Atoms in the center of a material are surrounded by other atoms, while atoms located on the surface of a material are only interacting with the atoms closer toward the center. In other words, some the surface electrons could interact with atoms surrounding the material. Depending on the "willingness" of these electrons to interact with the surrounding, we use the term "surface energy" to quantify the ability of the surface to react. In other words, high surface energy means that the surface is reactive.
According to thermodynamics all systems try to reach their lowest energy levels. The same rule is also valid for any material surface. Thus, a surface with high surface energy reacts with atoms (molecules) from the surrounding in an attempt to form a surface with lower energy level. This phenomena explains why metal surfaces become oxidized over time, because the metal oxide has a lower surface energy than the clean metal surface.
What happens if a surface has lower energy than the surrounding molecules? Will we then decrease the surface energy if we coat the surface with the molecules with the higher surface energy? Of course not. In this case we say that the surface has low wettability. A good example on low wettability is a metal surface coated with a film of wax. If we place water on such a surface, the water drop forms a bead that does not spread out over the surface. By looking at the bead from the side we can see that the angle between the tangent of the water bead where it contacts the metal surface and the metal surface under the bead is large. In this case we talk about a large contact angle. If we remove the wax film and place a drop of water on the surface, the drop spreads out over the metal surface and now it forms a small contact angle between the tangent and the metal surface under the water coated surface. In this case the wetting is good. The reason is simply that the clean metal surface has a higher surface energy than the water, and therefore water spreads easily in the surface.
The above concept is very important to understand from a dental point of view. When we try to bond a resin to a tooth surface, the tooth surface has often a rather low surface energy. The low surface energy has been achieved over time as a result of nature's strive to decrease energy. If we place a resin on such a surface, the resin may not spread if the surface energy of the resin is higher than the tooth surface. To avoid problems with spreading we etch the tooth surface. By doing so we remove the passivating layer and make the surface much more active. At the same time it also becomes rough, something that will enhance micro-mechanical retention. After the surface has been etched, rinsed and dried, its surface energy is higher and the resin will spread easily in an attempt to coat the surface and decrease the surface energy.
What can go wrong during the bonding procedure? A major danger is that the surface is contaminated with saliva before the resin is placed. If saliva reaches the newly etched tooth surface, the saliva will coat the surface and decrease the surface energy. This will in turn mean that when the resin is placed, the surface energy of the resin might be higher than the surface energy of the saliva coated tooth surface and the resin will not spread. Under such conditions the saliva coated surface needs to be reetched to remove the saliva film before the resin is placed.
From the above we can draw an important conclusion. Every time we want to use an adhesive it is important that the adhesive wets the surface we want to bond to as efficiently as possible. Intimate wetting will optimize the chance for chemical interaction (chemical bond formation) and mechanical interlocking. To achieve such a condition, the surface energy of the adhesive should be low and the surface energy of the adherent should be high. The latter require that the bonding procedure is conducted in as clean environment as possible.
