CHAPTER 6: Design Principles for Robust Superoleophobicity and Superhydrophobicity

The basic design parameters for superoleophobicity have been studied with a model pillar array surface fabricated on a silicon wafer by the Bosch etching process followed by fluorosilanation. The model surface was found to be both superoleophobic and superhydrophobic. The formation of the Cassie–Baxter state with hexadecane was confirmed by examining the replica of the liquid–solid interface. By varying the geometry of the pillar and surface coating, we established that the key drivers for superoleophobicity are chemistry, surface roughness, and re-entrant structure at the solid–liquid interface. The robustness of the surface design was investigated with a combination of experimental studies of model surfaces, simulation, and modelling. Results reveal that there may be conflicting surface requirements, depending on the application. For example, a surface with low solid-area fraction will lead to small hysteresis and low adhesion, a desirable feature for superoleophobicity, but such a surface will have relatively poor mechanical abrasion resistance and low wetting breakthrough pressure. A design space map is presented which will give a view of the latitude window for designing a complex and demanding surface. The design principles for robust superhydrophobicity are shown to be similar. Finally, gaps and shortfalls in implementation are discussed. Issues on large-area, large-scale manufacturing of superhydrophobic and superoleophobic surfaces are summarized.