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Thin liquid film near the gas-liquid-solid three phase contact line is the core area of oil and gas production, phase-change heat transfer, and material synthesis systems. Although there are many experimental studies on fluid dynamics in the contact line region, the prediction of contact angle is still difficult, and the bottleneck lies in the special structure of thin liquid film in the contact line region. Because the microstructure of thin liquid film is not well understood, the prediction of dynamic contact angle is always controversial. At present, the main controversial points focus on whether the microscopic contact angle changes with speed, and whether the microscopic contact angle is the same as the macroscopic contact angle. Therefore, it is crucial to monitor the dynamic process of the microscopic contact angle in the thin liquid film region of the contact line. In this work, the wetting system of 50 nm liquid droplets on different solid surfaces is constructed by molecular dynamics simulation, and the structure and migration mechanism of thin liquid film are studied. The structure of the precursor liquid film in the completely wetting droplet advancing contact line region and the nanoscale convex structure in the partially wetting droplet advancing contact line region are obtained. The precursor liquid film is 2–3 molecular layers in thickness, leading the droplet to move forward. However, there is no precursor liquid film in a partially wetting system, and the convex nano-bending larger than 10 nm is formed in the wetting process, resulting in the microscopic contact angle. By comparing the difference between the absolute smooth surface dynamic wetting process and the actual solid surface dynamic wetting process, the dynamic evolution law of the micro contact angle and the macro contact angle with time are obtained for the first time in the simulation. The liquid molecules in the contact line region are tracked and statistically analyzed by means of particle tracer. It is revealed that the liquid molecules in thin liquid film change from sliding mode to rolling mode with speed increasing under the action of solid surface friction, and then the air entrainment at the bottom of the contact line leads to slip and sputtering. The research results are expected to provide theoretical guidance for the following three directions: 1) improving heat transfer efficiency of micro and nano device based on wettability control; 2) improving the imbibition displacement efficiency of shale oil micro-nano matrix based on wettability regulation; 3) constructing universal contact angle prediction model.
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Keywords:
- contact line /
- thin liquid film /
- wetting /
- interface /
- molecular simulation
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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图 1 液滴在固体基底表面铺展的分子动力学模型, 液滴直径为50 nm. 构建了两种固体基底, 分别是绝对光滑固体基底A, 表面无摩擦; 另一种为晶格结构的固体基底B, 单晶Pt(110)表面, 原子尺度粗糙, 表面有摩擦
Fig. 1. Molecular dynamics simulation model of droplet spreading on solid substrates, with a droplet diameter of 50 nm. Two kinds of solid substrates were constructed, one was an absolutely smooth solid substrate A with no friction on the surface; the other was lattice structure solid substrate B with friction, which has atomic scale roughness, single crystal Pt(110).
图 2 固体A, B表面上不同浸润性液滴在1600 ps时刻动态铺展结构对比, 白色拟合虚线根据液滴球冠状理论所做, 红色为接触线真实形貌, 左上角分别对应各体系平衡态 (a)完全浸润液滴前进接触线区域存在数层分子厚度的前驱液膜; (b)部分浸润液滴前进接触线区域存在纳米凸起结构; (c)部分浸润液滴在绝对光滑固体A表面前进接触线区域没有纳米凸起结构
Fig. 2. Comparison of dynamic spreading structures of liquid droplets with different wettability on the surface B at 1600 ps, the white fitted line was based on the droplet spherical crown theory, and the red was the real morphology of the contact line, the upper left corner corresponds to the equilibrium state of each system: (a) There was several molecular-layer thick precursor films in the vicinity of advancing contact line of complete wetting droplet; (b) there was convex nanobending in the vicinity of the advancing contact line of the partially wetting droplet; (c) no convex nanobending structure appeared in the vicinity of the advancing contact line on absolutely smooth surface A.
图 3 部分浸润液滴在固体A, B表面铺展过程接触角随时间变化. 在绝对光滑A表面, 微观接触角始终保持与静态接触角相同(64°), 不随时间发生变化. 在单晶B表面, 宏观接触角与微观接触角均随时间发生变化, 微观接触角大于宏观接触角, 达到平衡时, 接触角相同
Fig. 3. Contact angle changed with time during the spreading process of partial wetting droplets on the surface of solid A and B. On an absolutely smooth A surface, the microscopic contact angle was always the same as the static contact angle (64°) and did not change with time. On the surface of single crystal B, both the macro contact angle and the micro contact angle change with time, and the micro contact angle was larger than the macro contact angle, and the contact angles were similar to equilibrium angle when the equilibrium was reached.
图 4 液滴在固体B表面不同速度下接触线区域红色粒子运动模式, 液滴主体粒子标记为蓝色, 接触线区域液体分子标记为红色, 固体标记为褐色, 氮气分子标记为粉色 (a)自发铺展液滴, 从t0时刻到(t0+7320) ps时刻平均接触线移动速度0.007 nm/ps; (b)从t0时刻到(t0+1660) ps时刻平均接触线移动速度0.03 nm/ps; (c)从t0时刻到(t0+240) ps时刻平均接触线移动速度0.21 nm/ps. 接触线区域液体分子的运动模式随速度提高, 出现从滑动为主向滚动为主的转变模式, 进一步发生气体入侵导致接触线滑移
Fig. 4. Droplet contact line particles movement mode at different velocities on the surface of solid B, the droplet body particles are marked in blue, the liquid molecules in the contact line area are marked in red, the solids are marked in brown, and the nitrogen molecules are marked in pink: (a) Spontaneous spreading droplet with an average contact line moving speed of 0.007 nm/ps from time t0 to time (t0+7320) ps; (b) average contact line moving speed is 0.03 nm/ps from time t0 to time (t0+1660) ps; (c) average contact line moving speed is 0.21 nm/ps from time t0 to time (t0+240) ps. The liquid sliding and rolling modes at the advancing contact line coexist, and tend to be the rolling mode as the speed increases. As the speed increases, air entrainment at the bottom of the contact line causes slippage.
图 5 (a)平均接触线移动速度0.007 nm/ps时, 接触线处标注红色原子分布百分比方差为34.72%; (b)平均接触线移动速度0.03 nm/ps时, 接触线处标注红色原子分布百分比方差为15.21%
Fig. 5. (a) When the average contact line moving speed is 0.007 nm/ps, the variance of the distribution of atoms marked in red at the contact line is 34.72%; (b) when the average contact line moving speed is 0.03 nm/ps, the variance of the distribution of atoms marked in red at the contact line is 15.21%.
表 1 不同固体表面粗糙度、液固原子间(液态氩、固态铂)相互作用参数和对应的体系平衡接触角
Table 1. Surface roughness of different solids, interaction parameters between liquid-solid atoms (argon in liquid state, platinum in solid state) and corresponding equilibrium contact angles of systems.
表面 均方根粗
糙度/nmεls/(kcal⋅mol–1) σls/nm 平衡接
触角/(°)A 0 0.6130 0.294 64 B 0.039 0.1346 0.294 64 1.6996 0.294 0 
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表 2 气液固原子间相互作用参数
Table 2. Parameters of gas-liquid-solid atomic interaction.
原子a 原子b εab /(kcal⋅mol–1) σab/nm N Ar 0.0343 3.3575 N Pt 0.0854 2.8925 N N 0.0740 3.3100
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[1] Wang H, Garimella S V, Murthy J Y 2007 Int. J. Heat Mass Tran. 50 3933
Google Scholar
[2] Wang H 2019 Langmuir 35 10233
Google Scholar
[3] Yang W, Brownlow J W, Walker D L, Lu J 2021 Water Resour. Res. 57 e2020WR029522
Google Scholar
[4] Mehrizi A A, Wang H 2017 Int. J. Heat Mass Tran. 107 1
Google Scholar
[5] Starov V M 2013 Colloid and Polymer Sci. 291 261
Google Scholar
[6] Rao S H, Li Z C, Deng Y J, Huang X, Lu H L 2022 Chem. Eng. J. 450 138120
Google Scholar
[7] Hu H, Sun Y 2013 Appl. Phys. Lett. 103 263110
Google Scholar
[8] Blake T D 2006 J. Colloid Interf. Sci. 299 1
Google Scholar
[9] Yu J P, Wang H 2012 Int. J. Heat Mass Tran. 55 1218
Google Scholar
[10] Mohammad Karim A 2022 J. Appl. Phys. 132 080701
Google Scholar
[11] Blake T D, Fernandez-Toledano J C, Doyen G, De Coninck J 2015 Phys. Fluids 27 112101
Google Scholar
[12] Cox R G 1986 J. Fluid Mech. 168 169
Google Scholar
[13] Werder T, Walther J H, Jaffe R L, Halicioglu T, Noca F, Koumoutsakos P 2001 Nano Lett. 1 697
Google Scholar
[14] Noble B A, Mate C M, Raeymaekers B 2017 Langmuir 33 3476
Google Scholar
[15] Lukyanov A V, Likhtman A E 2016 ACS Nano 10 6045
Google Scholar
[16] Yuan Q, Zhao Y P 2010 Phys. Rev. Lett. 104 246101
Google Scholar
[17] Herminghaus S, Pompe T, Fery A 2000 J. Adhes. Sci. Technol. 14 1767
Google Scholar
[18] Yu J P, Wang H, Liu X 2013 Int. J. Heat Mass Tran. 57 299
Google Scholar
[19] Deng Y J, Chen L, Yu J P, Wang H 2015 Int. J. Heat Mass Tran. 91 1114
Google Scholar
[20] Chen L, Yu J P, Wang H 2014 ACS Nano 8 11493
Google Scholar
[21] Liu Q, Chen L, Deng Y J, Wang H 2017 Chem. Phys. Lett 680 17
Google Scholar
[22] Giacomello A, Schimmele L, Dietrich S 2016 Proc. Natl. Acad. Sci. 113 E262
Google Scholar
[23] Marchand A, Chan T S, Snoeijer J H, Andreotti B 2012 Phys. Rev. Lett. 108 204501
Google Scholar
[24] Semal S, Blake T, Geskin V, et al. 1999 Langmuir 15 8765
Google Scholar
[25] Santiso E E, Herdes C, Müller E A 2013 Entropy 15 3734
Google Scholar
[26] Hoang A, Kavehpour H P 2011 Phys. Rev. Lett. 106 254501
Google Scholar
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