  We examine the dynamics of two-dimensional colloidal systems using numerical
simulations of a system with a drive applied to a thin region in the middle of
the sample to produce a local shear. For a monodisperse colloidal assembly, we
find a well defined decoupling transition separating a regime of elastic motion
from a plastic phase where the particles in the driven region break away or
decouple from the particles in the bulk, producing a shear band. For a
bidisperse assembly, we find that the onset of a bulk disordering transition
coincides with the broadening of the shear band. We identify several distinct
dynamical regimes that are correlated with features in the velocity-force
curves. As a function of bidispersity, the decoupling force shows a
nonmonotonic behavior associated with features in the noise fluctuations, power
spectra, and bulk velocity profiles. When pinning is added in the bulk, we find
that the shear band regions can become more localized, causing a decoupling of
the driven particles from the bulk particles. For a system with thermal noise
and no pinning, the shear band region becomes more extended and the average
velocity of the driven particles drops at the thermal disordering transition of
the bulk system.
