Ultrahydrophobic surfaces

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... 

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 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. 

M. Sbragaglia and A. Prosperetti, A note on the effective slip properties for microchannel flows with ultrahydrophobic surfaces, Phys. Fluids, 19, 043603 [2007). 

Fresnais J, et al. (2004) Polyethylen ultrahydrophobic surface synthesis and original properties. EurPhys JApplPhys 2G-. 209—214. 

Ou and Rothstein (2005) developed an ultrahydrophobic surface with micrometer-sized ridges (20-30 pm wide) placed 20-120 pm apart aligned in the flow direction. They demonstrated maximum 25% drag reduction for flow inside these channels. From //-PIV measurements, they showed the existence of slip flow in the air-water interface between the ridges, while the flow over the ridges obeys the no-slip condition. They attributed the existence of slip flow to drag reduction. 

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