Coalescence-enhanced melting in the incipient stage of surface melting

Melting is a fundamental process, but the atomic-level comprehension of surface melting remains unclear. In this study, we introduce a parameter called the per-atom crystallinity parameter and utilize molecular dynamics simulations to investigate the melting behavior of aluminum surfaces. Our computational findings reveal that the melting initiates at the free surface and then spreads in a manner depending on the surface orientation. We identify two distinct mechanisms that govern the spreading of melting: the self-growth of liquid nuclei and the coalescence of neighboring liquid nuclei. Our simulations demonstrate that the coalescence mechanism predominates on the Al(110) surface, resulting in a flat melting front. In contrast, the melting front on the Al(111) surface exhibits a spherical-like shape due to the weaker influence of the coalescence mechanism. Overall, this study provides a more comprehensive understanding of the initiation of melting on crystalline surfaces.