Super-hydrophobic surfaces

The lotus-like and honeycomb-like aligned CNT films with a combination of micro- and nanostructures were also reported (Sun et al., 2003 Choi et al., 2003). They all displayed super-hydrophobic properties as shown in Fig. 9.14. The well-aligned CNT-polymer films or coatings have potential on applications such as super-hydrophobic surfaces to textiles, coatings, gene delivery, micro-fluid channels, non-wetting liquid transfer, and so forth. 

Some recent examples including uses as super hydrophobic surfaces [142], medical composites of spun fibers, generating scaffolds for cell attachment [138], thermoplastic elastomers [139], as a supramolecular electrolyte in a dye-sensitized solar cell [190], as a method to align polymer chains [191], or as supramolecular polymer composites [192] have been discussed previously. Still there is ample space to be explored and there definitely will be many more patents and applications in this field. 

Erbil, H.Y., Demirel, A.L., Avci, Y. and Mert, O. (2003). Transformation of a Simple Plastic into a Super-Hydrophobic Surface. Science, 299, 1377-1380. 

Lim HS, Han JT, Kwak D, Jin M, Cho K. 2006. Photoreversibly switchable super hydrophobic surface with erasable and rewritable pattern. J Am Chem Soc... 

Ogawa, T., Ding, B., Sone, Y. and Shiratori, S. 2007. Super-hydrophobic surfaces of layer-by-layer structured film-coated electrospun nanofibrous membranes, 18 ... 

The use of cold-plasma can be applied to modify the fibres surface properties in two opposite directions, namely an increase in the hydrophilic character when the treatment is carried out in Argon or air [18, 76] and, on the contrary, the formation of a super-hydrophobic surface when fluorinated gases are used [18, 77-80]. A very recent publication illustrates clearly the latter approach [80]. In this study, paper sheets were plasma irradiated in the presence of CF4 and the surface modification characterized by XPS, ATR-FTIR as a function of the power parameters and the CF4 pressure. Interestingly, no significant difference was observed in the extent of modification to either side of the sheet. Figure 18.2 shows a typical XPS spectrum of one of these modified surfaces. 

Optimizing Super-Hydrophobic Surfaces Criteria for Comparison of Surface Topographies 41... 

As mentioned in the Introduction, the recent interest for reducing friction — promoting slip — at solid-liquid interfaces was initially motivated by the ever growing field of microfluidic devices where the role of channel surfaces is considerably enhanced compared with the macroscale. It is in this particular context that super-hydrophobic surfaces have been introduced, and we have presented in Section 2 a review of the different theoretical and experimental works showing their remarkable frictional properties in laminar (low Reynolds numbers) flows. 

Generally speaking, the investigations in the field of super-hydrophobic surfaces have evolved dramatically in the last few years. However, still considerable work needs to be done, for example, techniques for the development of double roughness surfaces in a single production step. With techniques like these, the applications and, most of all, the fabrication of such surfaces would be much easier and more cost-effective. 

Given the background for the self-cleaning properties known for the microrough and highly hydrophobic surfaces of a number of plants and animals (see Refs [4-11]), different approaches to the creation of super-hydrophobic surfaces on textiles have been discussed. 

Figure 1. Wetting and elecirowetting diagrams with (a) macroscopic view of a planar hydrophobic surface, (b) microscopic view of the contact line for a planar hydrophobic surface, (c) macroscopic view of Cassie and Wenzel wetting states for a structured super-hydrophobic surface and (d) microscopic view of electrowetting on a structured super-hydrophobic surface. The above diagrams are qualitative representations. The diagrams are not representative of aU possible approaches since many liquids, dielectric thicknesses, and structure morphologies are possible.

Feng L, et al. Super-hydrophobic surfaces from natural to artificial. Adv Mater 2002 14(24) 1857-60. 

FIGURE 9.13. Naturally occurring super-hydrophobic surfaces, as seen in an electron microscope. Shown are a drosera loaf (left) and a lotus leaf (right). The horizontal bars represent 5 i.m and 20 im, respectively. (From C. Neinhuis and W. Barthlott, in Annals of Botany, 79, p. 667 (1997), published by Academic Press. Reproduced by permission.)... 

FIGURE 9.19. Take off of a water drop following a violent impact on a super-hydrophobic. surface. The drop cxperioncos fragmentation and highly non-linear o.scillation.s (courtesy Denis Richard and Christopho Clanet). 

Liu, Z., Gou, Y., and Cheng, J. W. S. 2008. Frost formation on a super-hydrophobic surface under natural convection conditions. Int. J. Heat Mass Transfer 51 5975. 

Shirtcliffe, N. J., McHale, G., Newton, M. I., and Perry, C. C. 2004. Wetting and Wetting Transitions on Copper-Based Super-Hydrophobic Surfaces. Langmuir 20 10146. 

---