Superhydrophobicity

Incorporation of structural hierarchy enhances the superhydrophobicity further. Jxmg et at have compared the nanostructured and hierarchical surfaces of CNTs [221]. The hierarchal structure is formed by applying CNT on microstructures Si replica. With a contact angle of 170°, the hierarchical structured surfaces exhibit higher superhydrophobic nature as compared to that of nanostructured surface (contact angle 166°). 

Behavior of a liquid drop on a rough surface. Left, liquid penetrates into the spikes (Wenzel state) right liquid suspends on the spikes (Cassie-Baxter state) [72], 

SEM image of a lotus leaf showing the two-scale roughness [46], 

Slip of a water droplet from an inclined hydrophobic surface (left) where the water drop crawls over the dust particles and an inclined superhydrophobic surface (right) where the dust particles are collected and taken 

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The paper by Davies et al. (2006) reports results of a numerical investigation of the laminar, periodically repeating flow in a parallel-plate micro-channel with superhydrophobic walls. In particular, the influence of the Reynolds number and the vapor cavity size on the overall flow dynamics was explored. A schematic of the near-wall and cavity regions is shown in Fig. 3.18. 

Fig. 3.18 Schematic of the near-wall and cavity regions for liquid flow over a superhydrophobic surface exhibiting micro-rib structures and flow perpendicular to the ribs...

Gobas, F.A.P.C., Clark, K.E., Shiu, W.Y., Mackay, D. (1989) Bioconcentration of polybrominated benzenes and biphenyls and related superhydrophobic chemicals in fish role of bioavailability and elimination into the feces. Environ. Toxicol. Chem. 8, 231-245. 

In order to achieve stationary and highly spherical microdroplets, the possibility to use superhydrophobic nanostructured surfaces has also been explored to make lasing22 and Raman lasing microdroplets23, where the high contact angle makes it possible to make long-term measurements on nearly spherical microdroplets at rest. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

Sennaroglu, A. Kiraz, A. Dundar, M. A. Kurt, A. Demirel, A. L., Raman lasing near 630 nm from stationary glycerol water microdroplets on a superhydrophobic surface, Opt. Lett. 2007, 32,2197 2199... 

N. J. Shirtcliffe, G. McHale, M. I. Newton, C. C. Perry and P. Roach, Porous materials show superhydrophobic to super-hydrophilic switching, Chem. Commun., 2005, 3135. 

Hydrophobic ionic liquids, 26 850, 860-861 Hydrophobicity. See also Superhydrophobicity of silicone fluids, 22 578 of silicones, 22 603 solvent, 20 517 of surfaces, 22 111—112 Hydrophobic materials, encapsulation of, 11 545-546... 

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 other type of superhydrophobicity is the Cassie-Baxter form, wherein the liquid does not fully penetrate the surface topography, but instead will sit atop surface asperities and air pockets. For nonwetting surfaces, this is a more likely scenario, since it may be more energetically favorable for the hquid to be in contact with air than a repelhng surface. This type of wetting can be described by... 

Fig. 17. Fluorous patches direct the pairing of protein segments in lipid micelles. The hydrophobic peptides partition into lipid micelles, forming a-helices. Then, the superhydrophobic hexafluoroleucine residues seek each other, causing self-association into dimers and higher order aggregates. Fluorine is light, while the backbone of the a-helices is dark. From Ref. [81], with permission.

W. I. Milne, G. H. McKinley, and K. K. Gleason, Superhydrophobic Carbon Nanotube Forests, Nano Lett. 2003, 3, 1701. 

Recently it has been demonstrated by Guittard et al. that the electrochemical polymerization of semifluorinated thiophenes (226) [407], 3,4-ethylenediox-ythiophenes (227) [408], fluorenes [408], and 3,4-alkylenedioxypyrroles (230) [409] allowed the deposition of semiconducting polymer films with excellent antiwetting properties (superhydrophobic and lipophobic, see Fig. 73). Additional aromatics inserted between fluorinated tail and polymer chain (compounds 228, 231) improve mesogenity and in this way decrease the mobility of the Rp-chains, preorganize the molecules, and thus improve the antiwetting properties [388, 410]. 

Figure 7.9 Water drop on a superhydrophobic surface showing a high apparent contact angle app- The combined effect of hydrophobicity and roughness on the right length scale, causes the Lotus effect.

Sun T, Wang G, Feng L et al (2004) Reversible Switching between superhydrophilicity and superhydrophobicity. Angew Chem Int Ed 43 357-360... 

The linear correlation between BCF and Kow apparently breaks down for chemicals with a log Kow greater than approximately 6 (Figure 9.4), resulting in a "parabolic" or "bilinear" type relationship between the BCF and Kow (Bintein 1993, Meylan et al., 1999). For these superhydrophobic chemicals, the BCF appears to be much lower than expected from the chemical s octanol-water partition coefficient. A loss of linear correlation between the BCF and Kow can be caused by a number of experimental artifacts (described in section 9.4.3) and physiological processes, including metabolic transformation, fecal egestion, and growth. 

Furstner R, Barthlott W, Neinhuis C, Walzel P (2005) Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir 21 956-961... 

Genzer J, Efimenko K (2000) Creating long-lived superhydrophobic polymer surfaces through mechanically assembled monolayers. Science 290 2130-2133... 

As surrounding liquids oils or a specially liquid, fluorinert, can be used [100], For the actual experiments, a vegetable oil was chosen. It has a density lower than most aqueous solutions so that such droplets sink in the oil medium towards the panel surface. The droplets were applied using a normal pipet. Such prepared droplets are almost spherical on the polypropylene film. Often superhydrophobic materials were used as substrates. 

Superhydrophilic and superhydrophobic surfaces are more effective at stabilizing two-phase microflows. These surfaces can be obtained by creating roughness utilizing titanium nanoparticles. Titanium modification of a microchannel yields nanometer-scale surface roughness, and subsequent... 

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