P. B. Weisensee, E. J. Torrealba, M. Raleigh, A. Jacobi, W. P. King, “Hydrophobic and Oleophobic Re-Entrant Steel Miscrostructures Fabricated Using Micro Electrical Discharge Machining”, J. Micromech. Microeng. 24 (2014) 095020
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 μm. 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 γlg = 72.4 mN/m), RL-68H oil (γlg = 28.6 mN/m), or Isopropanol (IPA) (γlg = 21.7 mN/m). 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.