Adhesion

Adhesion is the tendency of certain dissimilar molecules to cling together due to attractive forces.

Mechanisms of adhesion

Five mechanisms 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 oxygen, nitrogen or fluorine atoms of the two materials share a hydrogen nucleus (hydrogen bonding).

Dispersive adhesion

In dispersive adhesion, also known as adsorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a regions of 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).

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, stickers, and rubbing dirt on golf balls.

See also

* Adhesion (medicine)
* Cell adhesion
* Cohesion
* Rail adhesion
* Scratch tester Method of testing the adhesion of coatings to substrates

References

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