Melting of Colloidal-Crystal Films

We studied the melting of multilayer colloidal crystals composed of diameter tunable microgel spheres with short-ranged repulsive interactions confined between two glass walls. Samples are annealed into large crystalline domains so that the finite size effects are negligible. Different melting behaviors were observed in three thickness regimes: 1. Thick films ($>$ 4-layer) melt from grain boundaries in polycrystals and from surfaces in single crystals. The liquid-solid coexistence regime decreases with the thickness and vanishes at 4 layers. 2. Thin films (2 to 4-layer) melt homogenously from both grain boundaries and surfaces. One-step melting is observed in 2-, 3- and 4-layer triangular and square lattices. 3. Monolayers melt in two steps with a middle hexatic phase.