Van der Waals forces are attraction between opposite poles on different atoms. Individually they are fairly weak, but they combine to bonding strength when acting among countless atoms. Why, then, do we need adhesives at all? If we press two solid objects together closely enough, won’t van der Waals forces hold the objects together?
No. They generally won’t. The reason is that the molecules of the two objects’ surfaces must be within a few angstroms of each other – and an angstrom is only 10 billionth of a meter. A polished material, for example, has greater angstroms. This makes the actual areas of molecule contact minimal, even if the surfaces are similar. An adhesive, by making contact with molecules of both surfaces, holds them together. For the most intimate and extensive contact, the ideal adhesive is a liquid. It should also solidify into a strong material that won’t shear off easily.