Freezing of two-dimensional hard disks.

We argue that structural rearrangements experienced by an assembly of hard disks under increasing disk density are accompanied by the mutual caging of each disk by its three alternating Voronoi nearest neighbors. This caging becomes effective at a packing fraction eta=pisqrt[3]8 approximately 0.680 when the average gap width between neighboring disks in the system shrinks to about 15% of the disk diameter. The freezing occurs when the fraction of caged disks is about 40%.

Attraction-driven disorder in a hard-core colloidal monolayer.

Monte Carlo simulation techniques were employed to explore the effect of short-range attraction on the orientational ordering in a two-dimensional assembly of monodisperse spherical particles. We find that if the range of square-well attraction is approximately 15% of the particle diameter, the dense attractive fluid shows the same ordering behavior as the same density fluid composed of purely repulsive hard spheres. Fluids with an attraction range larger than 15% show an enhanced tendency to crystallization, while disorder occurs for fluids with an attractive range shorter than 15% of the particle diameter. A possible link with the existence of "repulsive" and "attractive" states in dense colloidal systems is discussed.