  Under steady shear, a foam relaxes stress through intermittent rearrangements
of bubbles accompanied by sudden drops in the stored elastic energy. We use a
simple model of foam that incorporates both elasticity and dissipation to study
the statistics of bubble rearrangements in terms of energy drops, the number of
nearest neighbor changes, and the rate of neighbor-switching (T1) events. We do
this for a two-dimensional system as a function of system size, shear rate,
dissipation mechanism, and gas area fraction. We find that for dry foams, there
is a well-defined quasistatic limit at low shear rates where localized
rearrangements occur at a constant rate per unit strain, independent of both
system size and dissipation mechanism. These results are in good qualitative
agreement with experiments on two-dimensional and three-dimensional foams. In
contrast, we find for progessively wetter foams that the event size
distribution broadens into a power law that is cut off only by system size.
This is consistent with criticality at the melting transition.
