TABLE OF CONTENTS Adhesion is an interfacial phenomenon that occurs at the interfaces of adherends and adhesives. This is the fact underlying the macroscopic process of joining parts using adhesives. An understanding of the forces that develop at the interfaces is helpful in the selection of the right adhesive, proper surface treatment of adherends, and effective and economical processes to form bonds. This chapter is devoted to the discussion of the thermodynamic principles and the work of adhesion that quantitatively characterize the surfaces of materials. Laboratory techniques for surface characterization have been described which allow an understanding of the chemical and physical properties of material surfaces.
Two solid or liquid phases in contact have atoms/molecules on both sides of an imaginary plane called the interface. The interfacial particles differ energetically from those in the bulk of each phase as they are present on the boundary of the respective phase and interact with the particles of the other phase. The composition and energy vary continuously from one phase to the other through the interface. This region has a finite thickness, usually <0.1 ?m. [1 ,2 ]
The molecules of a liquid are held together by attraction forces. The sum of all attractive forces on any molecule present in the bulk of a liquid averages zero. The net force (also known as cohesion force) on a surface molecule is a non-zero quantity in the direction towards the bulk (Figure 2.1).
