Adhesion is the attraction between two dissimilar phases. Current theory divides adhesion into the three categories: fundamental adhesion, practical adhesion and thermodynamic adhesion.
- Fundamental adhesion is the sum of all interfacial intermolecular interactions between the two phases. It corresponds to the energy required to break chemical bonds at the weakest plane of the two-phase system. The fundamental adhesion cannot be directly measured since there are always other factors affecting in a measurement system.
- Practical adhesion is a function of fundamental adhesion and other factors, including internal stresses and the error caused by the measurement method. Practical adhesion can be measured by using variety of different techniques, such as Scotch tape test or stylus method.
- Thermodynamic adhesion is defined as the reversible work done in creating a unit area of the interface between two phases (see figure below). Typically, the unit is mN/m.
WAB = γA + γB – γAB
where WAB is the thermodynamic work of adhesion, γA the surface tension of substance A, γB the surface tension of substance B and γAB the interfacial free energy.
For two solid phases, the work of adhesion is not useful since there are various unknown parameters in the equation. If, however, one of the phases is liquid and the other one solid, the work of adhesion can be defined with Young-Dupré equation:
WAB = γB (1 + cos θ)
where γB is the surface tension of the liquid and θ is the contact angle between the liquid and solid. Now, the work of adhesion can be calculated by measuring surface tension of the liquid and contact angle with Attension tensiometers.