Bioinspired Geometry-Gradient Metal Slippery Surface by One-Step Laser Ablation for Continuous Liquid Directional Self-Transport.

Liquid directional self-transport on the functional surface plays an important role in both industrial and academic fields. Inspired by the natural cactus spine and pitcher plant, we have successfully designed a kind of geometry-gradient slippery surface (GGSS) based on aluminum alloy materials which could actively achieve directional self-movement and also antigravity self-movement of various liquid droplets by topography gradient. The mechanism of liquid directional self-transport was theoretically explored through the mechanical analysis of the triple contact line, which was mainly related to the competition between the driven force induced by Laplace pressure and the adhesive force induced by viscous resistance. The adhesive force between the droplet and the surface was quantitatively measured using a homemade experimental apparatus and the results showed that the lateral adhesive force on the GGSS is much smaller than that on the original surface. Additionally, a series of quantitative experiments were conducted to explore the influence of droplet volume and vertex angle on the transport distance and velocity. Finally, we achieved the antigravity self-transport of the droplet on the inclined GGSS to further verify the self-transport ability of the GGSS. We believe that the proposed GGSS with liquid directional self-transport ability in the present work would provide some potential opportunities in modern tribo-systems to optimize the lubricating qualities, especially the lubrication and friction at the extreme contact interface.