Colloidal rotation near the colloidal glass transition.

We compare, using single-particle optical imaging, trajectories of rotation and translation for micron-sized spheres in index-matched colloidal suspensions near their glass transition. Rotational trajectories, while they show intermittent caged behavior associated with supercooled and glassy behavior, explore a sufficiently wider phase space such that in the averaged mean-square angular displacement there appears no plateau regime, but instead sub-Fickian angular diffusion that follows an apparent power law in time. We infer translation and rotation time constants, the former being the time to diffuse a particle diameter and the latter being the time to rotate a full revolution. Correlation between time constants increases with increasing volume fraction, but unlike the case for molecular glasses, the rotation time constant slows more weakly than the translation time.