Melting of microgel colloidal crystals

We experimentally studied the melting of colloidal crystals composed of diameter-tunable microgel spheres by bright-field and confocal video microscopies at the single-particle level. Thick (> 4 layers) and thin (≤ 4 layers) films exhibited dramatically different melting kinetics. Thick films melted heterogeneously from grain boundaries in polycrystals and from surfaces in single crystals, while thin films melted homogeneously even in polycrystals. A novel heterogeneous melting at dislocation was observed in 5- to 12-layer films. The equilibrium phase behaviors in three thickness regimes were all different: thick films had a crystal-liquid coexistence regime which decreased with the film thickness and vanished at 4 layers, thin crystalline films melted into liquids in one step, while monolayers melted in two steps with an intermediate hexatic phase.