Re-entrant structures for superhydrophobic and superoleophobic surfaces


Wonjae Choi1, Anish Tuteja, atuteja@mit.edu2, Robert E. Cohen2, and Gareth Mckinley, Gareth@mit.edu1. (1) Department of Mechanical Engineering, Massachusetts Institute of Technology, MA, (2) Department of Chemical Engineering, Massachusetts Institute of Technology, Bldg. NE-47, Room 583, 500 Technology Square, Cambridge, MA 02139
It is not possible to make a super-oleophobic surface via patterning surfaces with simply vertical or slanted microtextures. The low interfacial tension of hydrocarbon oils result in partial wetting (contact angles θ < 90o) on almost all smooth substrates; including fluorinated surfaces. Simple application of Cassie-Baxter or Wenzel models then predict that texturing a surface will reduce the contact angle. However, Herminghaus has presented that hydrophobicity can be achieved even from hydrophilic material within the context of the Cassie-Baxter model when the textured surface exhibits strongly re-entrant structures. In the present work, we manufacture a severely undercut pillar array as the re-entrant structure and create super-hydrophobic ¢Herminghaus surfaces¢ from extremely hydrophilic SiO2. We then exploit these techniques to produce super-oleophobic surfaces (low hysteresis and contact angle > 160o even for octane) from silanized SiO2. To understand the underlying physics, the effects of pillar shapes on the oleophobicity are assessed.