From Molecules, through Mechanisms, to Mechanics

We normally observe adhesion at the macroscopic level, say by peeling a film from a surface as in Fig. 3.9. In order to understand the force which is necessary to pull the fihn off, we have to connect this mechanical picture with the molecular adhesion forces which we know to be universal at the nanometer level. 

The macroscopic mechanism looks simple, because we grip the film and pull it off by applying a force (Fig. 3.9(a)). However, it is obvious that the applied force is not acting directly at the contact. The film has to bend and apply leverage to the adhesive region. So there is a complex mechanical system operating. To understand this we need mechanics. In particular, we apply the continuum mechanics theory in Chapter 14. 

If one looks closer, using a microscope, at the point where the polymer film is detaching from the oxide surface, at the micrometer level (Fig. 3.9(b)), then it is evident that there is a crack traveling along the interface between the polymer and the glass. This crack is the mechanism by which the polymer material detaches from the surface. All brittle adhesive joints fail by cracking. This is a mechanism which involves elastic deformations and creation of new surfaces. It can be analyzed by the energy balance theory described in Cluster 7. 

This overview explains why there have been enormous problems of communication between adhesion scientists in different applications. Engineers 

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