Adhesion is the phenomenon wherein some molecules stick together due to attractive forces. Adhesion can be thought of as a surface force when layers of different types of molecules cling together due to adhesive forces.
- Adsorption: Also known as dispersive adhesion, adsorption occurs when materials are held together by Van der Waals forces, or the attraction between molecules with separate regions of positive and negative charge.
- Chemical: Atoms or molecules may form a chemical bond such as a covalent or ionic bond. The exchange or sharing of electrons produces adhesion between the constituents of the molecule.
- Diffusive: If two materials can exchange molecules by diffusion, they may become bound to each other. Polymers are one type of molecule that can exhibit diffusive adhesion.
- Electrostatic: In certain circumstances, a conducting material may pass electric charge to a material that cannot freely return the charge to the conductor. This creates an electrical attraction between the materials, similar to that found between the plates of a charged capacitor.
- Mechanical: Molecules in separate surfaces may entangle or fill gaps in the opposite surface that exist due to the surfaces' roughness. This entanglement can cause the surfaces to stick together.
Theoretical and experimental efforts to analyze adhesion forces continue. Ghatak et al. recently examined a method of measuring adhesion strength using a peel test and accounting for the adhesive film thickness and the flexibility of the surface in the analysis. This group proposed new dimensionless parameters that could be used to characterize adhesive behavior across materials. Their work was in agreement with the results of other research groups that had examined the "work of adhesion" parameter.