MOVING CONTACT LINE IN DROPLET WETTING: FROM ADSORBING TO SLIDING

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 < 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.