Structure of jammed ellipse packings with a wide range of aspect ratios

Motivated in part by the recent observation of liquid glass in suspensions of ellipsoidal colloids, we examine the structure of jammed two-dimensional ellipse packings over a much wider range of particle aspect ratios ( α , the ratio of the major and minor axis lengths) than has been previously attempted. We determine the jamming densities J ( α ) to high precision, and find empirical analytic formulae that predict J ( α ) to within less than 0.1% for all 1≤ α ≤10, for three different particle dispersities. Then we explore how these packings' local structural order varies with α . We find that the densest packings possess unusually-well-defined nearest-neighbor shells, including both a higher fraction f Z =6 of particles with exactly six contacts and a previously-unreported short-range order marked by "kinetically suppressed" regions in their positional-orientational pair correlation function g ( r ,Δ ). We also show that the previously-reported approach to isostaticity (coordination number Z J → Z iso ≡ 6) with increasing α is interrupted and then reversed as local nematic order increases: Z J ( α ) drops towards 4 as ellipses are more often trapped by contacts with a parallel-oriented neighbor on either side and a perpendicularly-oriented neighbor on either end. Finally we show that J / s (where s is the saturated RSA packing density) is nearly α-independent for systems that do not develop substantial local hexatic or nematic order during compression. Motivated in part by the recent observation of liquid glass in suspensions of ellipsoidal colloids, we examine the structure of jammed ellipse packings over a much wider range of particle aspect ratios than has been previously attempted.