Analysis of the wetting state of super-repellent fabrics with liquids of varying surface tension

Abstract

In designing a super-repellent surface that is not wet to liquids with a lower surface tension than water, micro and nano-scale surface roughness have a great impact in addition to low surface energy. In this study, a super-repellent fabric was fabricated using oxygen plasma etching and plasma enhanced chemical vapor deposition (PECVD) with hexamethyldisiloxane (HMDSO). The influence of dual roughness on wettability in micro and nano-scale structures was analyzed using the contact angles of test reagents whose surface tension ranges from 33–72 dyn cm⁻¹. The treated fabrics produced dual scale roughness, and exhibited contact angles greater than 160° against the test liquid whose surface tension was greater than 42 dyn cm⁻¹. The Cassie–Cassie theoretical model, which is based on the non-wetting assumption of either micro-scale or nano-scale roughness, explained well the actual water contact angles on the treated fabrics. For the liquid with 42 dyn cm⁻¹ surface tension, the wetting behavior followed behavior between the Cassie–Wenzel state and Cassie–Cassie state depending on the aspect ratio of the nano-scale roughness. With an increased etching time of 7 min or longer, the actual contact angles were measured to be larger than those predicted using the Cassie–Cassie model, which may be a result of the formation of partial re-entrant structures at the tips of the nano-pillars. Self-cleaning effects were demonstrated for solid particles adhered on the treated fabrics such as silicon carbide and Sudan Black B. Water was more effective in adhering to both particle types and rolling off the surface than isopropyl alcohol solution.