  Colloidal mixtures represent a versatile model system to study transport in
complex environments. They allow for a systematic variation of the control
parameters, namely size ratio, total volume fraction and composition. We study
the effects of these parameters on the dynamics of dense suspensions using
molecular dynamics simulations and differential dynamic microscopy experiments.
We investigate the motion of the small particles through the matrix of large
particles as well as the motion of the large particles. A particular focus is
on the coupling of the collective dynamics of the small and large particles and
on the different mechanisms leading to this coupling. For large size ratios,
about 1:5, and an increasing fraction of small particles, the dynamics of the
two species become increasingly coupled and reflect the structure of the large
particles. This is attributed to the dominant effect of the large particles on
the motion of the small particles which is mediated by the increasing crowding
of the small particles. Furthermore, for moderate size ratios, about 1:3, and
sufficiently high fractions of small particles, mixed cages are formed and
hence the dynamics are also strongly coupled. Again, the coupling becomes
weaker as the fraction of small particles is decreased. In this case, however,
the collective intermediate scattering function of the small particles shows a
logarithmic decay corresponding to a broad range of relaxation times.
