ABSTRACT
Nucleation is an important physical phenomenon observed in the kinetics of phase transformation. It lies at the border of macroscopic and microscopic processes and provides a guiding principle in the synthesis of many materials, with particular relevance in nanoscience. All first order phase transitions, such as gas-liquid (below the critical temperature) and liquid-solid, must proceed via nucleation because a macroscopic free energy barrier separates the bulk parent (or, the old, metastable) phase from the bulk daughter (or, the new stable) phase. Thus, the old phase cannot transform itself in bulk into the new phase by a large scale fluctuation. Instead, the new phase has to be nucleated within the old phase. Even then, there exists a barrier, called the nucleation barrier, that is to be surmounted by a spontaneous formation of an embryo of the new phase as a local fluctuation within the old phase. Such fluctuations become common when the old phase is sufficiently metastable with respect to the new phase. The nucleation free energy barrier is determined by the free energy gap between the parent metastable phase and the stable new phase, and the surface tension that resists the formation of the embryo. The initial theory of nucleation was developed by Becker, Doring and Zeldovich (BDZ). This theory applies to homogeneous nucleation and has been widely used, although its quantitative success in the prediction of nucleation rates seems to have remained limited over the years. In the case of heterogeneous nucleation where nucleation occurs in the presence of a surface or a seed of the new phase, the nucleation free energy barrier can be much smaller and the rate of nucleation much larger. In this chapter, we discuss the BDZ theory of homogeneous nucleation and also modifications necessary for heterogeneous nucleation. We discuss experimental methods and computer simulations that guided recent advances in nucleation theory. The ideas and theoretical expressions developed in the area of nucleation finds wide use in many branches of natural and biological sciences. We also discuss Ostwald’s step rule in the case where multiple phases can form from a given solution or melt.
