Abstract
In this work, we investigate the fabrication of stainless-steel substrates decorated with laser-induced periodic surface structures (LIPSS) of both hydrophilic and hydrophobic wettability through different post-processing manipulation. In carrying out these experiments, we have found that while a CO2-rich atmosphere during irradiation does not affect final wettability, residence in such an atmosphere after irradiation does indeed increase hydrophobicity. Contrarily, residence in a boiling water bath will instead lead to a hydrophilic surface. Further, our experiments show the importance of removing non-sintered nanoparticles and agglomerates after laser micromachining. If they are not removed, we demonstrate that the nanoparticle agglomerates themselves become hydrophobic, creating a Cassie air-trapping layer on the surface which presents with water contact angles of 180°. However, such a surface lacks robustness; the particles are removed with the contacting water. What is left behind are LIPSS which are integral to the surface and have largely been blocked from reacting with the surrounding atmosphere. The actual surface presents with a water contact angle of approximately 80°. Finally, we show that chemical reactions on these metallic surfaces decorated with only LIPSS are comparatively slower than the reactions on metals irradiated to have hierarchical roughness. This is shown to be an important consideration to achieve the highest degree of hydro-philicity/phobicity possible. For example, repeated contact with water from goniometric measurements over the first 30 days following laser micromachining is shown to reduce the ultimate wettability of the surface to approximately 65°, compared to 135° when the surface is left undisturbed for 30 days.
Highlights
Accepted: 8 April 2021The irradiation of solids with linearly-polarized laser radiation has long been known to result in the formation of laser-induced periodic surface structures (LIPSS) [1]
We note that the dynamic water contact angles are typically better suited to describe the wettability of a surface, and we include this data as Supplementary Materials, the nature of our experimental method leads us to centre our discussion around both datasets, as explained later
We have taken a systematic look at the effect of surface handling in the early times after irradiation on the final wettability of stainless-steel decorated with laser-induced periodic surface structures
Summary
The irradiation of solids with linearly-polarized laser radiation has long been known to result in the formation of laser-induced periodic surface structures (LIPSS) [1]. Kietzig et al (2009) reported the creation of superhydrophobic surfaces from smooth, hydrophilic, steel alloys some time after laser irradiation without the need for such chemical coatings They noted that the water contact angle measured on their micromachined samples evolved from hydrophilic to highly hydrophobic over the first 2–3 weeks following exposure to the femtosecond laser beam, with the samples exposed to lab air. This exposure led to a surface with a higher carbon content and a higher water contact angle than a surface exposed to lab air [6] This technique of exposing a micromachined metal to a CO2 -rich environment has since been reported as an experimental method to fabricate superhydrophobic surfaces [16,17]. These are all important considerations as metal laser micromachining technologies get scaled and transferred to industrial applications
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