Melting of Colloidal Crystals*

We experimentally studied the melting and freezing behaviors of colloidal crystals composed of diameter tunable poly-N-isopropylacrylamide (NIPA or pNIPAM) microgel spheres by bright-field and confocal video microscopies. The melting behaviors of three-dimensional (3D), two-dimensional (2D) and multilayer thin films of both single crystals and polycrystals were systematically studied with single-particle dynamics. Thick films ($>$4 layers) melt heterogeneously, while thin films ($<$5 layers) melt homogeneously even in polycrystals. A novel heterogeneous melting at dislocation is discovered in 5- to 12-layer films. The equilibrium phase behaviors are different in three thickness regimes: thick films have a liquid-solid coexistence regime which decreases with the film thickness and vanishes at 4 layers, thin films melt into the liquid phase in one step, while monolayers melt in two steps with an intermediate hexatic phase. These results provide new challenges in theory.