The nanoscale structures together with the few microscale features decorating the spikes result in a pronounced increase of the overall roughness. The increase of local surface roughness is beneficial for the enhancement of surface
hydrophobicity. It is assumed that the surface of sample B prepared with this procedure possesses the hydrophobic self-cleaning function due to the second length scale morphology. It is well known that a hydrophobic surface generally refers to a surface with a water contact angle larger than 90°. When a surface has VX-689 a water contact angle larger than 150°, it is called a superhydrophobic surface. AMN-107 supplier Figure 3 3D topological AFM image (5 × 5 μm 2 ) of sample B. AZD1152 The initial understanding on a superhydrophobic surface is mainly from lotus leaves , which consist of micro- and nanostructures with self-cleaning capability by instinct. In nature, it is very common that a hydrophobic surface is obtained from the self-cleaning phenomenon. For instance, the Compositae petal leaves with a water contact angle of 128° shows a hydrophobic self-cleaning function. In this paper, the silicon wafer has been modified with metal-assisted wet etching. After modification, the water contact angle on the surface of black silicon
clustered by nanospike and few microspike structures is adequate to achieve self-cleaning. According to the experimental measurement, Farnesyltransferase the mean static contact angle of sample B is approximately 118°, while that of sample A is approximately 82°. The textured silicon (sample B) with a dualistic structure can imitate Compositae petal leaves ideally. The water contact angles in such cases may be interpreted by describing the Cassie-Baxter wetting state, where liquid drops do not completely penetrate the nanostructures and air pockets are trapped inside the spikes underneath the liquid drop [22–24]. A relationship that describes the contact angle on the textured surface is expressed
by the equation cos θ CB = f cos θ + f − 1, where θ CB is the liquid–solid contact angle on the textured surface, θ is the static contact angle on the flat surface, and f is the fraction of the liquid–solid contact area. Therefore, depending on the value of the f factor, the surface can be either hydrophilic or hydrophobic. According to the above equation, the smaller the value of f, the higher the increase of the contact angle. So it is essential to make a smaller contact area in order to obtain the higher contact angle. For example, the surface hydrophobicity can be improved in the preparation of a nanostructured silicon section. The result is consistent with the reports that black silicon was obtained by a photochemical procedure based on anisotropic etching .