Ideal vitrification, barrier hopping, and jamming in fluids of modestly anisotropic hard objects

Abstract

Our recent theory for the glassy dynamics of fluids and suspensions of hard nonspherical objects is applied to several modestly anisotropic shapes. The role of bond length and aspect ratio is studied for diatomics, triatomics, and spherocylinders. As spherical symmetry is broken the ideal kinetic glass transition volume fraction of all objects increases linearly with aspect ratio with the same slope, in surprising agreement with the jamming phase diagram of hard granular ellipsoids. The ideal glass boundary of all shapes is a nonmonotonic function of aspect ratio which is also in qualitative accord with the jamming behavior of spherocylinders and ellipsoids. The maximum glass volume fraction shifts to higher values, and larger aspect ratios, as the object becomes smoother. Suggestions for why the nonequilibrium jamming and kinetic ideal glass formation (dynamical crossover) boundaries are similar are advanced. Beyond the ideal glass volume fraction the nonequilibrium free energy acquires a localization well and entropic barrier. Although its form is highly nonuniversal, if different shapes are compared at constant barrier height then a good collapse is found. Collapse of the volume fraction dependence of the barrier height for different shapes is also predicted for modest shape anisotropy, but increasingly fails as the aspect ratio exceeds 2. For a given volume fraction the mean barrier hopping times are nonmonotonic functions of aspect ratio. The functional form of this dependence, and order of magnitude variation with aspect ratio, is distinct for each object.