Abstract
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 realizing 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, 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 increasing laser power, which results in the disappearance of ridges. However, the 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.
Highlights
Natural biological surfaces, such as lotus leaf, rice leaf, fish scale, and butterfly wings have attracted so much attention over the last few decades, mainly due to special wettability that formed during long-time evolution and natural selection
For a given tilt angle α, there is a downslope gravitational force Fd on the water droplet: Fd = Fg sin = gV sin where Fg is the gravity of water droplet, ρ is the density of water, g is the gravitational acceleration, and V is the volume of water droplet
The retention force of substrate to water droplet in two directions are F1 and F2, which is the consequence of contact angle hysteresis and causes droplets to adhere to surfaces [18]
Summary
Natural biological surfaces, such as lotus leaf, rice leaf, fish scale, and butterfly wings have attracted so much attention over the last few decades, mainly due to special wettability that formed during long-time evolution and natural selection. Surface with anisotropic hydrophobicity can realize unidirectional droplet transportation, which has tremendous applications for flow control, liquid transport, cell directing, drug delivery, and microfluidic devices [1,2,3,4,5]. The natural surfaces that possess capabilities of transporting liquid directionally exist in rice leaf, ryegrass leaf, spider silk, shorebird’s beak, butterfly wing, desert beetle, Nepenthes peristome, and cactus spine [6, 7]. The sliding angles of rice leaf are different in two directions (e.g., 4° in the direction that parallels to the edge, 12° in the direction that perpendicular to the edge) due to the anisotropic distribution
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