Nature's billions of years of evolutionary history provides an inexhaustible and effectively inspiration for humans in the production of functional surfaces for specific applications in life and materials science. Recently, the nature-inspired surface structures with extreme wettability properties such as superhydrophobic and superhydrophilic surface have been widely studied for innumerable applications in energy, water, biological and daily activities. However, until now, the development of extreme wettability applications is mainly limited by the process complexity, long fabrication time, coating with toxic materials, and easily damaged surface structure. To reduce the fabrication time, and simplify the process for industrial applications, an eco-friendly postprocess has been developed.
Specifically, the superhydrophobic surfaces on the laser-textured metal substrates were fabricated extremely rapidly by a simple surface modification. The fabrication time was reduced from many days with aging in ambient air to only a 10 min heat treatment with nontoxic silicone oil. Hydrophobic organic group absorption has been accelerated on the silicone oil heat-treated surface and has created a low-energy surface. In addition, the potential of using the laser areal fluence parameter was demonstrated, that could be an alternative to single-laser process parameters such as scanning speed, power, and step size, to fine-tune the water adhesion behavior. Therefore, a surface that integrates different water adhesion behaviors can be easily fabricated for more complex practical applications such as controlled microdroplet transportation, microfluidic systems, and certain biomedical processes. The robustness of superhydrophobic surfaces was confirmed by abrasion tests, knife-scratch tests, chemical durability tests, and aging tests, and their repairability was evaluated for product applications in practice.
Besides, 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. In particular, the superhydrophobic aluminum surface with the micro-nanoscale hierarchical structure has shown excellent corrosion resistance after electrochemical polarization and impedance experiments.
The obtained results indicated that the proposed green manufacturing process for extreme wettability surfaces with superhydrophobic on metals and superhydrophilic/superhydrophobic patterns on aluminum has the potential to replace other existing fabrication methods with many outstanding advantages. It can be widely used in industry for several applications in near future.