Surface energy wetting equilibria/tensions

Wetting Criteria, Surface and Interface Free Energies, and Work of Adhesion In a solid-liquid system, wetting equilibrium may be defined from the profile of a sessile drop on a planar solid surface. Young s equation [36], relating the surface tension Y of materials at the three-phase contact point to the equilibrium contact angle 6, is written as... 

As these other chapters show, there is a well-developed thermodynamic theory of wetting and spreading. This relates surface energies and surface tensions to contact angles and extent of wetting. Where the relevant surface energies and contact angles are known, or can be deduced, the extent of contact between a specific adhesive and substrate at equilibrium can be predicted. The essential idea of the adsorption theory of adhesion is that whenever there is contact between two materials at a molecular level, there will be adhesion. 

Components of interfacial tension (energy) for the equilibrium of a liquid drop on a smooth surface in contact with air (or the vapor) phase. The liquid (in most instances) will not wet the surface but remains as a drop having a definite angle of contact between the liquid and solid phase. 

From equation (8) it can be seen that sohds and liquids will form equilibrium shapes in an effort to minimize their surface area and thus the free energy of the system. Indeed, crystal faces with the closest packing of surface atoms have the lowest surface area and tend to be the most stable. When one considers a two-component system, with one material on top of the other, the interaction between the two will be defined by the surface tensions. The surface tensions of some selected solids and hquids are listed in Table 2. From these values, it can be easily predicted which materials will be capable of wetting another. In general, most liquids have lower surface tensions than clean solids and will therefore spread to cover them. 

We now consider the case of complete wetting (spreading power 5 > 0) for the case where there are van der Waals interactions, which tend to thicken the film. For a finite amount of fluid spreading on an infinite solid substrate, the equilibrium film profile will therefore not be a monolayer, but a pancake with a maximum thickness that is determined by the balance of the surface tensions and the van der Waals energies (see Fig. 4.3). 

---