Two-dimensional surface melting with an intermediate quasi-hexatic layer.

Melting in two-dimensional (2D) systems exhibits physics absent in three dimensions, yet its mechanism has been debated for decades. The prevailing Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory describes 2D melting as two continuous transitions in the bulk with an intermediate hexatic phase. While this framework can describe bulk behavior at the transition, it does not account for the influence of free surfaces. Here, using single-particle imaging of optically-driven 2D colloidal solids, we report that melting with free surfaces proceeds via a double wetting pathway, where an inhomogeneous layer with hexatic order forms between the bulk solid and an outer quasi-liquid layer. Prior to melting, the thicknesses of both wetting layers grow into the bulk following a power-law divergence. The quasi-hexatic layer penetrates the bulk ahead of the quasi-liquid layer, giving rise to separate solid-hexatic and hexatic-liquid transitions. Different from the KTHNY scenario, in which dislocations nucleate as bound pairs in the bulk, the free surface acts as a prolific source that continuously emits dislocations into the solid, driving the invasion of the wetting layers. These results present a surface-initiated pathway for 2D melting, offering insights into colloidal matter, 2D materials, and condensed matter physics.