Entry by Meredith Duffy, AP225, Fall 2011
crystallization of a supersaturated solute, when it begins to precipitate out of solution by forming small regions of solid phase on the surface containing the liquid phase. These small regions of new phase are called nucleation sites, which are typically heterogeneous, i.e. composed of an impurity such as dust or a grain boundary, but can be homogeneous, i.e. made of the same material as that which is precipitating out of solution. Homogeneous nucleation requires supercooling or superheating to overcome the energy barrier to forming nuclei, whereas heterogeneous nucleation is aided by the difference in surface energy due to the impurity or defect. Supersaturation also provides a driving force in many nucleation examples.Because heterogeneous nucleation involves impurities or defects, it must of course depend upon the properties of these defects. Nonzero contact angles (i.e. not perfectly wettable surfaces) between phases encourage nucleation.
It is worth noting that nucleation rate is distinct from cystal growth rate. Nucleation rate refers to the rate of formation of nuclei of critical radius, i.e. radius large enough to overcome the free energy barrier to stable growth, and is a function of this energy barrier, temperature, the number density of potential nucleation sites, and molecular diffusion rate.
The schematic to the right shows the differences in the free energy barrier that must be overcome before stable nucleation occurs for homogeneous vs. heterogeneous nucleation sites. R* refers to the critical nucleus radius. Below this point, nuclei will randomly form and dissolve, but once this radius is reached, future growth of the nucleus will lead to a decrease in free energy, so spontanteous crystal growth occurs.