Many physical systems exhibit plastic flow when subjected to slow steadyshear. A unified picture of plastic flow is still lacking; however, there is anemerging theoretical understanding of such flows based on irreversible motionsof the constituent ``particles'' of the material. Depending on the specificsystem, various irreversible events have been studied, such as T1 events infoam and shear transformation zones (STZ's) in amorphous solids. This paperpresents an experimental study of the T1 events in a model, two-dimensionalfoam: bubble rafts. In particular, I report on the connection between thedistribution of T1 events and the behavior of the average stress and averagevelocity profiles during both the initial elastic response of the bubble raftand the subsequent plastic flow at sufficiently high strains.
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