Molecular dynamics simulation of compression-induced solid-to-solid phase transitions in colloidal monolayers

The compression of two-dimensional colloidal monolayers, consisting of polystyrene particles trapped at an oil-water interface and interacting via dipole-dipole potentials, is investigated by the molecular dynamics technique. In particular, the pair correlation function and global orientational order parameter of the monolayers are calculated as a function of the particle coverage. The simulation results exhibit a sequence of hexagonal-to-rhombohedral-to-hexagonal phase transitions of the monolayers under anisotropic compression. The influence of defects in the monolayers on the solid-to-solid phase transitions is also examined. The simulations show that the stability of the rhombohedral phase is relatively sensitive to lattice defects, while, under the same conditions, the hexagonal phase is very stable. Finally, the simulation results are compared with recent experimental observations, and the implications of the present computer simulations for diffusion mechanisms and protein folding studies are discussed briefly.