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丁怡, 王意乐, 孙莉杰, 靳亚康, 陈龙泉. 液滴润湿固体表面的移动接触线: 从吸附到滑动. 力学学报, 待出版. DOI: 10.6052/0459-1879-24-104
引用本文: 丁怡, 王意乐, 孙莉杰, 靳亚康, 陈龙泉. 液滴润湿固体表面的移动接触线: 从吸附到滑动. 力学学报, 待出版. DOI: 10.6052/0459-1879-24-104
Ding Yi, Wang Yile, Sun Lijie, Jin Yakang, Chen Longquan. Moving contact line in droplet wetting: from adsorbing to sliding. Chinese Journal of Theoretical and Applied Mechanics, in press. DOI: 10.6052/0459-1879-24-104
Citation: Ding Yi, Wang Yile, Sun Lijie, Jin Yakang, Chen Longquan. Moving contact line in droplet wetting: from adsorbing to sliding. Chinese Journal of Theoretical and Applied Mechanics, in press. DOI: 10.6052/0459-1879-24-104

液滴润湿固体表面的移动接触线: 从吸附到滑动

MOVING CONTACT LINE IN DROPLET WETTING: FROM ADSORBING TO SLIDING

  • 摘要: 液滴润湿固体表面是自然界和日常生产、生活中常见的流体力学现象. 针对液滴润湿固体表面过程中移动接触线的运动模式, 有必要开展深入的物理力学研究. 采用分子动力学模拟方法, 研究了3种不同润湿性固体表面上水滴浸润的动力学过程, 展示了全时域液滴铺展动力学行为及其相应的移动接触线标度律, 并划定了液滴润湿过程的4个阶段: 第1阶段中( t/\tau _\texti \leqslant 0.05 , 其中 \tau _\texti 为惯性-毛细特征时间), 液滴与固体表面实现点接触, 接触角为180°; 第2阶段中( 0.05 \leqslant t/\tau _\texti \leqslant 1 ), 液滴动态接触角快速减小, 接触半径随时间以1/2幂律关系快速增长, 且不受表面润湿性的影响; 第3阶段中( 1 < t/\tau _\texti \leqslant 3 ), 液滴动态接触角继续减小, 接触半径持续增长, 表面亲水性越强浸润速率越快; 最后进入长期弛豫阶段( t/\tau _\texti \gt 3 ), 疏水表面上的液滴铺展基本结束, 接触半径和接触角均趋于定值, 而亲水表面上液滴动态接触角的减小明显减缓, 接触半径呈指数增长. 进一步标定了固-液-气三相接触线的尺寸, 通过深入分析位于固-液-气三相接触线处水分子在相空间中的运动轨迹, 发现了两类接触线运动模式: 吸附和滑动. 具体而言, 在液滴润湿固体表面的第1和第2阶段中, 接触线以吸附模式产生和增长; 第3阶段中吸附和滑动模式并重; 第4阶段中接触线则以滑动为主. 文章揭示了液滴润湿固体表面的分子级运动细节, 有助于进一步深入理解固-液界面微尺度流动.

     

    Abstract: It is common in both nature and daily life that a droplet wets a solid surface − a typical fluid phenomenon. In order to understand the dynamics of moving contact line during the wetting, comprehensive physical mechanics study is desired. By employing large-scale molecular dynamics simulations, how a water droplet wets solid surfaces with various wetting properties are investigated. The droplet spreading behaviors in a full time domain and the corresponding scaling laws of moving contact line are shown, and four distinct stages are found: In stage I ( t/\tau _\texti \leqslant 0.05 , \tau _\texti is the inertia-capillary characteristic time), the droplet contacts with the surfaces in a point manner, and the contact angle is 180°; In stage II ( 0.05 \leqslant t/\tau _\texti \leqslant 1 ), the contact angle dramatically decreases, while the contact radius increases in a power-law fashion and the power takes 0.5 and does not affect by the surface wettability; In stage III ( 1 \lt t/\tau _\texti \leqslant 3 ), though the contact angle still decreases and the contact radius keeps increasing, but their decreasing/increasing rates become larger on more wettable surfaces; Finally it enters the long-term relaxation stage ( t/\tau _\texti \gt 3 ), the wetting on the hydrophobic surface is almost finished since the contact angle and the contact radius remain constant, while the decrease of contact angle of a droplet on a hydrophilic surface is very slow and the contact radius exponentially grows. By labelling the size of three-phase contact line and analyzing the trajectory of water molecules in the contact line, two modes of moving contact line are proposed: adsorbing and sliding. It is found that: in the first two stages, the moving contact line takes the adsorbing mode, and both the adsorbing and sliding modes are of equal importance in the third stage, while in the last stage the moving contact line is dominated by the sliding mode. The molecular details in this study are helpful to understand microscale flows at the solid-liquid interface.

     

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