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Classical density functional theory applied to inhomogeneous solids: A study of clusters, interfaces and non-classical nucleation pathways

par Schoonen, Cédric
Président du jury Losada Perez, Patricia
Promoteur Gaspard, Pierre
Co-Promoteur Lutsko, James
Publication Non publié, 2024-12-05

Thèse de doctorat

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Résumé :

Nucleation is the initial event triggering a first-order phase transition, where a cluster of the new phase appears out of the original phase. It is a poorly understood phenomenon despite playing a key role in many scientific and industrial contexts. The classical theory of nucleation (CNT) has been challenged by discoveries of complex crystallization mechanisms that invalidate its simplifying assumptions. The one-dimensional cluster growth dynamics in CNT, where the number of particles is used as the single coordinate, contrasts with the multistep crystallization mechanisms, which involve several order parameters (e.g. mass and crystallinity). The macroscopic representation of the cluster is also a major limitation as it does not capture its microscopic details. However, recent advances in classical density functional theory (cDFT) opened the door to solid phase calculations, allowing one to determine the structure of solids down to the molecular scale. In this work, we use these new methods of cDFT to investigate the properties of inhomogeneous solid structures relevant to the study of crystallization. We perform calculations of crystal-fluid surface free energies, which are important parameters of the classical models, and direct cDFT calculations of solid clusters to characterize their structure and free energies. We then use these clusters as input in calculations of complete crystallization pathways, starting from a dilute phase, using the tools of mesoscopic nucleation theory (MeNT). MeNT is a powerful framework combining the microscopic level of detail offered by cDFT and a realistic model of the nucleation dynamics derived from fluctuating hydrodynamics.The pathways we compute depict a crystallization mechanism in multiple steps, starting with the accumulation of mass in a dense liquid droplet and then the appearance of solid-like order in the droplet interior. One such crystallization pathway featured a metastable amorphous solid intermediate, adding to the complexity of the nucleation mechanism. Despite exhibiting non-classical structural features, we show that the nucleation dynamics is rather classical. A careful analysis of the dynamics reveals that the ordering step takes a minor role compared to the accumulation of mass and that the number of particles can be used as the only order parameter to describe the nucleation dynamics.