Spontaneous directional transportations of water droplets on surfaces driven by gradient structures.

Spontaneous directional transportation of droplets on solid surfaces driven by structure gradients has attracted much attention due to its large-scale applications, such as heat transfer, microfluidic devices, water collection, and separation. It also provides new insight for theoretical research into the interactions between droplets and solid surfaces. This review article summarizes recent progress in the spontaneous directional transportation of droplets on surfaces with structure gradients. Currently, surfaces with structure gradients can be divided into three types: wedge corners with a gradient opening angle, wedge-shaped surfaces, and conical substrates. This review focuses on their basic theory, detailed transport processes, fabrication methods, influence factors and application development. Finally, a perspective of this mode of transportation for future development is proposed.