Separation process, adhesion analysis

The studies on adhesion are mostly concerned on predictions and measurements of adhesion forces, but this section is written from a different standpoint. The author intends to present a dynamic analysis of adhesion which has been recently published [7], with the emphasis on the mechanism of energy dissipation. When two solids are brought into contact, or inversely separated apart by applied forces, the process will never go smoothly enough—the surfaces will always jump into and out of contact, no matter how slowly the forces are applied. We will show later that this is originated from the inherent mechanical instability of the system in which two solid bodies of certain stiffness interact through a distance dependent on potential energy. 

Surface analysis is a complex matter. The complexity starts by the definition of the term surface itself. Not everyone understands the same by surface. We often hear and speak about the surface of Earth or Mars, about the inner surfaces of a zeolite, about the existence of surface processes such as corrosion or adhesion, and so on. Depending on the field or the particular application, we talk about different things that have, however, a common characteristic. In all the cases, the surface is a border region that separates the solid (the bulk) from the environment (liquid or gaseous). It is the thickness of this border region, and what happens within this thickness, that determines the characteristics of many processes that are relevant in both basic science and technology. This thickness may extend from a few atomic layers (and thus be the subject of surface science ) to a few nanometers (surface analysis) or afraction of a micrometer (thin film analysis). Thus, in this chapter the term surface represents the external part of a solid having a thickness from a fraction of a monolayer to a few nanometers. 

Generalized fracturie mechanics (GFM) addresses this problem by expressing the adhesive failure parameters in terms of the interfacial energy and the mechanical hysteresis properties of the bulk phases. This then permits the surface and bulk contributions to adhesive failure energy to be separated, allowing a more profound analysis of the fracture process and the way it is controlled by rate, temperature, and other environmental factors. This will be illustrated by a study of the effects of moisture exposure on adhesive joints. 

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