  We study the expansion of harmonically trapped bosons in a two-dimensional
lattice after suddenly turning off the confining potential. We show that, in
the presence of multiple occupancies per lattice site and strong interactions,
the system exhibits a clear dynamical separation into slowly and rapidly
expanding clouds. We discuss how this effect can be understood within a simple
picture by invoking doublons and Bose enhancement. This picture is corroborated
by an analysis of the momentum distribution function in the regions with slowly
and rapidly expanding bosons.
