Passive Directional Water Transport on Superhydrophobic Gradient Textures Fabricated by Direct Laser Interference Patterning

In this work, the fabrication of a novel gradient surface microstructure using two‐beam direct laser interference patterning is presented. Periodic structures are produced on stainless steel surfaces exhibiting simultaneous gradients in spatial period (2.0–4.8 µm) and surface roughness (Sa: 0.6–0.2 µm) over millimeter‐scale lengths. Water droplets with a volume of 10 µL are released at different heights above the surfaces to characterize their capability of directional water transport. Surfaces combining gradients in both spatial period and roughness exhibit a transport distance improvement of 160% compared with reference surfaces containing only a spatial‐period gradient. When droplets are released from a height of 2.2 mm, a maximum lateral displacement of 7.59 ± 0.09 mm along the spatial period gradient direction is achieved, which is ascribed to a gradient in the pinning force gradient due to the ridges asperities. Further examinations with inclined surfaces reveal that the gradient‐induced pinning asymmetry can effectively counteract the gravitational force component at a tilt of 2.5°, reducing the net downhill displacement to nearly zero. The experiments performed in this work provide a novel approach to enhance the effects of surface gradients under superhydrophobic condition and explore their application potential for the directional water transportation of microliter droplets.