## 4 Passive Anti-Biofouling Nanoc Coatings
### Fouling Release Strategies
#### 4.2.2 Biomimetic Hierarchical Micro- and Nanostructured Superhydrophobic Surfaces
From the top-down fabrication techniques, we mention: (1) templating, which is a replication technique allowing the creation of a surface topography that is a complementary (negative) replica of an appropriate specially prepared template [125; 126], (2) lithography, including photolithography, electron beam lithography, and ion beam lithography which all share the principle of transferring an image from a mask to a receiving substrate [127], (3) chemical and plasma etching [128; 129], (4) anodic oxidation [130], and (5) laser ablation [131]. The bottom-up approach is exactly the opposite of the top-down approach, since it starts with atoms and molecules and builds up to larger nanostructures. In a bottom-up structuring process, the micro/nanotopographic features of the surface are created by the controlled sequential deposition of material onto a substrate. The deposition techniques can be based on (1) purely physical processes such as evaporative methods, physical vapor deposition, and electrospinning [132; 133]; (2) physicochemical processes such as molecular self-assembly [134; 135], and (3) chemical processes. The chemical deposition processes involve the formation of a thin solid film on a substrate by a chemical reaction of some precursors that may initially be either in a solution such as in the sol-gel methods [135; 136],the hydro- or solvothermal syntheses [137], and electrochemical deposition [138; 139; 140], or in the vapor phase such as in the chemical vapor deposition (CVD) method [141]. [PERSON] et al. [142] reported two cost-effective approaches to the fabrication of superhydrophobic polymer films with biomimetic Lotus-like hierarchical topography and self-cleaning properties using the templating technique. In one strategy, the first hierarchical level of the template consisting of a two-dimensional array of silicone microdomes was created through lithographic surface patterning. Next, the substrate was covered with a thin film of Ti, followed by the electron beam physical vapour deposition of an aluminum film 1 \(\upmu\)m thick. The second hierarchical level was achieved through surface nanotexturing the aluminum film by anodisation and etching. It is known that when anodised in an acidic medium, aluminium oxidises to form a nanoporous alumina layer [143]. The nanopores were further broadened to 200 nm by chemical etching with a solution of phosphoric acid. A negative replica of the so-prepared silicon-anodised aluminium oxide (AAO) mold was fabricated by nanoimprint lithography [144]. In order to reduce the surface energy and to facilitate demodling, the silicon-AAO template was first subjected to CVD of 1H,1H,2H,2H-perfluorodecyl trichlorosilane. Then, a 500 \(\upmu\)m thick film of polypropylene (PP) was placed on the silicone-AAO template and heated to 180 \({}^{\circ}\)C with a concomitant application of 60 Bar pressure for 5 min so that the polymer melt could infiltrate into the mold. Next, the polymer-mold assembly was cooled down to a temperature of 90 \({}^{\circ}\)C, the pressure was released, and demolding was carried out by simply pilling off the template. In the second approach, the ordered array of microdome structures forming the first hierarchical level of the mold was prepared in an aluminium plate approximately 1 mm thick with the aid of a computer numerical control machine tool. An ion track-etched nanoporous polycarbonate membrane was used as a nanotemplate. The polycarbonate membrane was placed over the aluminium mold. Next, the polypropylene film to be imprinted was placed on the top of  
Figure 4: The Cassie model presumes that in the grooves, the air is trapped below the liquid droplet resulting in a composite solid to liquid-to-vapor interface. Figure 5: Scanning Electron Microscopy (SEM) micrographs of Lotus leaf revealing a hierarchical dual-scale roughness. Reproduced from reference [116] with the permission of the Royal Society of Chemistry. Copyright (2009) Royal Society of Chemistry. Figure 3: The Wenzel model presumes homogenous wetting.