Nanostructured Superhydrophobic Surfaces

Superhydrophobic surfaces are water-repellent, having a high water contact angle (typically 150°) and a very low roll-off angle that is, the inclination angle at which a 

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Sun T, Tan H, Han D, Fu Q, Jiang L. No platelet can adhere-largely improved blood compatibility on nanostructured superhydrophobic surfaces. Small 2005 1 959-63. 

Here we describe two simple fabrication processes to modify the surface of the device to achieve a very high water contact angle. In the first approach, the device was first coated with a thin film of hydrophobic materials, fluoropolymer in this case, and then oxygen plasma was used to create superhydrophobic surfaces. However, only in some cases, the chemical properties of the hydrophobic materials could be altered by the oxygen plasma treatment [19]. Therefore, a second technique has been developed where the nanostructures can be created on the device surfaces by a nanoimprint process [20]. Both of these approaches are compatible with the micro-fabrication process. 

An alternative approach to fabricate a superhydrophobic surface on a device is to utilize the nanoimprint technique to create nanostructures on the chip surfaces, which are coated with a thin film of hydrophobic materials. The fabrication scheme for a superhydrophobic surface using nanoimprint is illustrated in Fig. 2. To conduct nanoimprint lithography, the first step is to fabricate the stamp for nanoimprint. Previously [24-26], we demonstrated a simple technique to fabricate nanoimprint stamp by nanosphere lithography. In this process, a monodispersed polystyrene dispersion with 400 nm diameter beads (Bangs Laboratories, Inc., Fishers, IN) was... 

To create a superhydrophobic surface on the ITO glass by the nanoimprint process, a 1 pm thick layer of polymer (Teflon AF) was coated on the ITO glass. Then the nanoimprint stamp was pressed against the polymer coated ITO glass under 70 mbar pressure at 150°C for 30 min. After removing the stamp, nanostructures with desired dimension can be fabricated on the device surfaces. 

H. Teisala, M. Tuominen, M. Aromaa, M. Stepien, J. M. Makela, J. J. Saarinen, M. Toivakka, and J. Kuusipalo, Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces, Langmuir, 28,3138-3145(2012). 

V. Jokinen, P. Suvanto, A.R. Garapaty, J. Lyytinen, J. Koskinen and S. Franssila. Durable superhydrophobicity in embossed CYTOP fluoropolymer micro and nanostructures. Colloids Surfaces A, 434, 207-212 (2013). 

Many experiments have been performed on the subject, with somewhat contradicting results. Experimental work focused mostly on a bare (smooth) surface more recent investigations have turned toward rough and structured surfaces, in particular superhydrophobic surfaces. We refer the reader to comprehensive review articles " for a detailed account of early experimental work. In our chapter we mention only what we believe is the most relevant recent contribution to the subject of flow past "simple" smooth hydrophobic and rough hydrophilic surfaces, which clarified the existing controversies in the field. We focus, however, more on the implication of micro- and nanostructuring on fluidic transport, which is still in its infancy and remains to be explored. 

Fabrication of superhydrophobic surfaces has been an area of active research since the mid 1990s. In general, the same techniques that are used for micro- and nanostructure fabrication, such as fithography, etching, and deposition, have been utifized for producing superhydrophobic surfaces. Among especially interesting developments is the creation of switchable surfaces that can be turned from hydrophobic to hydrophUic by... 

Bormashenko, E., Stein,T.Whyman, G., Bormashenko.Y., Pogreb, R., 2006.Wetting properties of the multiscaled nanostructured polymer and metaUic superhydrophobic surfaces. Langmuir 22, 9982. 

Feng,J.,Tuomien, M.T., Rothstein, J.P., 2011. Hierarchical superhydrophobic surfaces fabricated by dual-scale electron-beam-lithography with well-ordered secondary nanostructures. Adv. Funct. Mater. 21, 3715. 

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. 

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