How do adhesives hold two materials together?

Adhesives hold materials together by first wetting and spreading across the surfaces to achieve intimate molecular contact. Once this contact is established, several mechanisms generate intrinsic adhesive forces across the interface.1

The primary mechanism is adsorption, where intermolecular or valence forces between the surface layers of the adhesive and the adherend create a bond. Other proposed mechanisms include:1

• Mechanical interlocking: The adhesive flows into surface pores or around projections on the material.1
• Interdiffusion: The liquid adhesive dissolves and diffuses into the adherend.1
• Adsorption and surface reaction: Adhesive molecules chemically react with the solid surface.1
• Electronic attraction: Electrostatic forces develop between materials with differing electronic band structures.1

These interactions occur within a transitional zone known as the interphase, which controls the strength and durability of the joint. Most synthetic adhesives rely on polymers to provide the necessary strength, flexibility, and surface interaction. Ultimately, these bonds rely on short-range Coulomb forces, such as van der Waals forces, which require work to overcome when separating the joined materials.123