Thermodynamic or Ideal Adhesion

FIGURE 19.1. In its simplest form adhesion can be defined as the phenomenon that allows a mechanical force (in excess of any frictional forces) to be applied to two contacting objects without those two objects becoming separated. 

The following discussion will attempt to put into focus the interrelationship between the various definitions as well as illustrate some of the more important aspects of the subject. The subject is quite broad, with an extensive Uterature, so the coverage will be limited in scope. 

We have already encountered the concept of thermodynamic adhesion and its related terms such as the work of adhesion. The term is applied to a defined model system and does not take into consideration conditions before or after the formation of the interface, the presence of random flaws or defects in the system, or the bulk physical properties of the components, all of which are of primary importance in the practical application of the concept of adhesion. It is related to molecular interactions such as van der Waals, dipolar, and electrostatic forces but does not consider mechanical or chemical interactions as defined above. It is therefore not a very useful concept in terms of practical adhesion problems, but it serves as a good theoretical tool and to indicate a maximum force or work that a given interface may be expected to transmit before failure (i.e., separation) occurs. 

Because, in theory at least, file concept of ideal or thermodynamic adhesion applies equally well to liquid and solid phases, it is of interest to see how a calculation of such an ideal value compares with reality. The complete expression for the work of adhesion between two phases with each phase completely saturated by the other, denoted by A(B) and B(A) is 

TABLE 19.1. Ideal and Real Cohesive (Tensile) Strengths of Some Common Materials (ro = 0.4 nm) 

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