Nanostructured coatings for wetting control: design, fabrication and properties

Abstract

My work focused on the fabrication of superhydrophobic surfaces (SHSs) by deposition of Al2O3 nanoparticles (NPs) and chemical modification with fluoroalkylsilanes (FAS). I synthesized Al2O3 NPs via sol-gel method in water or isopropyl alcohol, assessing the differences between the suspensions in physicochemical properties and coating effectiveness on different materials. In some cases, the dispersant played a role in the homogeneity and performance of the coating. I explored two deposition techniques, with dip coating proving more effective than spray coating. After NPs deposition, few thermal treatments were required. I assessed the influence of treatment temperature (T) on the coating, especially for a sensitive material like copper. The crucial step of the process was the immersion in boiling water to form a flower-like boehmite nanostructure. After grafting FAS chains, the surface became superhydrophobic. I characterized the coating in terms of wetting properties (static contact angle CA and contact angle hysteresis CAH) with water and other liquids and it proved superhydrophobic and highly oleophobic. I also assessed coating durability by performing ageing tests in chemically and mechanically aggressive conditions. The coating displayed remarkable resistance, still delivering excellent liquid repellence after the tests. Furthermore, I determined surface morphology with FESEM observations throughout the process, while colleagues at Università La Sapienza investigated the surface chemical composition with XPS analysis and DFT calculations. FAS molecules formed an ordered and stable monolayer on the boehmite coating. Further characterizations concerned coating adhesion to different substrates and roughness. As SHSs have a wide range of potential applications, it is useful to assess their behavior in “dynamic” conditions. For example, the study of single drop impacts is crucial in applications like sprays or ink-jet printers. Thus, I observed the impact of water and hexadecane drops on SHSs with different morphology and chemical composition. Surprisingly, it was not possible to easily correlate wetting properties and drop impact outcome, as surfaces with the same CAs provided different drop impact results. Parameters related to the liquid (velocity, surface tension, viscosity) and to the surface (morphology, roughness and, unexpectedly, chemical composition) played relevant roles in determining impact outcomes. I also studied the drop shedding behavior on the SHSs at different T and relative humidity (RH) conditions. At room T and low RH, a sessile drop could be easily shed by a low-speed airflow. On the other hand, high RH and T < 0°C strongly hindered shedding, thus requiring much higher airflow speed U to occur. Furthermore, surface roughness proved influent in determining critical U. I also tried to quantify the delay in drop freezing provided by the superhydrophobic coating, but instrumental issues made it difficult to obtain reliable data. Moreover, I measured the evolution of CAH when the surface was kept in high RH environment. An interesting drop in CAH after long condensation times was observed, but further investigations are needed. Finally, I evaluated the anti-friction properties of the coating deposited on pump components. The coating provided a remarkable and long-lasting friction reduction, with positive effects on pump efficiency. In summary, the hybrid, nanostructured coating owned remarkable liquid-repellent properties, due to the combination of flower-like morphology and grafted FAS molecules. The coating displayed excellent durability in a wide spectrum of aggressive environments and proved efficient even in dynamic conditions (e.g. drop impact, drop shedding with airflow). However, limitations appeared in severe conditions like high pH, high RH and freezing T. Future work must be devoted to the optimization of the coating to widen its range of potential applications.