Nucleation of crystals and nano-particles from solution plays an important role in several areas of science and technology like manufacture of photonic band gap materials, formation of protein crystals for structure determination, and industrial production of pharmaceutical compounds etc. At present, limited understanding exists towards the underlying processes of particle formation from solution. In this work, we propose and offer evidence to the existence of a limit on the metastable zone width (MZW) and highlight the importance of rate of generation of supersaturation in nucleation.

While thermodynamics requires the phase transition to occur as soon as one crosses the equilibrium boundary (solubility), well known observations indicate that nucleation occurs at a relatively high supersaturation. Crystallizing solutions can sustain significant metastability. Traditionally, these metastable zones are measured through rapid generation of supersaturation, and the rate of generation of driving force influences the observed limit of metastability a system exhibits. This effect is often attributed to the so called unclear ‘kinetic effects'. The key questions then become, are there any limits to the metastability that a system can endure and if so what are the factors that determine such limits? What will be the influence of the‘kinetic effects' on nucleation when one generates the supersaturation at an infinitesimally slow rate such that the system is always at (pseudo)equilibrium?

In this work, we address the above questions by studying the nucleation behavior of various compounds under extremely slow rates of supersaturation. We achieve such slow rates of supersaturation by crystallizing the solutions in an evaporation based micro-device [1]. We present experimental evidence to the existence of a lower boundary on the MZW and explain this observation in terms of our current understanding of kinetic / thermodynamic processes underlying nucleation. This boundary on metastability can be considered as a ‘kinetic limit' of nucleation. We explain the origins of this ‘kinetic limit' from stationary cluster size distribution and suggest methods to predict nucleation behavior of compounds.

[1] Guangwen He, Venkateswarlu Bhamidi, Reginald B. H. Tan, Paul J. A. Kenis and Charles F. Zukoski, Cryst. Growth & Design, 6 (2006), 1175