Pinning and wetting stability of liquids on superoleophobic textured surfaces

Super water/oil repellency has been a very active research field and could be achieved via combination of surface texturing and chemical treatment. When a droplet is deposited on a superhydrophobic/superoleophobic surface, its contact line can be pinned somewhere on the structured surface instead of fully wetting the substrate, forming a solid-liquid-air composite interface. The focus of this article is to understand the effect of pinning and wetting stability through energy analysis and force balance. Textured surfaces with straight sidewall pillar, wavy sidewall pillar and hoodoo structures are discussed in detail. For the straight sidewall structure, it was found that the top pillar edge is a stable pinning site for water. Meanwhile, hexadecane fully wets the structure without any pinning. On the wavy sidewall structure, the protruding and concave corners are pinning sites for both water and hexadecane. However, the dominant breakthrough pressure comes from the energy barrier against contact line advancing along the re-entrant slope of the wave. On the hoodoo structure, there are two pinning sites (top and bottom corner of the hoodoo cap) for water, but only one pinning site (bottom corner) for hexadecane. The effects of solid area fraction and re-entrant angles on pinning stability are studied with the wavy sidewall structure. This study suggests that Gibbs energy analysis can be a viable approach in designing robust superoleophobic surfaces by enhancing the pinning stability and breakthrough pressure, which is strongly correlated to design parameters, for example solid area fraction, geometrical re-entrant angle and dimensions. This article contains supporting information that is available online.