Simple fabrication of asphalt-based superhydrophobic surface with controllable wetting transition from Cassie-Baxter to Wenzel wetting state

We report a simple method, using combustion of asphalt, to fabricate superhydrophobic surface that can change its wetting state from Cassie-Baxter state to Wenzel state. Although asphalt is a hydrophobic material mainly used in water-proof paving and construction, asphalt-coated surfaces exhibit hydrophilic behavior when exposed to heat and moisture. To change the wettability of asphalt, combustion reaction was used to create hierarchical roughness and low surface energy that are requirements for superhydrophobic surface. Combustion reaction of asphalt created carbon soot with hierarchical nano-structures by coating the rigid asphaltene layer with superhydrophobic soot layer, which changed the hydrophobic asphalt surface into Cassie-Baxter type superhydrophobic surface. Additional heat treatment with ethanol changed the wettability from Cassie-Baxter to Wenzel state. Changes in surface morphology and chemical composition of the fabricated surface were observed after the wetting transition. After the wetting transition to the Wenzel state superhydrophobic surface, the microsized structures on the surface was removed and the hydrogen bonding was increased. Droplet control using the fabricated Cassie-Baxter and Wenzel state surfaces was demonstrated. The fabricated Cassie-Baxter superhydophobic surface showed the static water contact angle of 157.2 degrees and contact angle hysteresis of 8.4 degrees while Wenzel state superhydrophobic surface showed static contact angle of 150.2 degrees and contact angle hysteresis of 48.6 degrees.