The reversible freezing and melting of colloidal crystal and glass

We present the observation of gas-liquid and gas-solid phase transitions in a close density matched system of charge stabilized polystyrene spheres suspended in the quasi binary 3-methylpyridine /H$_{2}$O/D$_{2}$O mixture. The reversible phase transitions are induced by using the temperature as control parameter. The temperature control parameter can be varied actively and accurately and applied to the same system to study the phase behaviors. The ``aggregation'' observed by Beysens is in reality a phase transition of the colloidal system. Density matching enables us to observe stable gas-liquid and gas-solid equilibriums. Thus the phases formed could be characterized by the measurement of the structure factor with Small Angle X-ray Scattering (SAXS): dense liquid, glass and face centered cubic ( fcc) crystal.