A sequential process for manufacturing nature-inspired anisotropic superhydrophobic structures on AISI 316L stainless steel

Surfaces with anisotropic superhydrophobicity have great potential applications in drug delivery and microfluidic devices due to their unique properties of drag reduction and unidirectional fluid transportation. Observations of natural biological surfaces have proven that directional microstructures are indispensable for realising anisotropic superhydrophobicity. However, current lithography-based manufacturing approaches have limited capabilities to scale-up for real-world industrial applications. This paper proposes a sequential process of laser ablation and chemical etching (LA-CE), for the first time, to manufacture ratchet-like microstructures on AISI 316L stainless steel by harvesting the advantages of both methods. The laser ablation will form a specified recast layer that will be covered by an oxide layer on the specimen, and these two layers can be easily removed in the chemical etching process to obtain the periodic ratchet-like microstructures. According to the experimental results, the direction of the microstructures is determined by the laser beam feed direction. Both the width and depth of microstructures increase with the increase of laser power, which results in the disappearance of ridges. However, increasing pitch will lead to the ridges appearing again. The specimen with a pitch of 25 μm machined at a laser power of 20 W has a maximum contact angle of 158.2°. Moreover, with a dip angle of 7°, this specimen shows a strong anisotropic superhydrophobicity, the droplet easily rolls off the surface in the laser beam feed direction; however, it is pinned tightly in the opposite direction.