Adhesive and Cohesive Forces

Both adhesive and cohesive forces act upon the adhesive and are determined by intermolecular interactions. The adhesive forces are caused by the interaction of molecules from the adhesive with molecules from the substrate surface. These forces describe the ability of the adhesive to adhere (stick) to the surface. The cohesion forces, on the other hand, are caused by the chemical bonds and intermolecular attraction between the adhesive molecules. These attractions are what hold the adhesive material itself together. An adhesive material must have both strong adhesive and cohesive forces. There is no point in the adhesive being very strongly bonded to the substrate if the material itself cannot handle the loads placed upon it.

Adhesive and cohesive forces.

Figure 1: Adhesive and cohesive forces.

A phenomenon which is related to cohesive forces the surface tension. The surface tension of a liquid is due to the cohesive forces within the bulk of the material and is defined as the energy required to increase the surface area of the liquid adhesive. Within the bulk of the liquid, the molecules will be evenly distributed. This means that molecules in the bulk of the liquid are surrounded by molecules at an equal distance and will experience equal intermolecular forces from all directions. The molecules on the surface, however, will be attracted towards the molecules within the bulk of the adhesive. Stronger cohesive bonds tend to minimise the surface area, hence increasing the surface tension. Strong adhesive forces and surface tension is what causes droplets of liquids to assume a spherical shape. Different liquids will have different surface tensions.

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