Hydrophobic and oleophobic re-entrant steel microstructures fabricated using micro electrical discharge machining

This paper presents the fabrication of metallic micro-mushroom re-entrant structures and the characterization of their hydrophobicity and oleophobicity. Five different microstructure geometries are introduced, with typical feature sizes in the range of 10–100 μ m. These microstructures are realized in steel, and are fabricated over the cm-scale using micro electrical discharge machining (mEDM). The liquid repellency of these surfaces is characterized using droplets of either water (surface energy γ lg = 72.4 mN m −1 ), RL-68H oil ( γ lg = 28.6 mN m −1 ), or Isopropanol (IPA) ( γ lg = 21.7 mN m −1 ). The water droplets form nearly perfect spheres, with contact angles in the range 146–162°, and contact angle hysteresis of 19–35°. The oil droplet contact angles are in the range 106–152° and the IPA contact angles are in the range 75–123°. Strong re-entrant features and close spacing are necessary to support a fully non-wetting state for use with oil and IPA. Water forms the highest contact angles with narrow, post-like, and widely spaced micro-mushroom geometries.