  We present Langevin dynamics simulations that study the collective behavior
of driven particles embedded in a densely packed background consisting of
passive particles. Depending on the driving force, the densities of driven and
passive particles, and the temperature we observe a dynamical phase separation
of the driven particles, which cluster together in tight bands. We explore the
mechanisms that drive this cluster formation, and determine the critical
conditions for such phase separation. A simple physical picture explains the
formation and subsequent growth of a jammed zone developing in front of the
driven cluster. The model correctly captures the observed scaling with time. We
analyze the implications of this clustering transition for the driven transport
in dense particulate flows, which due to a non-monotonic dependence on the
applied driving force is not straightforwardly optimized. We provide
proof-of-concept for a direct application of the clustering effect, and propose
a 'colloidal chromatograph'; a setup that permits the separation of colloids by
mass or size.
