How does glue work? How does it bond two things?

One might guess that the glue we use to mend a broken teacup works by chemical bonding or mechanical interlocking. The effects do play a part, but the essential reason adhesives work is this: If two materials are close enough together, they will adhere. The adhesion is due to a universal attraction between molecules in very close proximity. The forces of this attraction, called van der Waals forces, named after the Dutch physicist who postulated them, stem from the arrangement of electrons around an atom’s nucleus. Although electrons’ orbit is symmetrical paths, at any given instance their electrical charges are not evenly balanced. An atom has a positive and a negative electrical pole.

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.

More reading:
Adhesive (Wikipedia)
Van der Waals force (Wikipedia)

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