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

Surfaces with gradient structures supply actuation for water droplets to attain spontaneous directional transportation. In this review, three typical gradient structures (wedge corners with an opening angle gradient, wedge-shaped surfaces, conical substrates) have been summarized based on recent studies. The detailed process of this spontaneous directional transportation and its influence factors and applications are also discussed in the following sections.