Everything about Adhesion totally explained
Adhesion is the tendency of certain dissimiliar 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).
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.
Further Information
Get more info on 'Adhesion'.
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