Abstract

Small crystalline clusters melt differently from large bulk crystals. The melting kinetics and the strong surface effects in small crystals are not well understand due to the experimental challenges for the observations at small spatial and time scales. Here we directly observe the melting kinetics at the single-particle level for small tunable colloidal crystals confined in different sized and shaped micro-cavities by video microscopy. We find that defect-free crystals melt from edges via nucleation mechanism in large cavities and cavities not commensurate with the lattice. Generally, the melting volume fraction, i.e. the inverse of the effective melting temperature, is lower for smaller crystals. Poly-crystals melted heterogeneously from the intersection of grain boundaries and cavity walls. Single crystals melted heterogeneously from corners and surfaces in large cavities or in small circular and square cavities. By contrast, crystals in small (< 50 particles per layer x 6 layers ) hexagonal and triangular cavities melts catastrophically without a nucleation process. We further observed that the melting point can increase or decrease under different shape and size of the cavities. The measured vibration normal modes of these small crystals are also affected by the confinement. These results cast new lights on melting behaviors of small crystals under confinements, which have implications on nano-crystals, such as integrated circuit(IC), quantum dots and nano-catalysts.