Superhydrophobic natural surfaces usually have multiple levels of structure hierarchy, particularly microstructures covered with nano-roughness. The multi-scale nature of such a surface reduces the wetting of water and oils, and supports self-cleaning properties. In this work, in order to broaden our understanding of the wetting properties of technical surfaces, biomimetic surface patterns were fabricated on stainless steel with single and multi-scale periodic structures using direct laser interference patterning (DLIP). Micropillars with a spatial period of 5.5 µm and a structural depth of 4.2 µm were fabricated and covered by a sub-micro roughness by using ultrashort laser pulses, thus obtaining a hierarchical geometry. In order to distinguish the influence of the different features on the wettability behavior, a nanosecond laser source was used to melt the nano-roughness, and thus to obtain single-scale patterns. Then, a systematic comparison between the single- and multi-scale structures was performed. Although, the treated surfaces showed hydrophilic behavior directly after the laser treatment, over time they reached a steady-state hydrophobic condition. However, the multi-scale structured metal showed a contact angle 31° higher than the single-scale geometry when the steady-state conditions were reached. Furthermore, the impact of the surface chemistry was investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analyses. Finally, a hydrophobizing agent was applied to the laser treated samples in order to further enhance the water contact angles and to determine the pure contribution of the surface topography. In the latter case, the multi-scale periodic microstructures reached static contact angles of 152° ± 2° and a contact angle hysteresis of only 4° ± 2°, while the single-scale structures did not show superhydrophobic behavior. These results definitely suggest that multi-scale DLIP structures in conjunction with a surface chemistry modification can promote a superhydrophobic regime.

This review investigates the capabilities of laser-based interferometric methods for producing structures with multiple-scaled surface features imitating natural examples. Firstly, laser interference lithography is used to produce hierarchical patterns with length-scales in the micrometer and sub-micrometer range. Different strategies are discussed to produce a wide variety of periodic arrays, depending on the number of resist lasers used as well as the way in which the exposure steps are organized. After that, periodic patterns are fabricated on polymers using ns laser pulses from an UV-laser system. Additionally in this case, multiple-scale patterns are produced by using different strategies. A similar approach is described to treat metallic surfaces of steel X6Cr17 and a titanium alloy Ti6Al4V. The geometry of the produced microstructures was characterized using scanning electron microscopy and confocal microscopy. Measurement of water contact angle is performed for both polymer and metallic surfaces.

Direct laser interference patterning (DLIP) has proven to be a fast and, at the same time, high-resolution process for the fabrication of large-area surface structures. In order to provide structures with adequate quality and defined morphology at the fastest possible fabrication speed, the processing parameters have to be carefully selected. In this work, an analytical model was developed and verified by experimental data, which allows calculating the morphological properties of periodic structures as a function of most relevant laser-processing parameters. The developed model permits to improve the process throughput by optimizing the laser spot diameter, as well as pulse energy, and repetition rate. The model was developed for the structures formed by a single scan of the beam in one direction. To validate the model, microstructures with a 5.5 µm spatial period were fabricated on stainless steel by means of picosecond DLIP (10 ps), using a laser source operating at a 1064 nm wavelength. The results showed a difference of only 10% compared to the experimental results.