Dynamic behavior of drops crossing the boundary between two different wettability surfaces

A variety of surfaces with different wettability are prepared by coating aluminum sheets in this paper. The dynamic behavior of drops crossing the boundary between two surfaces with different wettability is recorded by a high-speed camera. When a drop is placed at the boundary between two different wettability surfaces, the drop can move from the surface with a larger apparent contact angle (region A) to the surface with a smaller apparent contact angle (region B). During the movement, the drop deforms and its advancing angle and receding angle change accordingly. Meanwhile, capillary waves form on the surface of the drop, and we find that there exists a certain relationship between the capillary waves and the dynamic contact angle. The crossing time (that is, the time required for the drop moving from the region A to the region B) is related to the wettability of each surface, wettability gradient of two regions, drop volume, surface tension and so on. A theoretical model is developed via energy analysis method and compared with the experimental results. This study provides a useful reference to design a surface with an excellent wettability gradient to induce the movement of drops.