Superhydrophobic layer

Superhydrophobicity is an important issue both for science and industry. Fabrication of such surfaces has been studied for more than ten years. Generally, there are some problems with obtaining stable superhydrophobic layers on solids. Many methods have been devised for such surface preparation [1]. 

Chen et al. [3] prepared superhydrophobic Co-based powder using the electrodeposition method to fabricate a superhydrophobic layer on stainless steel. This method is characterized by a short preparation time, water contact angle larger than 160°, and roll-off angle smaller than 2°. The powder was crystalline as confirmed by FTIR and XRD methods. 

Preparation of Superhydrophobic Layers Deposited on the Glass Surface... 

Table 15.1 Polymer particles used to obtain superhydrophobic layers and their diameters.

Images of the Superhydrophobic Layers Using Optical Profilometry... 

Table 15.2 Roughness parameters in [gm] for superhydrophobic layer of PS/ M1E4H10X deposited on glass slide.

CVD processes can also be used to grow polymers from nanocarbons. An important example is the coating of a CNT forest with a thin layer of poly(tetrafluorethlylene) (PTFE) via hot filament CVD to produce a superhydrophobic substrate [245]. Here, a vertically aligned MWCNT forest was prepared and placed in a CVD reaction chamber. Hexafluorpropylene oxide gas was then thermally decomposed to form the reactive radical difluorocarbene (CF2) and flowed over the CNT substrate along with a small amount of initiator where direct polymerization of PTFE onto the CNTs occurred [245]. 

The problems of supramolecular organization and distribution of bonded molecules are of great importance not only for the materials with a low surface coverage. These are also important for the preparation of the most dense bonded layers with the maximum shielding of the surface. Indeed, the complete modification of silica with alkylsilanes and preparation of superhydrophobic surfaces [44,45] are only possible under the conditions of island-like coverage of the surface with a. modifier. 

FIG U RE 8.6 A schematic diagram illustrating the preparation of superhydrophobic surfaces via layer-by-layer (LbL) coating of Ti02 nanoparticles and polyfaeryhc acid) (PAA) and fluo-roalkylsilane (FAS) surface modification. (Reprinted from Ogawa, T., et al., Nanotechnology 18, 165607, 2007. With permission.)... 

The fact that tuning the chemical nature alone of the soUd is unable to provide friction reduction beyond the submicrometer scale has led to the suggestion that one should try to get rid of the actual solid-liquid boundary by coating the surface with a bubble (a gas layer). Such a situation, where gas is trapped at the solid interface and partially replaces the solid-liquid contact, can be achieved in specific conditions (see Section 2.1) with the use of the so-called superhydrophobic surfaces. Such surfaces, which combine surface roughness and nonwettability to achieve unique static properties with water contact angles close to 180°, were indeed recently predicted [17] to exhibit also super-lubricating characteristics. 

Superhydrophobic surfaces based on poly(methyl methacrylate) (PMMA) with flower-like structures were prepared by controlling the aminolysis reaction of surface ester groups and the surface morphology of PMMA. The effects of the aminolysis time and the processing temperature in stearic acid solution on the surface wettability and surface morphology were examined in detail. The combination of rough surface morphology and the presence of hydrophobic carbon chtiins on the top layer was responsible for the super-hydrophobicity. 

It has been demonstrated that the preparation of superhydrophobic polymer surfaces is possible by simple plasma surface modification, either in one or two-step processes. The O2 plasma induces a variable roughness while CF4 plasma increases the roughness and creates an apolar layer. By the two-step treatment, several plasma parameters were found which allowed the preparation of superhydrophobic surfaces with controlled roughness and chemical structure. It has been shown that a superhydrophobic surface can be obtained even with a low roughness, around 20 nm. 

Verplanck et al. [62] made superhydrophobic silicon (Si) nanofiber surfaces of vertically aligned posts, shown in Fig. 10a, using a process based on chemical vapor deposition of silicon catalyzed by the metal particles. First a thin (4 nm) layer... 

Figure 3. Left A jet of tap water on a superhydrophobic glass slide. Interference at the air layer trapped underneath the water results in an intricate play of colors. Right A superhydrophobic glass slide in water after 27 days of immersion. The thin, homogeneous air layer leads to total reflectance of light.

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