Zheng Wenxiu, Sun Chengzhen, Wen Boyao, Bai Bofeng
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University Xi'an Shaanxi 710049 China
RSC Adv. 2019 Jan 22;9(6):3092-3101. doi: 10.1039/c8ra09715a.
The movement of the three-phase contact line with chain molecules in the liquid phase displays more complex mechanisms compared to those in the usual liquid-liquid-solid systems and even to the gas-liquid-solid systems controlled by the traditional single-molecule adsorption-desorption mechanisms. By introducing decane molecules with chain structures, we demonstrate from molecular dynamics insights that the moving mechanism of the contact line in a water-decane-silica system is totally different from traditional mechanisms. Three different wettability-related moving mechanisms including "Roll up", "Piston" and "Shear" are revealed corresponding to the hydrophilic, intermediate and hydrophobic three-phase wettability, respectively. In the "Roll up" mechanism, the decane molecules are rolled up by the competitively adsorbed water molecules and then move forward under the driving force; when the "Piston" mechanism happens, the decane molecules are pushed by the piston-like water phase owing to the comparable adsorption interactions of the two liquids on the solid surface; in the "Shear" mechanism, the contact line is hard to drive due to the stronger decane-silica interactions but the decane molecules far away from the solid surface will move forward. Besides, the time-averaged velocity of the moving contact line is greatly related to the moving mechanisms. For the "Roll up" mechanism, the contact line velocity increases first and then reaches a steady value; for the "Piston" mechanism, the contact line velocity has a maximum value at the start-up stage and then decreases to a stable value; for the "Shear" mechanism, the contact line velocity fluctuates around zero due to the thermal fluctuation of the molecules. Additionally, the mean distance from Molecular Kinetics Theory increases with decreasing hydrophilicity and the displacement frequency in "Roll up" mechanism is 2 orders of magnitude higher than that in the "Piston" mechanism, further demonstrating the different moving mechanisms from a quantitative point of view.
与通常的液 - 液 - 固系统甚至与由传统单分子吸附 - 解吸机制控制的气 - 液 - 固系统相比，液相中具有链状分子的三相接触线的移动显示出更复杂的机制。通过引入具有链状结构的癸烷分子，我们从分子动力学的角度证明，水 - 癸烷 - 二氧化硅系统中接触线的移动机制与传统机制完全不同。揭示了三种不同的与润湿性相关的移动机制，分别为“卷起”“活塞”和“剪切”，它们分别对应亲水性、中间性和疏水性三相润湿性。在“卷起”机制中，癸烷分子被竞争性吸附的水分子卷起，然后在驱动力作用下向前移动；当发生“活塞”机制时，由于两种液体在固体表面的吸附相互作用相当，癸烷分子被活塞状的水相推动；在“剪切”机制中，由于癸烷与二氧化硅之间的相互作用较强，接触线难以驱动，但远离固体表面的癸烷分子会向前移动。此外，移动接触线的时间平均速度与移动机制密切相关。对于“卷起”机制，接触线速度先增加然后达到稳定值；对于“活塞”机制，接触线速度在启动阶段有一个最大值，然后降至稳定值；对于“剪切”机制，由于分子的热涨落，接触线速度在零附近波动。此外，根据分子动力学理论，平均距离随着亲水性的降低而增加，并且“卷起”机制中的位移频率比“活塞”机制高2个数量级，从定量的角度进一步证明了不同的移动机制。
