A research of bionic hydrophilic micro-track on superhydrophobic surface for guiding droplets’ directional transportation

To achieve guiding droplets’ directional transportation on superhydrophobic surfaces, a simple and effective method was employed to prepare the superhydrophobic surface and rice leaf biomimetic hydrophilic micro-tracks. Firstly, NaOH solution was used to corrode the aluminum alloy surface to form a micro and nano structure. Then the aluminum alloy was soaked in stearic acid solution to make the superhydrophobic layer self-grow on the rough surface. Finally, a nanosecond laser system was used to remove the partial superhydrophobic layer to form rice leaf biomimetic micro-tracks for guiding the droplets. An anisotropic wetting surface was obtained using the superhydrophobicity of the superhydrophobic surface and the hydrophilicity of the hydrophilic micro-tracks. The anisotropic wetting and sliding characteristics of the droplets were changed by optimizing the parameters of micro-tracks. As the spacing distance between adjacent hydrophilic micro-tracks decreased, the contact angle in the parallel micro-tracks’ direction was significantly reduced, while the sliding resistance of the droplet along the micro-tracks path was increased. When the spacing distance between adjacent micro-tracks was 500 μm, the difference of two direction maximum sliding angle came to 50°, which was the best spacing distance for guiding directional droplet transportation. A superhydrophobic surface with variable density hydrophilic micro-tracks and curvilinear hydrophilic micro-tracks was further proposed. It was for guiding the droplets to gather along the micro-tracks to the hydrophilic endpoints. Through anisotropic sliding analysis, it was found that the anisotropic wetting and directional sliding of the droplets on the composite surface were due to the difference in surface energy properties and the pre-wetting effect caused by capillary action. The superhydrophobic surface composite hydrophilic micro-tracks for guiding directional droplet transport reported in this study has application potential in the fields of water collection, water drop mixing stain removal and etc.