Lotus Effect: Roughness-Induced Superhydrophobic Surfaces

Abstract

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. These surfaces with high contact angle and low contact angle hysteresis also exhibit a self-cleaning effect and low drag for fluid flow. These surfaces are of interest in various applications, including self-cleaning windows, exterior paints for buildings, navigation ships, textiles, and applications requiring a reduction in fluid flow, e.g., in micro/nanochannels. Superhydrophobic surfaces prevent formation of menisci at a contacting interfaces and can be used to minimize high adhesion and stiction. Certain plant leaves, notably lotus leaves, are known to be superhydrophobic due to their roughness and the presence of a thin wax film on the leaf surface, and the phenomenon is known as the “Lotus effect.” Extremely water-repellent superhydrophobic surfaces can be produced by using roughness combined with hydrophobic coatings. In this chapter, the theory of roughness-induced superhydrophobicity is presented followed by the characterization data of natural leaf surfaces and artificial superhydrophobic surfaces. Wetting is studied as a multiscale process involving the macroscale (water droplet size), microscale (surface texture size), and nanoscale (molecular size). This includes fundamental physical mechanisms of wetting, responsible for the transition between various wetting regimes, contact angle and contact angle hysteresis. Practical aspects of design of superhydrophobic surfaces are also discussed.