F. Veronesi, M. Raimondo, B.F. Jacob, E. Ciappi, F. Lagala, A. Ratti
Institute of Science and Technology for Ceramics CNR-ISTEC,
Keywords: liquid-infused surfaces, drag reduction, anti-fouling
Summary:Naval and maritime industries have huge economic and strategic relevance. Therefore, technological solutions for their issues are a crucial topic for the scientific community. Namely, skin-friction drag and biofouling are two of the most severe problems that sea vessels have to face. Therefore, surfaces with the ability to reduce skin-friction drag and biofouling in marine environment would be a massive breakthrough. Since their introduction in the early 2010s, Liquid-Infused Surfaces (LIS) have been intensively studied for their peculiar underwater behavior. In fact, they proved more durable than lotus-inspired Superhydrophobic Surfaces (SHS) when submerged, as the infused liquid phase is not displaced from water as opposed to the unstable air pockets trapped within SHSs. For this reason, LIS are excellent candidates as drag-reducing and antifouling materials to be applied in ships and hulls. So far, the studies on the skin-friction drag behavior of LIS were based on lab-scale equipment like Taylor-Couette rheometers or narrow channels. Therefore, a characterization of their behavior on larger scale is much needed to understand their potential in real environments. In this work, we have fabricated and tested LIS based on a randomly oriented, nanostructured boehmite coating. Then, fluorinated or fluorine-free organic molecules were grafted to the coating and different lubricant oils were infused, namely perfluoropolyethers, silicone oils and alkanes. The drag resistance behavior in turbulent conditions of such LIS has been assessed in experiments performed at CEIMM in Rome, a high-speed water channel. The wall shear force in a high-speed boundary layer over a large flat plate was measured with custom-made setups. Preliminary results shed a light on the design parameters that influence drag reduction properties and long-term stability of LIS in high-Reynolds number flows, typical of real marine environments. Moreover, the same surfaces were tested for their anti-fouling properties by immersion in Tyrrhenian Sea, off the coast of La Spezia (Italy), during the summer of 2019. Periodically, the samples were extracted and the growth of organisms was evaluated with a photographical method. Remarkably, LIS showed significant reduction in biofilm formation compared to uncoated surfaces. In this perspective, LIS represent a more eco-friendly anti-fouling solution compared to present commercial solutions based on harmful biocides.