Experimental drag reduction study of super-hydrophobic surface with dual-scale structures

Hydrophobic surfaces with micro- or nanoscale pillars have been attracting considerable interest from scientists. In nature, such surfaces can be found on lotus leaves or under the feet of pond skaters. One significant property of these surfaces is friction drag reduction (FDR). Many studies have been conducted to demonstrate this reduction in terms of laminar and turbulent flows. The slip-length hypothesis is often used to explain this phenomenon. In this study, processes with the advantages of simplicity and cost effectiveness were used to fabricate dual-scale structures. Durable super-hydrophilic and super-hydrophobic surfaces were easily obtained from these structures. FDR was measured on a super-hydrophobic surface and was compared to that on smooth and super-hydrophilic surfaces. The experimental results in a circulating water channel revealed the Reynolds number range within which substantial FDR can occur on a super-hydrophobic surface. The mechanism of FDR and the role of slip are discussed by comparing experimental results.