Self-assembly of nanostructures towards transparent, superhydrophobic surfaces

High transparency is important to the performance of optical equipment and devices, such as windows, lenses, solar panels, and safety goggles. As many of them are constantly exposed to various environmental conditions, it is highly desirable to develop a self-cleaning coating that can prevent microbial growth, fouling, corrosion and icing. One of these technologies is superhydrophobic coating. In this review, we discuss recent progress in design, synthesis and fabrication of transparent, superhydrophobic surfaces. First, we revisit different models of superhydrophobicity and present the potential challenges in the nanofabrication of transparent superhydrophobic surfaces. We then discuss the general fabrication methods, including the top-down fabrication methods and self-assembly approaches, to create roughness with a size in the sub-visible wavelength with or without post-hydrophobilization steps. While top-down fabrication offers well-defined size and shape of surface topography, self-assembly is more versatile and could enable mass-production of nano-roughness on a wide range of substrates at a lower cost. Therefore, we focus on discussion of different self-assembly methods, including sol-gel processes, microphase separation, templating, and nanoparticle assembly, to create transparent, superhydrophobic surfaces. The review concludes with perspectives on future directions and challenges in manipulation of surface nanoroughness, specifically, using nanoparticles, for both high transparency and superhydrophobicity and their potential applications.