Characteristics of Contact Line in Dynamic Wetting and Dewetting Processes on a Hydrophobic Surface

The adhesion of droplets to solid surfaces causes problems in many industrial fields. Solid surfaces can be classified as hydrophilic or hydrophobic on the basis of the equilibrium contact angle of a liquid droplet on the solid surface. Much attention has been paid to hydrophobic surfaces because liquids hardly stick to such surfaces, but only a few studies of the dynamic characteristics of liquids on these surfaces have been conducted. In this study, the extension-contraction method was used to investigate dynamic wetting and dewetting behaviors on a hydrophobic surface. The changes in the contact line displacements, contact line velocities, and dynamic contact angles when the water droplet volume was increased/decreased at a constant rate were measured using a high-speed camera. The results were analyzed based on the molecular kinetic theory, and the coefficients of contact line friction, average distances of molecular displacement, and frequencies of adsorption-desorption of molecules around the contact line during wetting and dewetting were calculated and compared. These parameters were compared with those obtained on hydrophilic surfaces, and the dynamic behavior of droplets on a hydrophobic surface was discussed.