Heterogeneous ice nucleation (HIN), i.e., ice crystallization on a foreign material, is the major mode of ice formation in nature. A deep understanding of the fundamental kinetics of HIN would help design materials to either suppress or promote ice crystallization. This will have significant impacts in a variety of areas, ranging from atmospheric sciences, food industry, aviation, to energy systems. In spite of recent progress, the inconclusive kinetics of HIN and controversial results in ice nucleation rate under the influences of foreign materials still need to be rationalized. Aiming at understanding the microscopic kinetics of HIN, molecular dynamics (MD) simulations are conducted in this thesis to explore how the HIN is affected by various parameters, including temperature, surface...[ Read more ]

Heterogeneous ice nucleation (HIN), i.e., ice crystallization on a foreign material, is the major mode of ice formation in nature. A deep understanding of the fundamental kinetics of HIN would help design materials to either suppress or promote ice crystallization. This will have significant impacts in a variety of areas, ranging from atmospheric sciences, food industry, aviation, to energy systems. In spite of recent progress, the inconclusive kinetics of HIN and controversial results in ice nucleation rate under the influences of foreign materials still need to be rationalized. Aiming at understanding the microscopic kinetics of HIN, molecular dynamics (MD) simulations are conducted in this thesis to explore how the HIN is affected by various parameters, including temperature, surface energy, and the surface roughness. In addition, the nucleation pathways, which is directly related to the HIN kinetics, are also explored.

For the roles of temperature and surface energy, MD simulations reveal that the stability of the hydrogen bond network of water at the interface is critical in ice crystallization. It is affected by the interplay between the thermal fluctuation of interfacial water molecules (temperature) and water-surface molecular interactions (surface energy). Under comparable thermal and surface effects, HIN is promoted. However, if the thermal effect or the surface effect dominates over the other, HIN is not favored. By varying the surface energy and temperature, a diagram of HIN events is presented, which shows that HIN occurs only in certain temperature and surface energy ranges.

Furthermore, an intermediate state, square ice, is observed at the early stage of ice nucleation at proper nucleation conditions. The square ice gives rise to a new, nonclassical, multistep pathway for HIN, i.e., from liquid water to hexagonal ice via square ice, which is different from the classical one-step pathway, i.e., direct change from liquid water to hexagonal ice. This new, multistep pathway may coexist with and can be more probable than the classical, one-step pathway though it may delay the ice nucleation process.

Finally, the effects of nanoscale grooves on HIN are investigated. It is found that the groove plays an important role in HIN. Surface grooves could either promote or suppress HIN, exhibiting a commensurate dependence on the width of the groove. At proper groove widths, ice nucleation can be enhanced by two orders of magnitude due to the formation of solid-like structures induced the grooves, which are the consequences of either nanoconfinement effects or ice-groove structural matches.

The studies in this thesis offer insight in understanding the microscopic picture of HIN, which also provides helpful information for designing surfaces to control ice nucleation.