The liquid repellency of a surface is principally governed by a combination of its chemical nature and topographical microstructure. Although flat low surface energy materials can often exhibit high water contact angle values, this is not normally sufficient to yield super-hydrophobicity (description reserved for materials upon which water drops move spontaneously across horizontal or near horizontal surfaces). In order for this behaviour to occur, the difference between advancing and receding contact angles (hysteresis) must be minimal. Effectively, the hysteresis can be regarded as the force required to move a liquid droplet across the surface; i.e. in the case of little or no hysteresis, very little force is required to move a droplet, hence it rolls off easily. Theoretical studies for idealised rough hydrophobic surfaces predict that contact angle hysteresis initially increases with surface roughness, until eventually a maximum value is reached; greater roughness scales beyond this lead to a fall due to the formation of a composite interface (liquid unable to completely penetrate the surface). The latter can be described by the Cassie-Baxter state, where the inherent surface roughness causes air to become trapped in voids (i.e. prevents liquid from wicking). Hence low contact angle hysteresis can be achieved by substrate roughening to produce a composite interface.

Low surface energy polymers form an important class of thin coatings. Such coatings are widely used for a broad variety of ‘anti’-applications like anti-wetting, anti-fogging or anti-fouling. They are important for microelectronics, textile, optical and even medical applications. Such polymer coatings have surface energies that are much lower than those of traditional solids and lower than that of water and even oil. Thus, even if oil were spilled on a low surface energy polymer film, it would not spread out to cover the surface, but the oil would form droplets instead, which could easily be removed.

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