Superhydrophobic & Oleophobic Surfaces

As per definition, superhydrophobicity is achieved when the static contact angle with water is over 150° and the contact angle hysteresis (glossary) is low. Characterization of superhydrophobic coatings is thus accomplished by measurements of both static (glossary) and dynamic contact angles (glossary). 

Oleophobic surfaces on the other hand are already being utilized in smartphones, for example, to give the display a certain amount of finger smudge resistance. They do not make your smartphone display fingerprint proof, but make it possible to simply wipe the grease off with a soft cloth. Superoleophobicity would add oil/water separation and oil droplet manipulation to the list of possible applications, to name a few. Superoleophobicity is similarly defined as superhydrophobicity but, instead of water drop, an oil drop must form an angle over 150 ° with the solid substrate. Oleophobicity, let alone superolephobicity, is much harder to achieve than superhydrophobicity. In oil, the forces between oil molecules are weak van der Waals forces. Due to this, oil molecules are not as strongly bound to each other as in water, and the surface tension of oil is much lower. For a surface to be hydrophobic, the surface free energy of the solid must be lower than the surface tension of water, about 72.8 mN/m but to be oleophobic the surface free energy has to be lower than 20 mN/m, which is a typical surface tension value for oil. Such a low surface free energy requires a special engineering of the surface properties, in terms of both chemistry and roughness.