We explore the effect of directionality on rotational and translational relaxation in glassy systems of patchy particles. Using molecular dynamics simulations, we analyze the impact of two distinct patch geometries, one that enhances the local icosahedral structure and the other one that does not strongly affect the local order. We find that in nearly all investigated cases, rotational relaxation takes place on a much faster time scale than translational relaxation. By comparing to a simplified dynamical Monte Carlo model, we illustrate that rotational diffusion can be qualitatively explained as purely local motion within a fixed environment, which is not coupled strongly to the cage-breaking dynamics required for translational relaxation. Nonetheless, icosahedral patch placement has a profound effect on the local structure of the system, resulting in a dramatic slowdown at low temperatures, which is strongest at an intermediate “optimal” patch size.
It should be noted here that in our study of the rotational correlations, we deliberately do not account for the symmetry of the particle. In other words, the rotational correlation function simply checks whether an average particle has rotated with respect to its original orientation, even if the new orientation has (different) patches pointing in the same directions as the original one. This implies that we are looking at the ability of the particle to rotate, rather than finding a new configuration that is fully independent of its starting orientation. If we were to take into account the particle symmetry, this would likely result in rotational correlation times that are on the time scale of the lifetime of the cages, since the preferred set of orientations for the central particle will adapt itself to the surrounding cage. As this choice would give us information about the (translational) dynamics of the cages, rather than about the true rotational freedom of the particles, we choose here not to take particle symmetry into account when measuring rotations.