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:
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.
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
atoms of the two materials share a hydrogen
nucleus (hydrogen bonding
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
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.
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
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.
- John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997
- A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987