Water/oil repellency and work of adhesion of liquid droplets on graphene oxide and graphene surfaces

The study examines the water/ethylene glycol (EG) repellency of graphene and graphene oxide sheets prepared by the chemical exfoliation of natural graphite powders. The graphene nanosheets were produced by reducing graphene oxide with EG under microwave irradiation. The graphene sheets were assembled into a thin paper, and a facile fluorination was used to coat a thin fluorine layer over the graphene paper. The graphene oxide paper is generally hydrophilic, whereas without aid of any fluorination, the resulting graphene paper displays superhydrophobicity (contact angle: 150.1 ± 2.3°) and low fraction in contact with solid (12.2%). Such low solid fraction may be attributed to the air pocket trapped in (i) the interspaces between graphene powders and (ii) the flake-like voids between graphene sheets, referred to as the Cassie state. The EG repellency of graphene paper can be significantly improved by surface fluorination. Taking into account Young-Duprè's equation incorporated with the Cassie parameter, the W_ad values of the graphene papers for water repellency were found to fall in the region of 9.62-12.5 mJ/m². The low W_ad value between the droplets and the graphene surface can be ascribed to the fact that porous graphene sheets offer an air cushion to repel the drop penetration, inducing the low work required for the movement of droplets on graphene paper. On the basis of the results, this study offers fundamentals on the water and EG repellency of graphene and graphene oxide surfaces.