 liquid film in sliding process; (b) density distribution along Z direction of four pure liquid films (C6H14, C12H26, C16H34, C18H40).
图 5 混合液体膜在上基板表面形成的分层内分子分布图 (a) C6C8; (b) C6C12; (c) C6C18
Figure 5. Distribution of the molecules in the layer formed on the surface of the base: (a) C6C8; (b) C6C12; (c) C6C18.
图 6 混合膜在滑动过程沿着Z方向密度分布
Figure 6. Distribution of density along Z direction in sliding process of mixed films
表 1 纯液体膜中上基板与液体膜间相互作用 (kJ/mol)
Table 1. Interaction between upper substrate and liquid film (kJ/mol)
模型 C6 C8 C10 C12 C14 C16 C18 作用能 2285 2434 2438 2484 2496 2408 2488 
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表 2 混合液体膜中上基板与液体膜间相互作用(kJ/mol)
Table 2. Interaction between upper substrate and mixed liquid film (kJ/mol).
模型 C6C8 C6C10 C6C12 C6C14 C6C16 C6C18 作用能 2396 2396 2893 2438 2400 2505
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[1] Lewis J B, Vilt S G, Rivera J L, Jennings G K, Mccabe C 2012 Langmuir 28 14218
Google Scholar
[2] Yang G, Jin F, Yu L, Zhang P 2015 Tribo. Lett. 57 12
Google Scholar
[3] Cheng H, Hu Y 2012 Adv. Colloid Interface Sci. 171–172 53
Google Scholar
[4] 潘鹤, 滕淑华, 丁翠翠 2012 物理化学学报 28 917
Google Scholar
Pan H, Teng S H, Ding C C 2012 Acta Phys. Chim. Sin. 28 917
Google Scholar
[5] Mcdermott M T, Green J B D, Porter M D 1997 Langmuir 13 25040
[6] Booth B D, Vilt S G, Mccabe C, Jennings G K 2009 Langmuir 25 9995
Google Scholar
[7] 刘蕾, 宋仕永, 张平余 2012 物理化学学报 28 427
Google Scholar
Liu L, Song S Y, Zhang P Y 2012 Acta Phys. Chim. Sin. 28 427
Google Scholar
[8] 张兆慧, 韩奎, 曹娟, 王帆, 杨丽娟 2012 61 028701
Google Scholar
Zhang Z H, Han K, Cao J, Wang F, Yang L J 2012 Acta Phys. Sin. 61 028701
Google Scholar
[9] Gosvami N N, Egberts P, Bennewitz R 2011 J. Phys. Chem. A 115 6942
Google Scholar
[10] Granick S 1991 Science 253 1374
Google Scholar
[11] Cui S T, Cummings P T, Cochran H D 2001 Fluid Phase Equilibria 183–184 381
Google Scholar
[12] Vasko A A, Kutsenko V Y, Marchenko A A, Braun O M 2019 Tribo. Lett. 67 49
Google Scholar
[13] Ewen J P, Gattinoni C, Zhang J, Heyes D M, Spikes H A, Dini D 2017 Phys. Chem. Chem. Phys. 19 17883
Google Scholar
[14] Jorgensen W L, Tirado-Rives J 1988 J. Am. Chem. Soc. 110 1657
Google Scholar
[15] Maxwell D S, Tirado-Rives J, Jorgensen W L 1996 J. Am. Chem. Soc. 118 11225
Google Scholar
[16] Damm W, Frontera A, Tirado-Rives J, Jorgensen L W 1997 J. Comp. Chem. 18 1955
Google Scholar
[17] 张兆慧, 李海鹏, 韩奎 2013 62 158701
Google Scholar
Zhang Z H, Li H P, Han K 2013 Acta Phys. Sin. 62 158701
Google Scholar
[18] Jiang B W, Keffer J D, Edwards J B 2006 J. Fluorine Chem. 127 787
Google Scholar
[19] Daw M S, Baskes M I 1984 Phys. Rev. B 29 6443
Google Scholar
[20] Savio D, Fillot N, Vergne P, Zaccheddu M 2012 Tribo. Lett. 46 11
Google Scholar
[21] Plimpton S 1995 J. Comp. Phys. 117 1
Google Scholar
[22] Sabzevari S M, Mcgraw J D, Woodadams P 2016 RSC Adv. 6 91163
Google Scholar
[23] Mazyar O A, Jennings G K, Mccabe C 2009 Langmuir 25 5103
Google Scholar
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