Surface Energy-Mediated Multistep Pathways for Heterogeneous Ice Nucleation

Heterogeneous ice nucleation (HIN) is the dominant mode of ice formation, while its kinetic pathways remain poorly understood. The classical nucleation theory suggests a one-step pathway, that is, direct change from liquid water to ice (e.g. hexagonal ice), which has been widely accepted. In this work, however, through molecular dynamics simulations, we observe an intermediate state, square ice, at the early stage of ice nucleation at certain surface energies. The intermediate square ice gives rise to a new nonclassical pathway for HIN: from liquid water to hexagonal ice via square ice. This multistep pathway may coexist with and can be more probable than the classical one-step pathway though it may delay the ice nucleation process. The new multistep pathway offers insights in controlling the kinetics of ice crystallization and understating the mechanisms of HIN.