Parametric Study of Ultrasuperhydrophobic Nanotextured Microstructured Surface Topographies Produced by Picosecond Laser Micromachining on H13 Tool Steel

Hierarchical nano/microscale surfaces offer properties that are of high interest to industry, as they can enable value-added functionalities such as controlled frictional, optical, aerodynamic, hydrodynamic, and other phenomena. Advanced laser-based structuring/texturing technologies, such as direct laser writing, laser-induced periodic structuring, and direct laser interference patterning, are most prominent for high-speed, large-area, and cost-effective fabrications of micro/nano grooves, riblets, dimples, pillars, pyramids, and their geometric combinations. The focus of this study is to explore how surface topography components are responsible for producing hydrophobic, superhydrophobic, and ultrasuperhydrophobic (contact angles 160°…175°) surfaces by single-step picosec-ond laser micromachining. Four functional surfaces, including microstructured square pyramids with side lengths of 10, 20, 30, 40 µm and nanotextured riblets with feature sizes of <1 µm, were machined on H13 tool steel, and the relationship between topographic characteristics and hydrophobic performance were studied. The results demonstrate that all features are synergistically responsible for the hydrophobic performance within a range of contact angles between 140° and 175°. The most critical role in obtaining superhydrophobic and ultrasuperhydrophobic performance was played by laser-induced nanoriblets on top of periodical microstructures. When nanoriblets were removed by flattening the top surfaces, wettability performance dropped from 175° to 139° contact angles. These results lay a scientific and engineering basis for hierarchical surface formation by laser processing and identify statistical metrics affecting surface wettability for the future development of fully controlled and optimized hydrophobic–hydrophilic surfaces.