Towards Amphiphobic Surfaces Fabricated by One Step Lithography Process

The phenomenon of superhydrophobicity observed on nature, including many plants and insects has attracted tremendous attention in recent years. The most famous case for this effect is represented by the lotus leaves, droplets can roll-off from these surfaces and dust particles, resulting in the famous self-cleaning effect (also known as “lotus” effect).

Great research efforts have been intensive during the last years on making self-cleaning surfaces. However, it remains a big challenge on these surfaces, and is the fact that normal superhydrophobic surfaces do not repel oils. For instance, in the case of the lotus leaf, although its leaf surface exhibits very high water contact angle and very small roll off angle, an oil droplet actually wets and spreads on the same lotus leaf. Most of the common contaminants and pollutants are based on organic materials, which adhere on the surfaces and do not repel easily.

Engineered surfaces that show low affinity toward most of the low surface tension liquids like oils is not achievable unless re-entrant or overhang structures in addition to surface roughness and low surface energy are provided. A liquid cannot enter a pattern with an overhang structure because of the capillary force at the solid/liquid interface near the structure. As a result, the liquid cannot touch the bottom of the pattern and a layer with trapped air is formed on the surface.

Fabrication of well-defined overhanging structures usually demands complex combination of several techniques. Here we report a novel one-step etching techniques to produce overhanging pillars without the needing of any additional steps.

The wetting properties of the overhanging structures was analysed using a wide range of different liquid possessing different surface tensions. The critical surface tension for achieving oleophobicity was experimentally established, obtaining a value as low as 23mN/m.