  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.

Quantum Gases