A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen
superhydrophobic, superoleophobic, oil-repellent, oleophobic
The combination of surface chemistry and roughness on multiple scales imbues enhanced repellency to the lotus leaf surface when in contact with a high surface tension liquid such as water. This understanding has led to the creation of a number of biomimetic superhydrophobic surfaces (i.e. apparent contact angles ( *) with water greater than 150° and low contact angle hysteresis). However, surfaces that display contact angles of * > 150° with organic liquids having appreciably lower surface tensions (i.e. superoleophobic surfaces) are extremely rare. Calculations suggest that creating such a surface would require a surface energy lower than any known material. In our recent work (Science, 318, 1618, 2007) we demonstrated how a third factor, re-entrant surface curvature, in conjunction with chemical composition and roughened texture can be used to design surfaces that display extreme resistance to wetting from alkanes such as decane and octane. Further, we also established the various design parameters that affect the robustness of the metastable composite (solid-liquid-air) interface, allowing for the creation of extremely non-wetting rough surfaces, even though their corresponding smooth surfaces may be easily wetted by a given liquid. Here, we extend that work by designing a number of different surfaces using electrospinning to produce nano-fibers, which incorporate re-entrant curvature. By systematically changing the various design parameters we assess the effects of surface geometry on both the apparent contact angle and hysteresis. This allows us to create, for the first time, surfaces that repel practically any liquid as demonstrated by apparent contact angles of 165°, 163° and 155° with methanol, octane and pentane respectively.