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 mu 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 gamma(lg) = 72.4 mN m(-1)), RL-68H oil (gamma(lg) = 28.6 mN m(-1)), or Isopropanol (IPA) (gamma(lg) = 21.7 mN m(-1)). The water droplets form nearly perfect spheres, with contact angles in the range 146-162 degrees, and contact angle hysteresis of 19-35 degrees. The oil droplet contact angles are in the range 106-152 degrees and the IPA contact angles are in the range 75-123 degrees. 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.