Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the atomic force microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.

Superhydrophobic surfaces, with a liquid contact angle greater than 150 , have important practical applications ranging from self-cleaning window glasses, paints, and fabrics to low-friction surfaces. Many biological surfaces, such as the lotus leaf, have a hierarchically structured surface roughness which is optimized for superhydrophobicity through natural selection. Here we present a molecular dynamics study of liquid droplets in contact with self-affine fractal surfaces. Our results indicate that the contact angle for nanodroplets depends strongly on the root-mean-square surface roughness amplitude but is nearly independent of the fractal dimension D f of the surface. DOI: 10.1103/PhysRevLett.97.116103 PACS numbers: 68.08.BcThe fascinating water repellents of many biological surfaces, in particular, plant leaves, have recently attracted great interest for fundamental research as well as practical applications [1][2][3][4][5][6][7][8]. The ability of these surfaces to make water bead off completely and thereby wash off contamination very effectively has been termed the lotus effect, although it is observed not only on the leaves of the lotus plant but also on many other plants such as strawberry, raspberry, and so on. Water repellents are very important in many industrial and biological processes, such as the prevention of the adhesion of snow, rain drops, and fog to antennas, self-cleaning windows and traffic indicators, low-friction surfaces, and cell mobility [9][10][11].Most leaves that exhibit strong hydrophobicity have hierarchical surface roughness with micro-and nanostructures made of unwettable wax crystals. The roughness enhances the hydrophobic behavior, so that the water droplets on top tend to become nearly spherical. As a result, the leaves have also a self-cleaning property: The rain drops roll away, removing the contamination particles from the surface, thanks to the small adhesion energy and the small contact area between the contaminant and the rough leaf [1].The hydrophobicity of solid surfaces is determined by both the chemical composition and the geometrical microor nanostructure of the surface [12 -14]. Understanding the wetting of corrugated and porous surfaces is a problem of long-standing interest in areas ranging from textile science [15] to catalytic reaction engineering [16]. Renewed interest in this problem has been generated by the discoveries of surfaces with small scale corrugations that exhibit very large contact angles for water and other liquids -in some cases, the contact angle is close to 180 . Such surfaces are referred to as superhydrophobic [17].The contact angle between a flat solid surface and a liquid droplet depends on the relation between the interfacial free energies per unit area: solid-liquid sl , solidvapor sv , and liquid-vapor lv . The Young equation sl lv cos sv results from the minimization of the total free energy of the system on a flat substrate surface. Complete wetting corresponds to 0 and typically happens on solids with a high surface energy sv . L...

We present an extensive but concise review of our present understanding, largely based on theory and simulation work from our group, on the equilibrium behavior of solid surfaces and nanosystems close to the bulk melting point. In the first part we define phenomena, in particular surface melting and nonmelting, and review some related theoretical approaches, from heuristic theories to computer simulation. In the second part we describe the surface melting/nonmelting behavior of several different classes of solids, ranging from van der Waals crystals, to valence semiconductors, to ionic crystals and metals. In the third part, we address special cases such as strained solids, the defreezing of glass surfaces, and rotational surface melting. Next, we digress briefly to surface layering of a liquid metal, possibly leading to solid-like or hexatic two dimensional phases floating on the liquid. In the final part, the relationship of surface melting to the premelting of nanoclusters and nanowires is reviewed.Comment: 54 pages, 26 figure