Bimodality of local structural ordering in extremely confined hard disks.

By combining computer simulations and a unit cell model approach, we study the apparent bimodality of local structural ordering in a system of confined hard disks. It is shown that a two-dimensional (2D) array of hard disks being confined laterally within a quasi-1D hard wall channel of the width commensurate the bulk 2D triangular lattice at disks close packing, possesses a bimodal probability distribution for the distance between the disk's left and right nearest neighbors. The observed feature aligns with the concept of locally favored structures intensively exploited in the studies of anomalous thermodynamic and kinetic behavior of hydrogen-bonding fluids, except that the reported case is driven by entropic bonding only. The bimodality is observed in a range of densities associated with the vicinity of freezing transition in bulk 2D hard disks, indicating a crossover from the "gas-like" to "liquid-like" state in confined quasi-1D hard disks. Such a phenomenon was not reported for bulk 2D hard disks and is physically unexpected for confined q1D hard disks.