Superhydrophobic states

Abstract

It is well known that the roughness of a hydrophobic solid enhances its hydrophobicity. The contact angle of water on such flat solids is typically of the order of 100 to 120 degrees, but reaches values as high as 160 to 175 degrees if they are rough or microtextured. This result is remarkable because such behaviour cannot be generated by surface chemistry alone. Two distinct hypotheses are classically proposed to explain this effect. On one hand, roughness increases the surface area of the solid, which geometrically enhances hydrophobicity (Wenzel model). On the other hand, air can remain trapped below the drop, which also leads to a superhydrophobic behaviour, because the drop sits partially on air (Cassie model). However, it is shown here that both situations are very different from their adhesive properties, because Wenzel drops are found to be highly pinned. In addition, irreversible transitions can be induced between Cassie and Wenzel states, with a loss of the anti-adhesive properties generally associated with superhydrophobicity.