Superhydrophobic Stability of Nanotube Array Surfaces under Impact and Static Forces

The surfaces of nanotube arrays were coated with poly­(methyl methacrylate) (PMMA) using an imprinting method with an anodized alumina membrane as the template. The prepared nanotube array surfaces then either remained untreated or were coated with NH₂(CH₂)₃Si­(OCH₃)₃(PDNS) or CF₃(CF₂)₇CH₂CH₂Si­(OC₂H₅)₃ (PFO). Thus, nanotube arrays with three different surfaces, PDNS, PMMA (without coating), and PFO, were obtained. All three surfaces (PDNS, PMMA, and PFO) exhibited superhydrophobic properties with contact angles (CA) of 155, 166, and 168°, respectively, and their intrinsic water contact angles were 30, 79, and 118°, respectively. The superhydrophobic stabilities of these three surfaces were examined under dynamic impact and static pressures in terms of the transition from the Cassie–Baxter mode to the Wenzel mode. This transition was determined by the maximum pressure (p^max), which is dependent on the intrinsic contact angle and the nanotube density of the surface. A p^max greater than 10 kPa, which is sufficiently large to maintain stable superhydrophobicity under extreme weather conditions, such as in heavy rain, was expected from the PFO surface. Interestingly, the PDNS surface, with an intrinsic CA of only 30°, also displayed superhydrophobicity, with a CA of 155°. This property was partially maintained under the dynamic impact and static pressure tests. However, under an extremely high pressure (0.5 MPa), all three surfaces transitioned from the Cassie–Baxter mode to the Wenzel mode. Furthermore, the lost superhydrophobicity could not be recovered by simply relieving the pressure. This result indicates that the best way to maintain superhydrophobicity is to increase the p^max of the surface to a value higher than the applied external pressure by using low surface energy materials and having high-density binary nano-/microstructures on the surface.