Potential skin-friction drag reduction & vibration controlin water repellent surfaces fabricated with different biomimetic approaches

In the last years, water-repellent surfaces have been widely studied in terms of design, processing and related properties (e.g. self-cleaning, anti-icing, reduction of frictional drug, etc). As far as the biomimetic fabrication approaches, the one inspired to the Lotus leaf envisages a two-tier, hierarchical structures enabling the entrapment of air within the solid surface, allowing a Cassie-Baxter wetting state when combined with low surface energy. So obtained superhydrophobic surfaces (SHSs) display water contact angle (WCA) larger than 150°, usually coupled with a small contact angle hysteresis (CAH). The approach inspired to the Nepenthes pitcher plant is based on the presence of a low surface tension liquid as outer layer filling the micro/nanoporosity of the material surface, thus creating a liquid-liquid interface with contacting drops. The resulting Slippery Liquid-Infused Porous Surfaces (SLIPSs) usually display lower WCAs compared to SHSs but the same reduced CAH. In addition, many papers proved SLIPSs more efficient than SHSs in peculiar conditions, e.g. at high pressure or underwater. With the aim of assessing the potential application of SHS and SLIPS for civil and military vessels, we performed experimental investigations in turbulent regime (Re?106) evaluating both the frictional drag and the vibration induced by the turbulent boundary layer by means of a purposely designed setup placed on the top of a high speed channel. A floating test surface (48x28 cm2), connected to a flexural load cell (0-20 N) and aligned trough a levelling screw system, allows the measurement of the frictional force exerted by the flow over the surface. On the other hand, the vibrational response of thin plates fixed to a rigid frame, have been measured by 8 piezoelectric ICP accelerometers. The tested SHSs consist of aluminum plates dip-coated in a isopropyl alcohol-based suspension of alumina nanoparticles, afterward immersed in boiling water to form boehmite AlOOH with a flower-like nanostructure. To lower the surface energy, fluoroalkylsilane molecules (FAS) were grafted to the surface. The as-produced SHSs had WCA = 164° and CAH = 6°. On the other hand, SLIPSs were fabricated via infusion of a fluorinated oil (Krytox 100, DuPont) within the pores of SHSs. SLIPSs had WCA = 121° and CAH = 4°. Drag measurements over random textured SHSs confirm the surface roughness as key parameter: the simultaneous air plastron monitoring clearly reveals that the lack in drag reduction is linked to an early air loss, only partially hindered increasing the roughness (from 1.5?m to 4?m ) via sandblasting. Meanwhile, for the SLIPSs drag reduction of 10-16% is observed across the tested flow velocity range (1.0-3.5 m/s).The analysis of the vibrational response shows a clear trend of SLIPSs at reducing the acceleration response in a quite large frequency range and up to 4 m/s. On the contrary, SHSs seem to induce a higher vibration level most likely due to the continuous planstron modification.