ABSTRACT
The term aggregation is derived from the Latin word aggregare, which means ‘‘forming a herd,’’ gathering cattle into a group. By extrapolation, it is applied to the gathering, active or passive, of many similar objects. In physics and chemistry, it has the meaning of the formation of larger structures from smaller units, and it is mostly restricted to physical (reversible) bonding of somewhat larger elements (cf. colloidal particles). Polymerization is usually not considered an aggregation process because it involves chemical binding, and the same applies to vulcanization, perhaps also because there is hardly any motion involved. Coagulation, such as can occur with emulsion droplets, is no aggregation, because in the process, the identity of the primary particle is irreversible lost. Aggregation is also used for micelle formation by proteins or lipids and for block copolymers forming supramolecular structures. Finally, the state of aggregation of matter in physics is synonymous with phase: solid, fluid, gas, or plasma. That does not imply that all of the processes involved in phase transitions and phase separation can be considered as aggregation or its inverse. Actually, the term aggregation is not used for any of the fundamental transitions [e.g., solid-liquid (melting-freezing), or liquid-gas (condensation-evaporation)]. However, many of the fundamental processes that establish the dynamical phase transition are aggregation processes, for example, the nucleation process in a first-order phase transition from gas to liquid or solid.
