Wetting-State-Induced Turning of Water Droplet Moving Direction on the Surface.

The spontaneous directional movement of water droplets on a wedge-shaped groove has gained extensive attention due to the advantage of not requiring energy input and its potential wide applications. However, manipulating the direction of movement of water droplets on a wedge-shaped groove has been not fully achieved, and the fundamental understanding of its underlying mechanism remains unclear. Here, molecular dynamics simulations and theoretical analyses are combined to reveal the mechanism of movement in opposite directions of a water droplet at the same position on the wedge-shaped groove interface. It is shown that the moving direction of the water droplet is related to its wetting state on the surface, i.e., the Wenzel and the Cassie states. A water droplet initially in the Wenzel and Cassie states will move toward the diverging and the converging ends, respectively. This phenomenon is attributed to the opposite roles played by the groove substrate and the upper layers in the two wetting states. Moreover, it is found that the water droplet is likely to move faster on a surface with a higher groove, larger opening angle and stronger hydrophobicity. These findings are expected to be of benefit for fully understanding droplet movement and shedding light on the regulation of the direction of movement of the droplets on the groove surface.