Slanted nanostructures capped with SiO2 nanospheres for asymmetric hydrophilicity inducing rapid unidirectional water spreading

Abstract

Two levels of nanostructures, consisting of slanted nanostructure and nanogroove, were fabricated on polymeric surface through a single step oxygen plasma processing with tuning incident angles. A polymeric sheet embedded with the SiO2 nanospheres was exposed to the oxygen plasma through Faraday cages to induce anisotropic etching. The nanopillars were slanted by changing the relative angle between plasma ions and polymer substrate from 10 to 80°. The nanogrooves were also formed among the slanted nanopillars along the trajectory of plasma ions. Together with the nanogrooves, the slanted nanopillars enhanced unidirectional water spreading. The wetting behaviors, with gradual transformation through four stages with different degree of hydrophilicity (super-, strong, moderate, and mild hydrophilicity), were observed. The surface with moderate hydrophilicity showed strong asymmetric wetting for unidirectional water spreading, against the gravity force, at very high velocity (up to 1.5 ∼ 4.17 mm/sec). This easy and simple fabrication method of slanted nanostructures could be utilized in a wide range of applications requiring long-lasting hydrophilicity with asymmetric wetting, such as, microfluidic, oil–water separation, water-harvesting, and biomedicines.