Factors determining wettability of superhydrophobic paper prepared by spraying nanoparticle suspensions

Superhydrophobic paper was prepared by spraying nanoparticle suspensions onto paper substrates. In this work, we examined factors that determine superhydrophobicity of nanoparticle films on paper. Since used oxide nanoparticles (i.e., SiO2, Al2O3 and TiO2) are naturally hydrophilic due to the presence of surface hydroxyl groups, they were treated with silane coupling agents to lower their surface energy. Type of the silane coupling agents affected the wettability of the nanoparticle film, and superhydrophobic paper was obtained when using silane coupling agents with longer alkyl chains than a hexyl group. In addition, type of oxide nanoparticles had an influence on the wettability of the nanoparticle films because both amounts of residual hydroxyl groups and coating behavior of nanoparticles depended on type of the inorganic oxides. A water droplet bounced on the superhydrophobic paper, indicating its high water repellency.