  The spreading of a bilayer gold film propagating outward from gold clusters,
which are pinned to clean Si(111), is imaged in real time by low energy
electron microscopy. By monitoring the evolution of the boundary of the gold
film at fixed temperature, a linear dependence of the spreading radius on time
is found. The measured spreading velocities in the temperature range of 800 < T
< 930 K varied from below 100 pm/s to 50 nm/s. We show that the spreading rate
is limited by the reaction to form Au silicide, and the spreading velocity is
likely regulated by the reconstruction of the gold silicide that occurs at the
interface.
