Simple strategy for producing superhydrophobic nanocomposite coatings in situ on a building substrate

Numerous superhydrophobic materials have been developed in recent years by using a combination of two strategies: reducing the surface free energy and roughening the surface. Most of these procedures have the serious drawback of involving tedious multistage processes, which prevent their large-scale application, such as on the external stone and similar material surfaces of buildings exposed to the weather. This paper describes an innovative synthesis route for producing superhydrophobic surface coatings. The coating can even be produced, outdoors, on the building by a low-cost process. We demonstrate that the addition of silica nanoparticles to a mixture of organic and inorganic silica oligomers in the presence of a surfactant produces a coating of closely packed particles. The effect of this is to trap air beneath the water droplets, thus significantly minimizing the contact area between droplet and surface. The organic component reduces the surface free energy of the material, resulting in a high static contact angle. This has the effect of repelling water because the water droplets that form simply roll rapidly down the coated surface. The surfactant plays a valuable role, acting as a sol-gel transition catalyst and, by coarsening the pore structure of the gel network, prevents the coating material from cracking.