Development of Robust Superhydrophobic Surfaces Through Laser-Induced Micro/Nanoscale Armor Textured Structures

Superhydrophobic surfaces have attracted the attention of researchers across the globe due to their utility in a range of applications such as biotechnology, anti-corrosion, anti-fouling, and heat transfer. Super hydrophobicity is typically achieved through low surface energy and high micro to nano-scale surface roughness, allowing reduced contact between the liquid and the surface of solid material. However, during mechanical loading, this increase in surface roughness leads to high localized pressures due to reduced contact area between the interacting surfaces. This increase in local pressures makes such surfaces fragile and susceptible to damage during mechanical abrasion wear. As a result, the material underneath gets exposed which may cause a change in the hydrophobic property of the surface under abrasion. Therefore, most superhydrophobic surfaces are fragile and prone to damage when exposed to harsh environments.<br/>In this study, we have proposed a methodology to achieve robustness in superhydrophobic surfaces on metals making them resistant to mechanical abrasion wear and can withstand harsh operating conditions, such as acidic and alkaline environments, sea water exposure, ultraviolet radiation (UV) exposure, exposure to water jet, and elevated temperatures. This was achieved by splitting the problem statement into two sections. The mechanical robustness is achieved through a single-step laser-induced micro-to-nanoscale surface texturing, obtaining interconnected armor grids of various shapes and sizes, whereas the low surface energy was achieved using the silica-based nanoscale superhydrophobic coating.<br/>The microstructure armor shapes investigated in this study include interconnected hexagonal, triangular, and square-shaped holes. Optimization was carried out by testing eight different sizes of each microstructure shape. The interconnectivity in microstructure shapes was inspired by nature as observed in honeycomb structures and fish-scale skin structures.<br/>The grid microstructure offers pockets to hold nano-coating and acts as protective armor for the superhydrophobic coating against abrasion wear. The water-repellent coating housed inside microstructure pockets offered superhydrophobic behavior to the surface. The influence of various geometric factors such as microstructure wall thickness, hole depth, and microstructure size on the superhydrophobic behavior was also investigated. The initial water contact angle (WCA) was measured to be around 168 degrees and after 1000 sandpaper abrasion cycles under loading, the surface retained its superhydrophobic property with WCA measuring above 150 degrees for most sizes of all microstructure shapes. The prepared surface retained its superhydrophobic property when exposed to; 30 days of UV radiation, 600 minutes of water jet impact test, 30 days in sea water, acidic and alkaline environments, and 30 cycles of thermal cycling testing as per ASTM D6644-15. The proposed methodology is a scalable process for achieving durable robust superhydrophobic surfaces and can be utilized for a range of materials in a variety of applications.