Adhesion

Adhesion is the tendency of certain dissimilar molecules to cling together due to attractive forces. In contrast, cohesion takes place between similar molecules.

Mechanisms of adhesion

Five mechanisms of adhesion have been proposed to explain why one material sticks to another:

Mechanical adhesion

Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale. This is similar to surface tension.

Chemical adhesion

Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons. A weaker bond is formed if a Hydrogen atom in one molecule is attracted to an atom of Nitrogen, Oxygen, or Fluorine in another molecule, a phenomenon called Hydrogen bonding.

Dispersive adhesion

In dispersive adhesion, also known as physisorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a regions of slight positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule (Keesom forces) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region (London forces).

In surface science, the term "adhesion" almost always refers to dispersive adhesion. In a typical solid-liquid-gas system (such as a drop of liquid on a solid surrounded by air) the contact angle is used to quantify adhesiveness. In the cases where the contact angle is low, more adhesion is present. This is due to a larger surface area between the liquid and solid and results in higher surface energy. The Work of Adhesion explains the interactive force between the liquid and solid phases and the Young-Dupree equation is used to calculate the Work of Adhesion. The contact angle of the three-phase system is a function not only of dispersive adhesion (interaction between the molecules in the liquid and the molecules in the solid) but also cohesion (interaction between the liquid molecules themselves). Strong adhesion and weak cohesion results in a high degree of wetting, a lyophilic condition with low measured contact angles. Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting.

Electrostatic adhesion

Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials.

Diffusive adhesion

Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.

What makes an adhesive bond strong?

The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that don't. Wetting depends on the surface energy of the materials. Well-known examples of adhesion are tape, glue, and stickers.

References

John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997
A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987

External links

Definition of dispersive adhesion Glossary definition of related surface science terms including dispersive adhesion, Work of Adhesion, wettability, cohesion, and surface energy, with diagrams.