Comment on “Bioinspired Reversible Switch between Underwater Superoleophobicity/Superaerophobicity and Oleophilicity/Aerophilicity and Improved Antireflective Property on the Nanosecond Laser-Ablated Superhydrophobic Titanium Surfaces”

Abstract

Laser-textured surfaces enabling reversible wettability switching and improved optical properties are gaining importance in cutting-edge applications, including self-cleaning interfaces, tunable optical lenses, microfluidics, and lab-on-chip systems. Fabrication of such surfaces by combining nanosecond-laser texturing and low-temperature annealing of titanium Ti-6Al-4V alloy was demonstrated by Lian et al. in ACS Appl. Mater. Inter . 2020 , 12 (5), 6573–6580. However, it is difficult to agree with (i) their contradictory explanation of the wettability transition due to low-temperature annealing and (ii) their theoretical description of the optical behavior of the laser-textured titanium surface. This comment provides an alternative view—supported by both experimental results and theoretical investigation—on how the results by Lian et al. could be interpreted more correctly. The annealing experiments clarify that controlled contamination is crucial in obtaining consistent surface wettability alterations after low-temperature annealing. Annealing of laser-textured titanium at 100 °C in contaminated and contaminant-free furnaces leads to completely different wettability transitions. Analysis of the surface chemistry by XPS and ToF-SIMS reveals that (usually overlooked) contamination with hydrophobic polydimethylsiloxane (PDMS) may arise from the silicone components of the furnace. In this case, a homogeneous thin PDMS film over the entire surface results in water repellency (contact angle of 161° and roll-off angle of 15°). In contrast, annealing under the same conditions but in a contaminant-free furnace preserves the initial superhydrophilicity, whereas the annealing at 350 °C turns the hydrophobicity “off”. The theoretical calculations of optical properties demonstrate that the laser-induced oxide layer formed during the laser texturing significantly influences the surface optical behavior. Consequently, the interference of light reflected by the air–oxide and the oxide–metal interfaces should not be neglected and enables several advanced approaches to exploit such optical properties.

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.…”

Quo Vadis LIPSS?—Recent and Future Trends on Laser-Induced Periodic Surface Structures

“…The impact of laser processing on surface wetting behavior became more clear already ten years ago through a study of Kietzig et al [ 44 ], reporting that metal surfaces are superhydrophilic (water contact angles ~ 0°) right after laser irradiation of different types of surface structures (LIPSS, Grooves, Spikes) due to laser-induced oxidation. On the timescale of several days or even longer, and without any topographic changes, the surfaces turn nearly superhydrophobic (water contact angles > 150°) due to the adsorption of hydrocarbon molecules or other contaminants from the ambient environment [ 44 , 45 , 46 ]. Since the presence of adsorbed molecules or contaminants at the surface is not necessarily stable against the storage conditions, surface cleaning, heat treatment [ 46 ], and other ageing effects, reliable industrial applications of these structures are difficult and may require additional stabilization steps.…”

“…On the timescale of several days or even longer, and without any topographic changes, the surfaces turn nearly superhydrophobic (water contact angles > 150°) due to the adsorption of hydrocarbon molecules or other contaminants from the ambient environment [ 44 , 45 , 46 ]. Since the presence of adsorbed molecules or contaminants at the surface is not necessarily stable against the storage conditions, surface cleaning, heat treatment [ 46 ], and other ageing effects, reliable industrial applications of these structures are difficult and may require additional stabilization steps.…”

Modulation and Control of Wettability and Hardness of Zr-Based Metallic Glass via Facile Laser Surface Texturing

“…A 2 h heat treatment ensured that both of the laser textured surfaces reached superhydrophobicity, with θ w values of 155.9 ± 1.2 • and 154.5 ± 1.9 • , respectively (Figure 6f,g). As discussed in the previous section, low-temperature heat treatment with an annealing temperature of ~150 • C has been proved to be an efficient approach to achieve wettability transition from superhydrophilicity to superhydrophobicity for various metallic materials such as aluminum, copper, stainless steel, and titanium [28][29][30][31][32][33][34][35][36]. However, this wetting state transition approach has never been utilized and confirmed for BMG.…”

“…In recent years, laser-based surface texturing has been proved to be one of the most efficient techniques to modify and control important surface functionalities, including surface wettability [18][19][20][21][22], reflectivity [23,24], anti-icing property [25,26], corrosion resistance [27], etc. For example, wettability transition from superhydrophilicity to superhydrophobicity has been achieved on various materials including aluminum [28], copper [29], stainless steel [30,31], and titanium [32][33][34][35][36] by combining laser surface texturing and lowtemperature annealing. In terms of laser texturing for BMG materials, there have been some recent research efforts on modification of surface properties of BMG via creation of laser-induced surface texture and change of surface chemistry [15,16,[37][38][39][40][41][42].…”