Effect of Chemical Solvents on the Wetting Behavior Over Time of Femtosecond Laser Structured Ti6Al4V Surfaces

Functionalization of Ti64 via Direct Laser Interference Patterning and Its Influence on Wettability and Oxygen Bubble Nucleation

The nucleation of bubbles on solid surfaces is an important phenomenon in nature and technological processes like electrolysis. During proton-exchange membrane electrolysis, the nucleation and separation of the electrically nonconductive oxygen in the anodic cycle plays a crucial role to minimize the overpotential it causes in the system. This increases the efficiency of the process, making renewable energy sources and the "power-to-gas" strategy more viable. A promising approach is to optimize gas separation by surface functionalization in order to apply a more advantageous interface to industrial materials. In this work, the connection between the wettability and bubble nucleation of oxygen is investigated. For tailoring the wettability of Ti64 substrates, the direct laser interference patterning method is applied. A laser source with a wavelength of 1064 nm and a pulse duration of 12 ps is used to generate periodic pillar-like structures with different depths up to ∼5 μm. The resulting surface properties are characterized by water contact angle measurement, scanning electron microscopy, confocal microscopy, and X-ray photon spectroscopy. It was possible to generate structures with a water contact angle ranging from 20° up to nearly superhydrophobic conditions. The different wettabilities are validated based on X-ray photon spectroscopy and the different elemental composition of the samples. The results indicate that the surface character of the substrate adapts depending on the surrounding media and needs more time to reach a steady state for deeper structures. A custom setup is used to expose the functionalized surfaces to oxygen-oversaturated solutions. It is shown that a higher hydrophobicity of the structured surface yields a stronger interaction with the dissolved gas. This significantly enhances the oxygen nucleation up to nearly 350% by generating approximately 20 times more nucleation spots, but also smaller bubble sizes and a reduced detachment rate.

Enhancing heat transfer at low temperatures by laser functionalization of the inner surface of metal pipes

The latent heat transfer during vapour condensation in the condenser section of passive heat transport devices such as the two-phase closed thermosiphon is limited by film condensation. Dropwise condensation provides an increase of the heat transfer coefficient by up to one order of magnitude and can be achieved with a water-repellant surface. The inner surface of pipes made from stainless steel was functionalized by laser surface texturing with ultrashort laser pulses and subsequent storage in a liquid containing long-chained hydrocarbons. The pipes were separated into half-pipes by wire eroding to enable laser texturing of the inner surface, and were then joined by electron beam welding after laser texturing. As a result, superhydrophobic and water-repellent surfaces with a contact angle of 153° were obtained on the inner surface of the pipes with a length of up to 1 m. The functionalized pipes were used in the condenser section of a two-phase closed thermosiphon to demonstrate a heat transfer rate of 0.92 kW at 45 °C, which is approximately three times the heat transfer rate of 0.31 kW of a smooth reference pipe.

Owens-Wendt Method for Comparing the UV Stability of Spontaneous Liquid-Repellency with Wet Chemical Treatment of Laser-Textured Stainless Steel

The liquid-repellent properties of AISI 304 stainless steel surfaces textured with a femtosecond laser were studied, both after spontaneous hydrophobization and when treated with stearic acid and octyltrimethoxysilane. Surface topography has been shown to play a critical role in determining these properties. Although textures containing only LIPSS exhibited poor liquid-repellency, the performance was significantly improved after engraving the microtexture. The most effective topography consisted of 45 µm-wide grooves with a pitch of 60 µm and protrusions covered with a rough microcrystalline structure. Liquid-repellency, chemical treatment efficiency, and UV resistance were compared using derived Owens-Wendt parameters. The surface of femtosecond-laser-textured steel after spontaneous hydrophobization was found to be significantly less stable under UV irradiation than surfaces treated with stearic acid or octyltrimethoxysilane modifiers.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.

Study of Micro- and Nanopatterned Aluminum Surfaces Using Different Microfabrication Processes for Water Management

Superhydrophobic surfaces demonstrate extreme water-repellence, promoting drop-wise over film-wise condensation, increasing liquid mobility, and reducing thermal resistance for heat-exchanger applications. Introducing topographic structures can lead to modified surface free energy, as inspired by natural systems like the lotus leaf, potentially allowing coating-free ice- and frost-free surfaces under certain conditions. This work presents a study of coating-free aluminum micro/nanopatterns fabricated using micromilling or laser-etching techniques and the resultant wetting properties. Our review and experiments clarify the roles of line-edge-roughness and microstructural geometry from each microfabrication technique, which manifests in technique-specific nano- to midmicro-scale roughness, producing a hierarchical structure in both cases. For micromilling, line-edge-roughness consists of jagged burrs of 1-8 μm thickness with 10-25 μm periodicity along the microlines with constantly changing height on the order of 1-20 μm. These effects simultaneously raise the water contact angle from 52° (unprocessed aluminum) up to 136° but with strong edge pinning effects. On the other hand, laser-etched surfaces exhibit line-edge-roughness with a microstructure of 3-20 μm width and 5-10 μm in height superimposed with evenly spread spikes of 50-250 nm. This results in a high contact angle (>150°) coupled with a low contact angle hysteresis (<15°), promoting superhydrophobicity on a coating-free aluminum surface. It is also shown that for certain cases, line-edge-roughness is more important for the resultant wetting properties than the structure geometry.

Ten Open Questions about Laser-Induced Periodic Surface Structures

Laser-induced periodic surface structures (LIPSS) are a simple and robust route for the nanostructuring of solids that can create various surface functionalities featuring applications in optics, medicine, tribology, energy technologies, etc. While the current laser technologies already allow surface processing rates at the level of m2/min, industrial applications of LIPSS are sometimes hampered by the complex interplay between the nanoscale surface topography and the specific surface chemistry, as well as by limitations in controlling the processing of LIPSS and in the long-term stability of the created surface functions. This Perspective article aims to identify some open questions about LIPSS, discusses the pending technological limitations, and sketches the current state of theoretical modelling. Hereby, we intend to stimulate further research and developments in the field of LIPSS for overcoming these limitations and for supporting the transfer of the LIPSS technology into industry.

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

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.

A New Method for Modeling the Cyclic Structure of the Surface Microrelief of Titanium Alloy Ti6Al4V After Processing with Femtosecond Pulses

A method of computer modeling of a surface relief is proposed, and its efficiency and high accuracy are proven. The method is based on the mathematical model of surface microrelief, using titanium alloy Ti6Al4V subjected to processing with femtosecond pulses as an example. When modeling the examples of microrelief, changes in the shape of segments-cycles of the studied surface processes, which correspond to separate morphological formations, were taken into account. The proposed algorithms were realized in the form of a computer simulation program, which provides for a more accurate description of the geometry of the microrelief segments. It was proven that the new method significantly increases the efficiency of the analysis procedure and processing of signals that characterize self-organized relief formations.