Isothermal-expansion melting of two-dimensional colloidal monolayers on the surface of water

Monodisperse distributions of 1.01 mu m and 2.88 mu m polystyrene microspheres in monolayers on the surface of water were used in a study of isothermal-expansion melting in two dimensions. The equation of state, defect structures, and the translational and orientational correlation functions were obtained from digitised particle positions as the particle-number density ranged from the ordered solid to the liquid phase. The 2.88 mu m system showed evidence of defect mediated melting and of an intermediate hexatic phase, in partial accord with theoretical results of other studies. Melting in the 1.01 mu m system appeared to proceed by a weak first-order transition. If so, the difference in melting behaviour of the two samples may reflect differences in defect core creation energies which can be traced to sphere size.