  Little is known about the coupling of rotation and translation in dense
systems. Here, we report results of confocal fluorescence microscopy where
simultaneous recording of translational and rotational particle trajectories
from a bidisperse colloidal dispersion is achieved by spiking the samples with
rotational probe particles. The latter consist of colloidal particles
containing two fluorescently labelled cores suited for tracking the particle's
orientation. A comparison of the experimental data with event driven Brownian
simulations gives insight into the system's structure and dynamics close to the
glass transition and sheds new light onto the translation-rotation coupling.
The data show that with increasing volume fractions, translational dynamics
slows down drastically, whereas rotational dynamics changes very little. We
find convincing agreement between simulation and experiments, even though the
simulations neglect far-field hydrodynamic interactions. An additional analysis
of the glass transition following mode coupling theory works well for the
structural dynamics but indicates a decoupling of the diffusion of the smaller
particle species. The shear stress correlations do not decorrelate in the
simulated glass states and are not affected by rotational motion.
