Surface tension

Consider a substance in any shape or form. The molecules of which it is made are continually acted on by neighbouring molecules. Those in the centre of this substance are in a state of dynamic equilibrium as they are completely surrounded by similar molecules. Those on the surface, however, are subjected to attractions on one side only, so they have a tendency to be pulled into the inner bulk of the substance.

The force which attempts to pull the surface molecules into the centre is termed the surface tension (symbol ), which is defined as the force tending to reduce the surface area of a liquid. An alternative definition of ? is the free energy per unit surface area.

The adsorption of one substance onto another is accompanied by a change in energy. This is reflected in several measurable parameters, for example heat liberation, contact angle, etc. Such phenomena have been analysed to form the basis of a theoretical understanding of the magnitude of sorption forces generated between solid (adherend) and liquid (adhesive) at a macro level.

When a liquid contacts a solid surface it will flow until the surface energies of the liquid and solid surface are balanced. This is because, in general, the surface energy of a solid is much higher than a liquid. Consequently, the lower-energy liquid molecules are pulled across the solid surface, reducing the surface free energy of the solid.

The surface tension of the liquid resists this flow process; hence the eventual equilibrium. The flow process is called wetting and is not the same as spreading, which requires the application of an external force to cause the liquid to flow.

In moving from state A to state B or C the surface free energy of the solid, s, is reduced by s - sl (interfacial tension) over the surface where the liquid contacts the solid. Since the liquid surface tension resists flow, then, after resolving the forces into vertical and horizontal components, the equilibrium can be described by:

To remove the droplet from the solid, some work is required and the energy needed to do this is equivalent to the work of adhesion, WA.Using the same method as above; on removal s and lv are gained and sl is lost, therefore:

This equation is not very useful because s and sl are practically impossible to measure, but by combining equation (1) with (2) then WA becomes directly proportional to the surface tension of the liquid involved:

If the liquid wets the surface perfectly, ie = 0, then cos is 1 and so WA is twice lv, which is an easily measured characteristic.

Unfortunately, liquids that perfectly wet a wood surface would not make good adhesives as they would soak into the wood, leaving a 'starved joint'.What is needed is a liquid that makes a contact angle of less than 90° (state C) so that it is easy to spread and penetrate.

Some penetration is needed to create some mechanical plugging of the adhesive in the cellular structure of the wood. It also helps to ensure that a bond is not dependent on any weakly-connected surface fibres.