  Foams are ideal model systems to study stress-driven dynamics, as
stress-imbalances within the system are continuously generated by the
coarsening process, which unlike thermal fluctuations, can be conveniently
quantified by optical means. However, the high turbidity of foams generally
hinders the detailed study of the temporal and spatial distribution of
rearrangement events, such that definite assessments regarding their
contribution to the overall dynamics could not be made so far. In this paper,
we use novel light scattering techniques to measure the frequency and position
of events within a large sample volume. As recently reported (A. S. Gittings
and D. J. Durian, Phys. Rev. E, 2008, 78, 066313), we find that the foam
dynamics is determined by two distinct processes: intermittent bubble
rearrangements of finite duration and a spatially homogeneous quasicontinuous
process. Our experiments show that the convolution of these two processes
determines the age-dependence of the mean dynamics, such that relations between
intermittent rearrangements and coarsening process can not be established by
considering means. By contrast the use of the recently introduced photon
correlation imaging technique (A. Duri, D. A. Sessoms, V. Trappe, and L.
Cipelletti, Phys. Rev. Lett., 2009, 102, 085702) enables us to assess that the
event frequency is directly determined by the strain-rate imposed by the
coarsening process. Surprisingly, we also find that, although the distribution
of successive events in time is consistent with a random process, the spatial
distribution of successive events is not random: rearrangements are more likely
to occur within a recently rearranged zone. This implies that a topological
rearrangement is likely to lead to an unstable configuration, such that a small
amount of coarsening-induced strain is sufficient to trigger another event.
