Green hydrophobic and superhydrophobic coatings and surfaces for water related applications: A review

Water-resistant coatings and surfaces are ubiquitous in modern life. They are used in consumer goods, food packaging, medical devices and other technical, as well as scientific applications. The introduction of per- and polyfluoroalkyl substances (PFAS) several decades ago was the key technology making this innovation possible. PFAS coatings are low surface energy, hydrophobic coatings, which exhibit low friction and antiwetting properties. PFAS materials and coatings have been extensively used to realize extreme anti-wetting surfaces, i.e. surfaces which can repel water (superhydrophobic), as well as other low surface tension liquids and oils (superoleophobic). Such materials and surfaces are ideal for innovative water related applications, such as atmospheric water collection, water desalination or other separation applications in which extreme wettability properties can significantly enhance the performance in these applications. However, PFAS are non-biodegradable, persistent in the environment and there is increasing evidence of negative ecotoxicological and health effects. To this end, various substitutes (natural hydrophobic materials such as waxes or synthetic hydrophobic ones such as hydrocarbons) have been developed and reported in the literature in response to these concerns. However, the uptake of these new coatings is limited, mainly due to the lack of systematic performance evaluation and the diversity of the results presented. This review paper focuses on presenting the most promising "green" coatings and surfaces for hydrophobicity and superhydrophobicity existing in the literature, focusing on the ones which are applied for innovative water related applications, such as atmospheric water collection and oil-water separation.