Facile creation of hierarchical PDMS microstructures with extreme underwater superoleophobicity for anti-oil application in microfluidic channels

Composition modification and surface microstructures have been widely utilized in interface science to improve the surface performance. In this paper, we observed a significant improvement of oil contact angle (CA) from 66 +/- 2 degrees to 120 +/- 4 degrees by introducing a radical silanol group on a flat PDMS surface through oxygen plasma pretreatment. By combining surface microstructures and plasma modification, we produced three kinds of superoleophobic surfaces: 20 mm pitch micropillar arrays, 2.5 mu m pitch micropillar arrays and gecko foot-like hierarchical microstructures. Among them, the hierarchical surface with high surface roughness showed extreme underwater superoleophobicity, which featured ultrahigh CA (175 +/- 3 degrees) and ultrasmall sliding angle (<1 degrees). Quantitative measurements demonstrated that these superoleophobic surfaces exhibited distinct adhesive behaviors, by which they were interpreted as Wenzel's, Cassie's and the Lotus state, respectively. A microfluidic channel with superoleophobic microstructures was further created by novel curve-assisted imprint lithography, and the characterization based on anti-oil contamination applications was carried out and discussed. We believe that the superoleophobic surfaces will power broad applications in oil microdroplet transportation, anti-oil channels and droplet microfluidic systems.