Surface Premelting and Wetting of Tunable Colloidal Crystals

We discovered a dye-induced attraction between colloidal spheres which can be finely tuned by temperature. It opens the way to study surface physics in colloidal model systems. We epitaxially grew high-quality colloidal crystals with free surfaces, and observed the surface premelting, grain-boundary-mediated 2D melting and isostructural solid-solid transitions at the single-particle level for the first time. We found that monolayer and bilayer crystals have distinct premelting behaviors due to their different lattice stabilities and bulk melting behaviors. Analogues to the surface liquid in premelting, we discovered a layer of square lattice on the surface of the bulk triangular lattice. Such novel surface wetting crystal should generally exist when two crystals can form a coherent interface. The thicknesses of both the surface liquid in premelting and the surface square lattice in wetting exhibit power-law growth as approaching the melting point.