Adhesion - Surface Energy

Surface Energy

Surface energy is conventionally defined as the work that is required to build a unit area of a particular surface. Another way to view the surface energy is to relate it to the work required to cleave a bulk sample, creating two surfaces. If the new surfaces are identical, the surface energy γ of each surface is equal to half the work of cleavage, W: γ = (1/2)W₁₁.

If the surfaces are unequal, the Young-Dupré equation applies: W₁₂ = γ₁ + γ₂ – γ₁₂, where γ₁ and γ₂ are the surface energies of the two new surfaces, and γ₁₂ is the interfacial tension.

This methodology can also be used to discuss cleavage that happens in another medium: γ₁₂ = (1/2)W₁₂₁ = (1/2)W₂₁₂. These two energy quantities refer to the energy that is needed to cleave one species into two pieces while it is contained in a medium of the other species. Likewise for a three species system: γ₁₃ + γ₂₃ – γ₁₂ = W₁₂ + W₃₃ – W₁₃ – W₂₃ = W₁₃₂, where W₁₃₂ is the energy of cleaving species 1 from species 2 in a medium of species 3.

A basic understanding of the terminology of cleavage energy, surface energy, and surface tension is very helpful for understanding the physical state and the events that happen at a given surface, but as discussed below, the theory of these variables also yields some interesting effects that concern the practicality of adhesive surfaces in relation to their surroundings.