Free-expansion melting of a colloidal monolayer

A two-dimensional melting transition has been observed in a freely expanding colloidal monolayer lattice formed of 1.01-..mu..m 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.