Extremely stable underwater superhydrophobicity via plasma micro-nanotexturing

Underwater superhydrophobic surfaces, inspired by Salvinia molesta leaves, can retain a stable air layer (plastron) that prevents wetting. However, their application in real-life maritime environments is hindered by external forces, such as hydrostatic pressure and water flow, which destabilize the plastron. Under the hypothesis that appropriately textured superhydrophobic surfaces can provide long-term plastron stability against challenging underwater conditions, superhydrophobic, hierarchically structured Poly(methyl methacrylate) (PMMA) surfaces were fabricated using plasma nanotechnology. Plastron stability was studied under external overpressures of up to 1500 mbar, simulating immersion depths of 15 m, and under continuous water flow rates up to 350 ml·min^-1, corresponding to a Reynolds number of 116. Experiments were conducted in water undersaturated with air, representing a worst-case scenario more demanding than immersion in air-saturated seawater. Plastron thickness was monitored in situ using White Light Reflectance Spectroscopy (WLRS), enabling real-time tracking of air layer dynamics. The plasma micro-nanotextured PMMA surfaces presented here retained durable superhydrophobicity under high overpressures and continuous water flows for extended durations. Notably, the surfaces preserved a stable plastron under a pressure equivalent to a 10-m water depth for at least two weeks of continuous operation. Additionally, mechanisms governing plastron lifetime are analyzed in-depth, and strategies for achieving durable, long-term underwater superhydrophobicity are discussed.