Anisotropy-independent packing of confined hard ellipses

Abstract We have examined the close-packed structure and ordering properties of hard ellipses between two parallel hard walls by using geometrical conjectures and Monte Carlo simulations. Starting from closely packed hard disk structures, we discover two competing densest packings of ellipses with the stretching of hard disks into different directions. We find that parallel and tilted close-packed structures alternate with widening pore width up to the two-dimensional bulk limit, where both structures produce the same close packing value ( η = π / 12 ). Our results highlight that the densest packing does not depend on the aspect ratio of the ellipse. This universal behavior is confirmed for three different aspect ratios (κ=2, 3, and 4) using replica exchange Monte Carlo simulation. Our simulation results show that even the global phase diagram is universal, where layering transitions between parallel structures and tilted-parallel structural changes are present.