  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
enhance local icosahedral structure and one which does not strongly affect
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.
