Anisotropic Wetting of Micro–Micro Hierarchical Structures Fabricated by Ultra-precision Raster Milling

Stability of the composite wetting state is important to a hydrophobic surface with micro-structures. Adding hierarchical structures on micro-structured surfaces can further enhance the stability of the droplet composite wetting state due to the increase of the energy barrier for the wetting transition. Although scientists have proposed some fabrication methods of hierarchical structures such as chemical etching, photolithography, and self-assembly methods, most of the manufacturing processes are complicated in chemical treatment, time-consuming, not suitable for fabricating large surfaces, or only applicable to engineering materials such as metals, semiconductors, and alloys but are limited for plastics. This chapter presents several mechanical machining methods that would be one of the best solutions to fabricate bare hydrophobic micro–micro hierarchical structured surfaces with good sliding performance and stable composite wetting state for mass production, due to numerical control of machine tool path during the material removal process. Anisotropic wetting of bare micro-hierarchical structured surfaces including: (i) regular micro-hierarchical structures, (ii) irregular micro-hierarchical structures, and (iii) multi-level hierarchical structures fabricated by highly controllable one-step mechanical machining methods, including ultra-precision raster milling (UPRM), end-fly-cutting servo (EFCS), and end-fly-cutting (EFC) are investigated. The experimental results indicate that the micro–micro hierarchical structured surfaces have great potential for mass production by plastic injection molding in microfluidic applications such as artificial self-cleaning surfaces.