A thermodynamically equilibrated fluid of hard spheroids is a simple model of liquid matter. In this model, the coupling between the rotational degrees of freedom of the constituent particles and their translations may be switched off by a continuous deformation of a spheroid of aspect ratio t into a sphere (t=1). A tiny deviation of t from unity breaks the symmetry of the particles. The role, which this particle shape symmetry breaking has for the structural and the thermodynamic properties of the fluid has hitherto remained unknown. 

We demonstrate, by experiments, theory, and computer simulations, that dramatic nonanalytic changes in structure and thermodynamics of the fluids take place, as the coupling between rotations and translations is made to vanish[1]. This nonanalyticity, reminiscent of a second-order liquid-liquid phase transition, is not a trivial consequence of the shape of an individual particle and does not show up in (the first orders of) the virial expansions. Rather, free volume considerations relate the observed transition to a similar nonanalyticity at t=1 in structural properties of jammed granular ellipsoids[2]. This observation suggests a deep connection to exist between the physics of jamming and the thermodynamics of the simple fluids. Furthermore, fingerprints of the observed nonanalyticity seem to show up in critical temperatures and concentrations of liquid-vapour transitions in attractive liquids[3], emphasizing the importance of the simpler athermal systems which we study.

[1] A. P. Cohen, S. Dorosz, A. B. Schofield, T. Schilling, and E. Sloutskin, Phys. Rev. Lett. 116, 098001 (2016).

[2] A. Donev et al., Science 303, 990 (2004).

[3] S. Varga, E. Meneses-Juárez, and G. Odriozola, Soft Matter 9, 11178 (2013).