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A molecular dynamics investigation of ice nucleation induced by electric fields Yan, Jingyi


This thesis aims to understand the influence of electric fields on ice nucleation. Molecular dynamics simulations are employed to investigate heterogeneous ice nucleation induced by electric fields, and why external electric fields promote freezing in liquid water models. The first project considers heterogeneous ice nucleation in systems, where water molecules experience an electric field in a narrow region over an entire surface. The specific focus is ice nucleation and growth processes. Different water models are considered, and the influences of temperature and field parameters are examined. We find no qualitative difference between the two water models. By analyzing structure, we show that a ferroelectric cubic ice layer freezes inside the field region, and unpolarized ice grows beyond the field region, at temperatures not far below the melting point. We explore ice nucleation by electric field bands, which act only over a portion of a surface. Field bands of different geometry nucleate ice, provided that the band is sufficiently large. Analysis of different systems reveals that ice strongly prefers to grow at the (111) crystal plane of cubic ice, and that ice nucleated by field bands usually grows as a mixture of cubic and hexagonal ice. Our results suggest that local electric fields could play a major role in heterogeneous ice nucleation, particularly for rough particles with many surface structural variations, that serve as ice nuclei in the environment. We also investigate the electrofreezing of water subject to a uniform field. The aim is to obtain an understanding of why electric fields facilitate ice nucleation. It is shown that the melting point of water increases significantly when water is polarized by a field. The increased melting point is mainly due to the favourable interaction of near perfectly polarized cubic ice with the applied field. Relevant to the mechanism of heterogeneous ice nucleation by local surface fields, our results suggest that local fields effectively increase the degree of supercooling of locally polarized liquid. This decreases the size of the critical nucleus in the region influenced by the field, facilitating ice nucleation.

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