Wetting transitions

The macroscopic parameter characterizing wetting

Wetting

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

 of a solid (or liquid) surface with a liquid is contact angle

Contact angle

The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...

. Various contact angles can co-exist on the same solid substrate. When contact angle experiences change we observe a wetting transition. Wetting transitions occurring on flat and rough surfaces should be distinguished.

1. Flat surfaces. When a liquid drop is put onto a surface, two situations may result. If the contact angle is zero a situation is referred as complete wetting. On the other hand, if the contact angle is between 0 and 180º, a situation called partial wetting. A wetting transition is a surface phase transition from partial wetting to complete wetting.
2. Rough surfaces.

The situation on rough surfaces is much more complicated. The main characteristic of the wetting properties of rough surfaces is the so-called apparent contact angle (APCA). It is well known that the APCA usually measured are different from those predicted by the Young equation. Two main hypotheses were proposed in order to explain this discrepancy, namely the Wenzel and Cassie

Cassie's law

Cassie's law describes the effective contact angle θc for a liquid on a composite surface . The law explains how simply roughing up a surface increases the apparent surface angle...

 wetting models. According to the traditional Cassie model, air can remain trapped below the drop, forming "air pockets". Thus, the hydrophobicity of the surface is strengthened because the drop sits partially on air. On the other hand, according to the Wenzel model the roughness increases the area of a solid surface, which also geometrically modifies the wetting properties of this surface. Transition from Cassie to Wenzel regime is also called wetting transition. Under certain external stimuli, such as pressure or vibration, the Cassie air trapping wetting state could be converted into the Wenzel state. It is well accepted that the Cassie air trapping wetting regime corresponds to a higher energetic state, and the Cassie–Wenzel transition is irreversible. However, the mechanism of the transition remains unclear. It was suggested that the Cassie–Wenzel transition occurs via a nucleation mechanism starting from the drop center. On the other hand recent experiments shown that the Cassie–Wenzel transition is more likely to be due to the displacement of a triple line under an external stimulus. The existence of so-called impregnating Cassie wetting state also has to be considered. Understanding wetting transitions is of a primary importance for design of superhydrophobic surfaces.