Rotational and translational diffusion in a 2D glass-former

We conduct simulations studying diffusive motion of ellipse tracers in 2D systems. The background system is composed of bidisperse disks, which have a glass transition when the area fraction exceeds 0.79. At lower area fractions, the ellipse rotational and translational motions are coupled to each other: their ratio is a constant, independent of the area fraction. At higher area fractions (close to the glass transition), we observe decoupling between translational and rotational diffusion. The decoupling depends on the ellipse aspect ratio: ellipses with an aspect ratio close to one rotate faster than might be expected, and those with an aspect ratio above six translate faster than might be expected. Diffusion constants are coupled for ellipses with intermediate aspect ratios around three.