2.25 Hydrophobic Materials

Abstract

Solar energy harvesting has many challenges, particularly for selective surfaces and cover glasses of photovoltaic panels. Recent changes in climate give rise to regular dust storms around the globe, which has an adverse effect on the optical characteristics of the solar energy harvesting devices. Dust removal from such surfaces involves with energy intensive processes and requires excessive clean water usage, which remains difficult in the areas where clean water scarcity is high. One of the solutions of such problem is introducing self-cleaning of surfaces. Hydrophobic characteristics are necessary to establish surfaces for self-cleaning. Hydrophobic/hydrophilic characteristics of surfaces are influenced by both, the surface texture and the surface energy of substrate materials. Surfaces consisting of a hierarchal texture covered with a low surface energy material show unusual water repellency, thus, termed as superhydrophobic surfaces. A water droplet on such surface rolls-off at small inclination angles, and picks and carries away small particles along its path, thereby, giving rise to the self-cleaning effect. These surfaces have attracted considerable attention in recent years due to their vast technological applications ranging from self-cleaning windows, textiles, and paints to low-drag surfaces for energy conversion. Most of the techniques that are introduced for the development of superhydrophobic surfaces require multistep procedures, longer processing times, and are not commercially scalable. Optical transmittance of surfaces is also of concern since most of the processes render surface opaque. Polydimethylsiloxane (PDMS), a transparent silicon-based organic polymer, is used in this chapter. Replication of textured surfaces by PDMS is introduced as a fast and cost-effective way of producing optically transparent superhydrophobic surfaces. Laser textured alumina tiles and photo lithographically etched silicon wafers are used as templates for replication studies. Another facile approach to obtain optically transparent superhydrophobic surfaces is through coating by silica nanoparticles. In this chapter, some fundamental aspects of surface hydrophobicity and assessment methods are presented. Later, two case studies are introduced for surface texturing. These cases include laser texturing of alumina surface and PDMS replication of textured surface toward achieving the surface hydrophobicity, and solvent crystallization of polycarbonate surface and influence of environmental dust particles on texture and optical properties of the surface. Analytical tools incorporating optical, electron scanning, and atomic force microscopes, X-ray diffraction, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy are used to assess morphological and elemental characteristics of the resulting surface. The sessile water droplet method is used to determine the water droplet contact angle and contact angle hysteresis. Microtribometer is used to determine friction coefficient of the treated surfaces. The optical transmittance is analyzed incorporating the UV–visible spectrometer. The characteristics of the textured are discussed and hydrophobic behavior of the surface together with optical transmittance are presented.