Universal Scaling of Nematic Order and Rotational Diffusion in Quasi 2D Ellipsoidal Colloids.

Ellipsoidal colloids frequently form nematic phases which can give rise to significant changes in both structure and dynamics, such as distinct glass transitions associated with rotational and translational motion. Experimentally, these systems are often studied in a quasi-2D environment and are subsequently modeled computationally by a genuinely 2D representation. Here we use molecular simulations to consider how small deviations away from a purely 2D representation affect the phase diagram and dynamical behavior of ellipsoidal particles. We compare actual 2D simulations with quasi-2D simulations in which ellipsoids are confined to a slit of specified width. The ability of ellipsoids to rotate even a small amount out of the plane leads to significant impact on the isotropic to nematic and nematic to crystal transitions; increasing slit width increases the packing fraction for the transitions. A similar shift of the concentration is seen for where the vanishing of rotational diffusion occurs. However, regardless of slit width or packing fraction, we show that a nematic order parameter can be used to map quasi-2D systems to a corresponding 2D case. As a consequence, we find that the rotational diffusion coefficient for all systems collapses to a universal master curve with the nematic order. These findings demonstrate that only the degree of orientational order is needed to predict the behavior of ellipsoidal colloids, whether genuinely 2D or only quasi-2D.