  We study transport dynamics of ultracold cesium atoms in a two-dimensional
optical lattice across the superfluid-Mott insulator transition based on in
situ imaging. Inducing the phase transition with a lattice ramping routine
expected to be locally adiabatic, we observe a global mass redistribution which
requires a very long time to equilibrate, more than 100 times longer than the
microscopic time scales for on-site interaction and tunneling. When the sample
enters the Mott insulator regime, mass transport significantly slows down. By
employing fast recombination pulses to analyze the occupancy distribution, we
observe similarly slow-evolving dynamics, and a lower effective temperature at
the center of the sample.
