Equilibrium Wetting Phenomena

In this chapter, we shall discuss equilibrium liquid shapes on solid substrates, which demands equilibrium of liquid-vapor, liquid-solid, and vapor-solid. Not always do authors take into account all the three equilibria. The vapor-solid equilibrium determines the presence of adsorbed liquid layers on solid surfaces for both complete and partial wetting. Even at this moment, we and everything around are covered by a thin water film. The thickness of aqueous films depends on the humidity in the room, and the adsorption is exactly at equilibrium with the surrounding humidity, no matter how low or high. 

The presence of liquid layers on solid substrates is determined by the action of surface forces (the disjoining pressure), which was discussed in Chapter 1. The disjoining pressure isotherm is normally dealt with because it is usually measured at a constant temperature. In the case of water and aqueous solutions, the disjoining pressure is S-shaped. Water and aqueous solutions are crucial for life. Does the peculiar shape of the disjoining pressure isotherm of water and aqueous solutions in some way determine our existence  

It is well known that all properties of water and aqueous solutions are vitally important for life. This means that the peculiar shape of disjoining pressure isotherms of water and aqueous solutions, in some unknown way, determines the existence of our life. At the moment we do not know how the process works. 

Further in this chapter, we investigate the influence of the combined action of disjoining pressure and capillary forces on the equilibrium shapes of liquids on solid substrates. 

1 THIN LIQUID FILMS ON FLAT SOLID SUBSTRATES 

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