Adhesion, << ad HEE zhuhn, >> is the property of two unlike substances that causes them to stick together. Adhesion occurs because of the attraction between all molecules and atoms.
Adhesive strength varies, depending on the characteristics of the substances coming together. The adhesion between the surfaces of two solid substances tends to be low, even if they seem perfectly flat and clean. The surfaces are actually rough when viewed through a microscope, and they touch each other at relatively few places. But if one of the substances is part or all liquid, contact between the surfaces is much greater.
The strongest adhesives are applied as thin layers of liquids. In many cases, these liquids are solutions of polymers, large molecules formed by the chemical linking of many smaller molecules into a long chain (see Polymer ). As these solutions set, their molecules become immovable. The adhesion can be so good that a strong force will cause the substance, but not the adhesive bond, to break.
See also Cohesion ; Gennes, Pierre-Gilles de .
