Hierarchical roughness

Figure 1 Partial representation of a signal transduction network, mediated by a cell surface receptor. The molecules are organized hierarchically (roughly, from top to bottom) according to their functions as adaptors, receptor-recruited enzymes, membrane-associated substrates, and effector kinases. Arrows represent activation mechanisms, whether by complex formation or covalent modification V bars indicate negative regulation.

Superhydrophobicity, electrospinning, nanofibers, oleophobicity, hierarchical roughness, robustness... 

Introducing Hierarchical Roughness to Electrospun Fiber Mats... 

Recently, several groups have reported superhydrophobic fibrous membranes with hierarchical roughness based on electrospun materials. Ma et al. [43] made superhydrophobic fiber mats with hierarchical roughness by decorating micrometer scale electrospun fibers with nanometer scale pores or particles followed by surface hydrophobizafion. Examples of the fiber surfaces in each case are shown in Fig. 6. The hydrophobicity increased in both cases compared to the mats of smooth fibers with the same fiber diameter and surface chenfistry. The fibers decorated with nanometer scale pores were made by elecfrospinning poly(methyl methacrylate) (PMMA) fibers from a solution in chloroform. The formation of the pore structure is associated with the use of a highly volatile solvent and has been explained... 

The superhydrophobicity of the Au particle array can be described in terms of both equations (1) and (2). First, the structure of the Au particle array shows significant wave-like surface, so that air can be trapped in the interstices between micro-particles, which will increase hydrophobicity according to equation (2). Second, surface nano-scale roughness for the individual micro-particles in the array is beneficial towards increasing water CA based on equation (1). Such hierarchically rough Au particle array with superhydrophobicity could be used for micro-fluidic devices or the nano-devices with water-repeUent behavior. Also, on such superhydrophobic surfaces, the contamination, oxidation, and current conduction can be inhibited, and the flow resistance in micro-fluidic channels can also be reduced. 

A Facile Colloidal Route for Superhydrophobic Films with Hierarchical Roughness... 

For the nanoparticle surfaces, a similar mixed wetting state as described above is assumed [11, 17]. Only on silica sphere arrays decorated by gold nanoparticles we observe CA > 150°, reduced hysteresis and SA < 5°, which are characteristic for superhydrophobic substrates. Although a detailed characterization of the wetting mechanism on these hierarchical surfaces lies outside the scope of this work, we assume that the droplets on the substrates with hierarchical roughness are neither in the Wenzel nor Cassie-Baxter state. Most likely they reside in a mixed state as presented by... 

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). 

Wettability of a rough surface can be described by the Wenzel [7] model if the surfaces are completely wetted by the liquid into the protrusions on the surface. The Cassie-Baxter [8,9] model gives an idea about wettability of rough hydrophobic surfaces. In the case of such surfaces, the air trapped into hierarchical roughness prevents water penetration into the surface protrusions. 

Nosonovsky M, Bhushan B. (2007) Hierarchical roughness optimization for biomimetic superhydrophobic surfaces. Ultramicroscopy 107 969-979. 

By modifying the surfaces or choosing appropriate polyelectrolytes, both superhydrophobic (contact angle 150°)" " and superhydrophilic 6<5° in less than 0.5 sec)" surfaces can be created. In nature, there are matty superhydrophobic surfaces, such as the sacred lotus leaf, taro leaf, grass leaf and water strider s legs. The superhydrophobicity of these materials is mostly related to the hierarchical roughness, voids, channels and hydrophobic structure of the suifaces." Thus, superhydrophobic surfaces may be used for self-cleaning purposes on A1 alloy surfaces, which in turn reduces corrosion rates. 

Nosonovsky, M., Bhushan, B., 2007a. Hierarchical roughness makes superhydrophobic states stable. Microelectron. Eng. 84, 382. 

---