Liquid-repellent surfaces for frictional drag reduction & noise attenuation

Skin-friction drag is a major source of energy dissipation in marine vehicles. In addition, self-induced noise generated by turbulent boundary layer can strongly limit the operation of on-board sensors. Therefore, designing surfaces with friction drag reduction and vibration control properties is a research topic of great interest for the marine transportation field, both in civil and military segments. Liquid-repellent surfaces have been identified as potential solutions for drag reduction and some papers have already proved their positive effects on drag1,2, but also their inherent limits3. Moreover, no one has ever investigated the potential of liquid-repellent surfaces in terms of self-induced noise attenuation. In our study, we first fabricated water-repellent surfaces following two well-known biomimetic approaches, namely the SuperHydrophobic Surface (SHS) approach and the Liquid-Infused Surface (LIS) one. Then, we performed experiments in a high speed water channel to evaluate frictional drag and vibration induced by the turbulent boundary layer (Re?106). Drag experiments were performed on floating aluminum surfaces (48x28 cm2) bearing a flexural load cell that measured the frictional force exerted by water flow. Meanwhile, in noise experiments a tray-shaped surface was fixed to a rigid frame while 8 piezoelectric accelerometers measured its vibrational response to water flow. On one hand, SHSs showed quick air plastron depletion during the tests, leading to no drag reduction and higher vibration level compared to a reference uncoated panel. On the other hand, LISs showed remarkable drag reduction (up to 16%) in the 1.0÷3.5 m/s velocity range and reduction of the acceleration response spectra at medium and high frequencies, regardless of flow velocity. These preliminary results suggest LISs as potentially relevant surfaces to reduce friction drag and self-induced noise, with perspective huge impact in marine transportation. Further experiments are needed to explain the observed noise attenuation and drag reduction effects, which indeed overcame expectations.