Green manufacturing of extreme wettability contrast surfaces with superhydrophilic and superhydrophobic patterns on aluminum

In nature, Stenocara beetles have a hydrophilic/hydrophobic contrast surface for water harvesting in the desert by mist condensation in hydrophilic regions and water transport through hydrophobic regions. This nature-inspired wettability contrast surface has been widely studied for water harvesting, efficient heat transfer using dropwise condensation, and liquid arrays for biomedical applications. However, most fabrication methods have used toxic chemical coatings with silanization or fluorination. In this study, extreme wettability contrast aluminum surfaces having stable superhydrophilic/superhydrophobic properties were fabricated with a green manufacturing process including laser beam machining, boiling water treatment, and silicone oil heat treatment without the use of toxic chemicals. The nano-microscale hierarchical structures including nanostructures prepared by boiling water treatment and microstructures prepared by laser texturing became superhydrophobic surface after hydrophobic organic absorption by silicone oil heat treatment. Additional formation of hydrophilic pseudo-boehmite (AlOOH) structure after boiling water treatment of newly laser-textured aluminum could create stable superhydrophilic patterns on the superhydrophobic surface. The fabricated superhydrophobic surface could minimize the AlOOH formation during the additional boiling water treatment for superhydrophilic surface by trapping the air between the solid surface and water. The mechanism of wettability transition was analyzed by surface morphology and surface chemistry. In addition, various superhydrophilic patterns on the superhydrophobic surfaces were fabricated with sub-millimeter precision for demonstration of water droplet arrays and aqueous liquid control.