Ultrahydrophobicity

Laminar Drag Reduction in Micro-Channels Using Ultrahydrophobic Surfaces... 

Fig. 3.15 Examples of ultrahydrophobic surfaces. Lithographically etched silicon surface patterned with 30 pm tall cubic micro-posts. Reprinted from Ou et i. (2004) with permission...

Fig. 3.17 Average pressure drop reduction as a function of flow rate for a series of different surfaces in a micro-channel having dimensions W = 2.54 mm, H = 127 pm, and L = 50 mm. The experimental data include a series of ultrahydrophobic surfaces with a regular array of square micro-posts with d = 30 pm with a spacing between micro-posts of w = 15 pm represented by triangles (A), <7 = 30 pm and w = 30 pm represented by squares ( ), J = 30 pm and w = 60 pm represented by circles ( ), and d = 30 pm and w = 150 pm represented by diamonds ( ). Reprinted from Ou et al. (2004) with permission...

Ou J, Perot B, Rothstein JP (2004) Laminar drag reduction in microchannels using ultrahydrophobic surfaces. Phys Fluids 16(12) 4635 643... 

It is known that fluorinated compounds dissolve considerable amounts of oxygen19 and that membranes containing ultrahydrophobic moieties show a high selectivity in gas-separation processes, e.g., in 02/N2 separation. 

Patton, R.L., McCulloch, B., and Nidd, P.K. (2000) Chromatographic separation of fatty adds using ultrahydrophobic silicalite. U.S. Patent 6,013,186. 

The rate of Ostwald ripening depends on the size, the polydispersity, and the solubility of the dispersed phase in the continuous phase. This means that a hydrophobic oil dispersed as small droplets with a low polydispersity already shows slow net mass exchange, but by adding an ultrahydrophobe , the stability can still be increased by additionally building up a counteracting osmotic pressure. This was shown for fluorocarbon emulsions, which were based on perfluo-rodecaline droplets stabilized by lecithin. By adding a still less soluble species, e.g., perfluorodimorphinopropane, the droplets stability was increased and could be introduced as stable blood substitutes [6,7]. 

As a model monomer for radical homopolymerization of hydrophobic monomers, styrene is described in many papers. The polymerization of acrylates and methacrylates is also well known. It could also be shown that the miniemulsion process also easily allows the polymerization of the ultrahydrophobic monomer lauryl methacrylate without any carrier materials as necessary in emulsion polymerization [71]. 

TEM and ultracentrifuge results showed (see Fig. 16) that this process results in effective encapsulation of the carbon with practically complete yield only rather small hybrid particles, but no free carbon or empty polymer particles, were found. It has to be stated that the hybrid particles with high carbon contents do not possess spherical shape, but adopt the typical fractal structure of carbon clusters, coated with a thin but homogeneous polymer film. The thickness of the monomer film depends on the amount of monomer, and the exchange of monomer between different surface layers is - as in miniemulsion polymerization - suppressed by the presence of an ultrahydrophobe. 

Oner, D., McCarthy, T.J., 2000. Ultrahydrophobic surfaces. Effects of topography length scales on wettability. Langmuir 16 (20), 7777—7782. 

Plasma etching. Beyond simple processes for the efficient removal of organic impurities, for example, from metal surfaces (plasma cleaning), a low-pressure plasma can be employed to etch microstructures into polymer surfaces or polymer coatings. This includes well-defined 3D patterns like wells or channels using masking techniques but also random stmctures like a desired roughness as a prerequisite for an ultrahydrophobic surface. 

Ou J, Rothstein JP (2005) Direct velocity measurements of the flow past drag-reducing ultrahydrophobic surfaces. Phys Fluids 17 103606-103610... 

In this work we used such kinds of multilayer surfaces to achieve ultrahydrophobic behavior of micro-structures and the two simple mathematical models described above to appraise the nature of our artificial sixrfaces. Although there have been many publications on dynamic wetting behavior and moving wetting lines which can describe the phenomenon in detail, the two relatively easy models as discussed above are sufficient to describe the behavior of our structures. 

In order to ensure good compatibility between POSS and the matrix, one must match the polarity or solubility parameter of the POSS and the solvent, polymer or coating in which it is to be dissolved. POSS polarities span the whole gamut from extremely hydrophobic fluoroalkyl POSS used to make ultrahydrophobic surfaces [12], through progressively more polar variants such as alkyl POSS (low polarity), phenyl POSS (medium polarity), and water soluble types including PEG POSS, POSS trisulfonic adds, and octaammonium POSS. 

This mechanism was used previously to stabilize fluoroalkane droplets by adding the ultrahydrophobe perfluorodimorphineopropane, the results being the production of an effective and stable blood substitute [7]. A variety of molecules can be used as hydrophobes, although they must be chosen such that they add useful properties to the final product for example, the molecule may be a dye, a plasticizer, or a cross-Knker, while for biomedical applications this component might also be a fluorescent marker or a drug. 

FIGURE 11.24 Ultrahydrophobicity a drop of water on an ullrahydrophobic P(MMA-co-sfMA-HlOFlO) surface (O. Fischer, IFF Dresden). 

Grundke, K. Nitschke, M. Minko, S. Stamm, M. Froeck, C. Simon, F. Uhlmann, S. Poschel, K. Motornov, M. Merging Two Concepts Ultrahydrophobic Polymer Surfaces and Switchable Wettability. In Contact Angle, Wettability and Adhesion Mittal, K. L., Ed. VSP Utrecht, The Netherlands, 2003 Vol. 3, 267-291. 

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