Because atoms have different energy levels and tend to bond to each other to neutralize electrical charges, nucleation theory suggests that at in any given moment in any medium atomic aggregates are created. Two characteristics directly influence the aggregation process: density (the population of atoms in solution) and mobility of the atoms (defined by characteristics of the medium). Condensed mediums with high atomic density and mobility facilitate the formation of aggregates, increasing the chances atoms will encounter. The atomic mobility is associated with the temperature of the medium, among other things. In a cool liquid both conditions are met: the density of liquid increases (phenomenon inherent to cooling) while the atomic mobility is maintained (considering the nature of the liquid medium). These aggregated molecules or ions are essentially unstable until they reach a certain critical volume.
Aggregates whose radii do not pass the critical radius (r) will dissolve, reincorporating into the liquid. If the aggregate volume radius does pass this value, they tend to grow and eventually form a new phase. This nucleation can occur on a solid’s surface or within its bulk forming new solid phases.
After the aggregates stabilize, they grow as long as there are atoms to attach to the surface. The kinetics of growth depend on restrictions imparted by the atoms that surround the aggregate – the tensions the will be created when this new arrangement of atoms increases the volume by incorporating neighbors. The temperature of the body greatly influences the speed of growth of the new phase.
Everything mentioned up until now is a short summary of Nucleation and Growth Phases. For a deeper understanding more information can be found in books on material science and online (www.cienciadosmateriais.org, for example). The names of Willard Gibbs(1839-1903), Max Volmer (1885-1965) and A. Weber(?), co-author of Max Volmer are recognized as the pioneers in Nucleation theory.
Figure 1.1: Max Volmer (left) and Willard Gibbs (right).
