Achieving superhydrophobic surfaces with tunable roughness on building materials via nanosecond laser texturing of silane/ siloxane coatings

Abstract

In this work, we employ a versatile laser-based top-down approach that allows to create superhydrophobic and hydrophobic surfaces, with controlled roughness and wetting properties, on marble and potentially other building materials. The process involves two stages: (1) application of an organically modified silica coating to reduce surface energy. (2) Controlled texturing of the coating by ablation using a nanosecond-pulse laser. In general terms, at higher laser fluence (energy per unity of area), the contact angles increased from 110 degrees of the non-textured surface to values around 155 degrees, following a nearly linear correlation with the measured roughness values. Starting from fluence values of 154 J cm-2, the surfaces displayed water repellence (hysteresis <= 10 degrees) and the micrographs showed the formation of sub-micrometric structures on top of the micro-roughness by melting and re-deposition of the coating material, suggesting the formation of a Cassie-Baxter wetting regime. Ablation at lower fluences created a random micro-roughness, leading to static contact angles of 135-145 degrees but no water repellence, which is indicative of a Wenzel wetting regime. At the highest fluence values tested, the increasing trends respect roughness and hydrophobic/water repellent properties are inverted due to the damages suffered by the coating. In terms of durability, the coating demonstrated a good adhesion to the stone surface, maintaining its superhydrophobic properties after repeated cycles of an "adhesive tape test". The sand falling test showed that water repellence is relatively sensitive to abrasion, although the hydrophobic character of the coating is maintained.