Nucleation theory beyond capillarity approximation
Jeudi 11 octobre 2012
à 14h en salle C. Brot
The phenomenon of nucleation is associated with the non-equilibrium first-order phase transitions transforming a metastable parent phase to a thermodynamically stable daughter phase. The transformation proceeds through the creation of small clusters of molecules (nuclei) of the daughter phase out of the parent phase by thermal fluctuations. Recent advances in nucleation experiments made it possible to reach nucleation rates as high as 1010-1018 cm-3s-1. Such rates usually correspond to extremely small critical nuclei – containing only about 10 to 50 molecules. These nano-sized fractal-like objects can not be adequately treated within the purely phenomenological models (Classical Nucleation Theory and its modifications) based on the so called capillarity approximation. The latter assumes that the surface energy of a cluster can be described in terms of the plain layer surface tension. Obviously, for small clusters the concept of macroscopic surface tension looses its meaning and this assumption fails. A challenging task for a theoretician is to propose a nucleation model which treats clusters of all sizes on the same footing. In my talk I will describe a realization of this program – the Mean-field kinetic Nucleation Theory (MKNT). This model treats small clusters using statistical mechanical considerations and provides a smooth interpolation to the limit of big clusters obeying the capillarity approximation. Comparison of MKNT with experiment for various microscopically diverse substances will be presented. Among various important features, the model leads to the Generalized Kelvin Equation signaling the pseudo-spinodal (as opposed to its classical analogue). If time permits I will briefly discuss the extension of these ideas to binary nucleation where the treatment of small clusters has to be complemented with the treatment of adsorption effects.
Fluides & Matériaux Complexes
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