The secret role of water's structure near surfaces in ice formation

Most ice on Earth forms via heterogeneous nucleation, as homogeneous nucleation requires significant supercooling. Despite its prevalence, the microscopic mechanisms behind this process remain unclear. We hypothesize that ice nucleation is primarily driven by low-dimensional structural preordering in interfacial liquid layers, rather than by the surface's direct affinity for bulk ice.

To test this hypothesis, we perform molecular dynamics simulations of ice nucleation on a simple cubic substrate with tunable hydrophilicity. We analyze layering, hydrogen-bond distortions, orientational order, and substrate-ice lattice matching to uncover the physical mechanisms that control nucleation pathways.

Bilayer hexagonal ice forms on all substrates, but the nucleation pathway depends sensitively on surface hydrophilicity. At low hydrophilicity, bylayer ice nucleates directly from interfacial water. As hydrophilicity increases, enhanced planarity and density promote sequential nucleation, with two-dimensional ice forming first in the contact layer, then in the second layer. Excessive hydrophilicity hinders this process by suppressing 2D ordering in the contact layer, reversing the nucleation sequence. Consequently, the nucleation rate is maximized at intermediate hydrophilicity. Furthermore, we find that crystalline preordering in the contact layer is strongest when the substrate lattice closely matches that of ice, minimizing the free energy barrier for nucleation. These results highlight how surface-induced liquid ordering - rather than simple templating - controls ice formation. This mechanism likely extends to tetrahedral liquids such as silicon, germanium, carbon, and silica, underscoring the universal role of interfacial liquid structuring in surface-assisted crystallization across natural and technological systems.