The Tuning of LIPSS Wettability during Laser Machining and through Post-Processing

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 135when the surface is left undisturbed for 30 days.

Ten Open Questions about Laser-Induced Periodic Surface Structures

“…One of the main difficulties of practical applications of LIPSS arises from the fact that the desired surface functionality is usually affected not only by the grating-like surface topography but also by its specific surface chemistry. The latter includes laser-induced chemical alterations, e.g., superficial oxidation [16,[55][56][57] caused by laser-processing in an air environment but also from redeposited material (debris) [58] or from post-irradiation adsorption of molecules such as hydrocarbons during sample handling and ambient storage [58][59][60][61]. Thus, there has been a long-lasting debate in the LIPSS-community regarding the relevance of the topography vs. the interfacial chemistry for surface functionalization.…”

Editorial: Special Issue “Laser-Generated Periodic Nanostructures”

“…We aimed to attract both academic and industrial researchers in order to collate the current knowledge of nanomaterials and to present new ideas for future applications and new technologies. By 8 August 2021, 22 scientific articles have been published in the Special Issue [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], see .…”

“…Three “Feature Papers” [ 15 , 16 , 17 ] were invited by the Guest Editors for the adequate framing of this Special Issue. An additional 19 papers were published as regular contributions [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ]. The publications in this Special Issue can generally be divided into three groups: (1) manuscripts related to self-organized laser-induced periodic surface structures [ 15 , 18 , 19 , 21 , 23 , 24 , 25 , 26 , 28 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], (2) manuscripts related to interference-based periodic surface structuring [ 17 , 22 , 27 , 29 ], or (3) manuscripts combining both approaches [ 16 , 20 ].…”

“…Several publications focus on the relevance of chemical surface alterations during or after laser-processing, including studies of superficial oxidation on the formation of specific types of LIPSS [ 18 , 23 , 24 , 25 , 30 ], chemo-capillary effects in DLIP [ 29 ], or even post-laser irradiation effects due to reactions with the environment [ 30 ]. Other publications study the fundamental mechanisms of structure formation on the basis of experimental and theoretical plasmonic approaches [ 15 , 21 , 26 ], and also in combination with a two-temperature model [ 28 ], advanced molecular dynamics simulations [ 22 ], or multi-physical hydrodynamic continuum considerations [ 31 , 34 ].…”

“…Wood et al [ 30 ] systematically explored the influence of the post-laser-processing environment (CO 2 atmosphere, boiling water) on the water wettability of LIPSS(LSFL)-covered stainless steel (AISI 316) surfaces that were previously scan-processed by fs-laser irradiation (800 nm, <100 fs, 1 kHz) in air. It was found that exposure to a CO 2 -rich environment after micromachining leads to an increased surface hydrophobicity, while residence in a boiling water bath instead leads to a hydrophilic surface.…”