Fabrication and properties of hybrid, liquid repellent coatings

The control of the wetting properties of solid surfaces is extremely relevant for a wide spectrum of industrial applications, i.e. building industry, electronics and renewable energies. Thus, huge efforts have been devoted to the fabrication of surfaces with tailored wetting properties. More specifically, many scientists focused on the mimicry of the so-called Lotus Effect, which can be observed on the surface of lotus leaves: water drops do not stick, rather they remain spherical and roll off very easily. Such behavior is due to the combination of a dual-scale, hierarchical surface structure (i.e. on the micro- and the nano-scale) with a chemical composition that leads to low surface energy. A surface with such characteristics will be able to trap air within its features and will not let water drops penetrate, thus hindering sticking and allowing removal. Therefore, in our studies we followed a biomimetic approach for the design of liquid-repellent surfaces. First, we developed a hybrid coating mimicking the Lotus Effect. The inorganic component of the coating was obtained via dip-coating the surface with a suspension of alumina nanoparticles; the resultant gel film was first heat-treated, then transformed into a boehmite AlOOH layer with a peculiar flower-like nanostructure by immersion in boiling water. We investigated the formation of a properly structured coating on different materials, adjusting process parameters (e.g. dispersant, treatment temperature) to achieve the best results in terms of wetting properties. In fact, after chemical modification with a fluoroalkylsilane (FAS) to decrease surface energy, the coating was superhydrophobic (SH), with a water contact angle (WCA) well above 150° and contact angle hysteresis (CAH) lower than 10°. XPS analyses confirmed the grafting of FAS chains to the surface, while DFT calculations provided a deeper insight into the formation of the FAS monolayer. We also extended the repellence to other liquids, causing great increase in contact angles with low surface tension liquids (e.g. alkanes and oils), thus the coating could also be defined as oleophobic. Furthermore, this coating showed excellent resistance to chemically aggressive environments, maintaining superhydrophobicity even after prolonged testing. We also pursued an alternative approach to liquid repellence. Once again we followed a biomimetic approach, taking the slippery surface of pitcher plant as a model. We started from the aforementioned hybrid coatings and added a fluorinated lubricant to fill the pores in the coating and to create a continuous liquid film. In such situation, a liquid drop is in contact with the liquid phase rather than with the solid surface. These innovative materials are called Slippery Liquid-Infused Porous Surfaces (SLIPSs). Even though they displayed much lower WCA than regular SH surfaces, they still had exceptionally low CAH (about 2°) and water drops showed no sign of adhesion. These surfaces have potential self-healing properties, as the infused oil should be able to repair damaged areas where the liquid film is depleted. Moreover, SLIPSs might show liquid repellence even in adverse conditions, e.g. under high pressures at which SH surfaces usually fail. The properties of these surfaces still have to be fully explored, but the first observations are extremely appealing. In summary, we fabricated and optimized two biomimetic coatings with notable liquid repellence. Their application in many industrial fields is yet to be achieved, but the huge potential advantages foster further research and investigations to improve their performance and properties.