Adhesion

Nico A.M. Schellart, Dept. of Med. Physics, AMC

Principle

Molecules in liquid state experience strong intermolecular attractive forces. When those forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces. This leads to an upward turning meniscus at the walls of the vessel (see Capillary action).

The attractive forces between molecules in a liquid can be seen as residual electrostatic forces and are called van der Waals forces or van der Waals bonds (see Cohesion).

More generally, and from a macroscopically point of view, adhesion is the molecular attraction exerted between bodies in contact.

Notice that in medicine, an adhesion has a completely other meaning. It is a fibrous band of scar tissue that binds together normally separate anatomical structures. It usually occurs as a result of surgery, infection, trauma or radiation.

Application

Adhesion is of particular interest to (medical) biologists to understand the workings cells and to engineers who wish to stick objects together.

More Info

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 of fluorine atoms of the two materials shares a hydrogen nucleus (hydrogen bonding).

Dispersive adhesion

This is also known as adsorption. Two materials may be held together by van der Waals 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 do not. Wetting depends on the surface energy (the disruption of chemical bonds that occurs when a surface is created) of the materials.