A two-dimensional melting transition has been observed in a freely expanding colloidal monolayer lattice formed of 1.01-μm polystyrene microspheres in an aqueous suspension between two parallel optical flats. The correlation functions computed from digitized images of the particle distribution within a fixed sampling area are consistent with a continuous two-step melting transition. However, direct observations of defect creation and evolution reveal that the system may melt via a first-order process.

• Received 16 September 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.2401