Adhesion

Adhesion is the molecular attraction exerted between bodies in contact. It is of particular interest to engineers who wish to stick things together and to biologists to understand the workings of cells.

Mechanisms of Adhesion

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

Mechanical Adhesion

Two materials may be mechanically interlocked. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.

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

Also known as Adsorption. Two materials may be held together by van der Waals forces. A van der Waals force is the attraction between two molecules that have positively and negatively charged ends. This positive and negative polarity may be a permanent property of a molecule (Keesom forces ) or universally occurs in molecules as the random movement of electrons within the molecules may result in a temporary concentration of electrons at one end (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. The electrons are passed if one conducting material binds its electrons less strongly than the other.

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. The driving force for this diffusion is typically the reduction in surface energy, though it could also be a reduction in the chemical potential.

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.

Railway usage

The term adhesion railway or adhesion traction describes the most common type of railway, where power is applied by driving certain of the wheels of the train and thus it relies on the adhesion between a steel wheel and a steel rail. The term is used when discussing mountain railways to distinguish from other forms of traction such as funicular or cog railway (rack-and-pinion). For example, the Bernese Oberland Railway "is a mixed rack and adhesion railway" [1].

On an adhesion railway most locomotives have a box containing sand which can be sprayed on to the rail to improve traction under slippery conditions.

See also

• Cell adhesion
• Cohesion

References

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