Temperature-dependent Kinetic Pathways of Heterogeneous Ice Nucleation: Competition between Classical and Non-classical

Ice formation is essential in diverse areas, ranging from climate changing, energy consumption to cell cryopreservation. Compared to ice nucleation in the bulk, the presence of foreign materials in heterogeneous ice nucleation (HIN) complicates the nucleation process, making HIN less comprehended. Here, we employ Markov States models (MSMs) and transition path theory to elucidate the kinetic pathways of HIN simulated by molecular dynamics simulations. Interestingly, our MSMs reveal that the classical one-step and non-classical two-step nucleation pathways can coexist with comparable fluxes at T=230K. We find that the classical one-step pathway with the direct formation of hexagonal ice is promoted by the favorable interactions from the surface. In stark contrast, the non-classical pathway containing intermediate formation of rhombic and hexagonal structures is facilitated by the entropy stabilization of the nucleus via disordered mixing of the two structures. Furthermore, we discover that, at elevated temperatures, the nucleation process shifts towards the classical pathway, mainly because the potential energy difference, which favors the classical pathway, prevails over the configurational entropy compensation. This study provides the underlying kinetics and mechanisms for HIN, shedding light on the possibility to control crystallization.