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Siegel der Universität
Department für Chemie - Arbeitsgruppe Prof. Strey

Crossing over from the Nucleation to the Spinodal Regime

Guram Chkonia, Februar 2008

The gas-liquid phase transition is examined by means of Molecular Dynamics
simulations of Lennard-Jones argon at T = 80.724 K. The different mechanisms initiating the phase transition, such as nucleation and spinodal decomposition are investigated in the range 8.5 ? S ? 35. Nucleation rates are obtained from mean first-passage times (MFPT) and the Yasuoka-Matsumoto method. The nucleation rates range from 1023 J/cm?3s?1 ? 1028. The results are compared to theoretical predictions from Classical Nucleation Theory (CNT. The difference between the methods of analysis is smaller than a factor of two. Both results show agreement to the CNT within one order of magnitude, which is a significantly smaller deviation than one observes at lower temperatures. The critical cluster size is obtained via MFPT, Nucleation Theorem (NT), Gibbs-Thomson equation (GT) and the barrier reconstruction. The implementation of the ten Wolde-Frenkel cluster definition improves the critical cluster sizes, obtained from the MFPT-method, significantly towards earlier works, and shows good agreement with critical sizes from barrier reconstruction and NT. Even in the vicinity of the spinodal, critical cluster sizes of 8 to 9 atoms can be found. The nucleation barrier is reconstructed by a novel method from the MFPT and the steady-state cluster probability distribution. The method suggests a vanishing work of formation at the spinodal at S ? 14. From the MFPT, a gradual coupling between nucleation and growth with increasing supersaturation is observed until no steady-state nucleation step is distinguishable. New definitions for nucleation rates are suggested for these cases.