Capillary flow and spreading processes

Before continuing with the specific topic of capillary flow, it may be useful to divert our attention to the broader topic of the spreading of a liquid on a surface. The spontaneous spreading of liquids is a topic of great practical interest because of its importance in many applications. For that reason, such phenomena will be discussed in more detail in later chapters. However, it will be useful to introduce the basic concepts at this point in order to clarify some concepts related to the current topic. 

FIGURE 6.6. The wetting of a solid by a liquid in the presence of a second fluid phase will be controlled by the relative interfacial interactions among the three phases. 

If a quantity of liquid is placed on a surface, which may be a solid or another liquid, one of two things may occur (1) the hquid may spread across the surface to form a uniform duplex film or (2) the liquid may form a drop (on a solid) or lens (on a liquid) with a finite, nonzero contact angle (Fig. 6.7). Thermodynamically, at constant temperature and pressure the change in the free energy of the system is given by 

The term (SG/SAb) gives the free energy change for the spreading of liquid B over the surface A and is called the spreading coefficient of B on A, Sbia, given by 

From the definitions of the works of cohesion and adhesion given in Chapter 2, it can be seen that 5b/a is the difference between the work of adhesion of B to A and the work of cohesion of B  

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