Ostwald ripening, in which large clusters grow at the expense of smaller on
es, is driven by differences in the chemical potentials of these clusters.
We demonstrate that ripening can also occur without chemical potential diff
erences between small and large particles, by observing the exchange of sin
gle atoms between linear chains of Ir atoms; on a W(110) surface these diss
ociate at a rate independent of length. Ripening takes place because of the
stochastic nature of individual atomic events, and is characterized by lar
ge fluctuations in the number of exchanges for a chain to disappear. This m
echanism should be generally applicable in the ripening of one-dimensional
chains, formed on metals as well as semiconductors, and for which the disso
ciation energy is insensitive to length.