Abstract
Hierarchical structures are promising geometries for superhydrophobic surfaces, however a processing method with a single laser light source that is capable of both one-pass and rapid processing has not been established. The purpose of this study was to propose a concept of direct laser processing of two-scale periodic structures exhibiting superhydrophobicity. We hypothesized that the molten material that occurs due to the expanding plasma and that is squeezed around the micro-holes could play an active role in the processing of two-scale periodic structures. Percussion drilling using a nanosecond pulsed laser (532 nm wavelength) was performed on a steel surface. Twenty four different test-pieces were prepared using pitch (16–120 μm), number of repetition shots (1–120), and fluence (2.49–20 J/cm2), as the parameters. As the results, micro-holes with bank-shaped outer rims were formed. The maximum apparent contact angle was 161.4° and the contact angle hysteresis was 4.2° for a pitch of 80 μm and 20 repetition shots. The calculated results for the apparent contact angles were consistent with the measured results. Finally, an equation for estimating the processing rate was proposed. We demonstrated that this direct processing method can achieve a maximum processing rate of 823 mm2/min.
Highlights
Nature-inspired surface engineering has flourished over the past two decades
A surface is categorized as being hydrophobic if its apparent contact angle, θ’, of water is greater than 90° [6] and superhydrophobic if θ’ of water is greater than >150° and the contact angle hysteresis, CAH, of water is less than 10° [7] or 5° [8]
The two-scale periodic structure consist from (i) a basically periodic surface structure formed by the pitch of the percussion drilling and (ii) a small-scale periodic surface structure formed by the molten material around the micro-holes
Summary
Novel biomimetic surfaces similar to those found in nature have been developed to realize improved physical properties of wettability for applications such as liquid repellency, low adhesion, self-cleaning, and drag reduction [1,2,3,4,5]. A surface structure that has two different-scales and exhibits such physical functions is known as a functional texture. Superhydrophobic surfaces can be produced by mimicking lotus leaves, which possess hierarchical structures with two-scales [9]. Several fabrication methods based on lithographic techniques have been proposed for producing two-scale periodic structures [4, 10, 11]. A fabrication method that can process two-scale periodic structures simultaneously, one-pass possessing, is yet to be established using a lithographybased approach
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