What causes crystals to “grow”?
The driving force for crystallization comes from the lowering of the potential energy of the atoms or molecules when they form bonds to each other. The crystal growth process starts with the nucleation stage. Several atoms or molecules in a supersaturated vapor or liquid start forming clusters; the bulk free energy of the cluster is less than that of the vapor or liquid. The total free energy of the cluster is increased by the surface energy (surface tension), however, this is significant only when the cluster is small. A cluster of radius smaller than a critical radius, r*will evaporate (or dissolve in the solution) a cluster of radius greater than r* will become stable, will increase its size by the addition of other atoms and is thus “growing”! The critical radius r* also defines a critical energy barrier, DG, that we need to overcome in order to obtain a stable nucleus that will keep growing, eventually become a large single crystal! Thermodynamics can help us describe the process.
