  Colloidal Janus spheres in water (one hemisphere attractive and the other
repulsive) assemble into two-dimensional hexagonal crystals with orientational
order controlled by anisotropic interactions. We exploit the decoupled
translational and rotational order to quantify the orientational dynamics. Via
imaging experiments and Monte Carlo simulations we demonstrate that the
correlations in the orientation of individual Janus spheres exhibit glasslike
dynamics that can be controlled via the ionic strength. Thus, these colloidal
building blocks provide a particularly suitable model glass system for
elucidating nontrivial dynamics arising from directional interactions, not
captured by the consideration of just translational order.
