Wetting in Solid-Liquid Systems

Wetting includes the spreading of a pure liquid over the surface of a solid, displacing the gas (or vapour) initially in contact with that surface [18, 33]. Hence the phenomenon involves three interfaces, namely Solid-Gas, Solid-Liquid, and Liquid-Gas ones. The spreading coefficient Ws is defined as [4, 33] 

When Ws is positive or zero, the liquid wets the solid material, i.e., spreads out spontaneously over its surface, providing there is enough liquid to eliminate a unit area of the Solid-Gas interface while exposing a corresponding amount of the Solid-Liquid and Liquid-Gas interfaces. 

When Ws is negative, the liquid remains as a drop having, at equilibrium, a definite angle of contact with the solid surface (the liquid does not wet the solid). This case is illustrated in Fig. 6.4. 

The equilibrium contact angle between the Uquid and the solid phases is determined by the following balance of interfacial tensions [4, 33, 34]  

The classical form of Young s equation, which describes the equilibrium balance of forces meeting at the three-phase contact line in the plane of the solid surface (see Fig. 6.4), is one of the most controversial expressions in Surface Science and there is 

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The factor ip is determined by the so-called Volmer equation (24) (XIV), which involves the contact angle, between the crystalline nucleus (c) and the impurity (s). The latter is controlled by the interfacial energy, o , of the different phases (HQuid, I, crystalline nucleus, c, and solid impurity, s) (XV) and corresponds to the angle of wetting in solid-liquid systems. 

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