The local microscopic structure of fluids of simple spheres is well known. However, the constituents of most real-life fluids are non-spherical, leading to a coupling between the rotational and translational degrees of freedom; this coupling does not allow the structure of simple dense fluids of ellipsoids to be obtained by classical experimental techniques. As a result, the fundamental role played by the rotational degrees of freedom in formation of local fluid structure remained unknown.

We employ real-time three-dimensional confocal microscopy to determine, for the first time by a direct experimental technique, the structure of dense fluids of ellipsoids[1]. We use molecular dynamics simulations and theory to reproduce the experimental structure and estimate the contribution of charge effects to the system[2], achieving perfect agreement between theory, experiment, and simulation. Further, we employ the same theoretical framework to examine the local order in these fluids as a function of the aspect ratio of the constituent particles t. Strikingly, the extent of (short-range) positional correlations exhibits a non-analytical point for the spheres t=1, where the positional order is maximal. This indicates that the behavior of fluids of spheres, where rotations and translations are decoupled, is qualitatively different from that of the fluids of rotationally-anisotropic particles, which are much more common. Moreover, these results suggest, quite unexpectedly, a connection between thermodynamically-equilibrated fluids of ellipsoids and disordered non-ergodic packings of M&M candies[3].

[1] A. P. Cohen, E. Janai, E. Mogilko, A. B. Schofield, and E. Sloutskin, Phys. Rev. Lett. 107, 238301 (2011).

[2] A. P. Cohen, E. Janai, D. C. Rapaport, A. B. Schofield, and E. Sloutskin, J. Chem. Phys. 137, 184505 (2012).

[3] A. Donev, I. Cisse, D. Sachs, E. A. Variano, F. H. Stillinger, R. Connelly, S. Torquato, and P. M. Chaikin, Science 303, 990 (2004).