We investigate heterogeneities in the dynamics of coarsening foams by performing both time and space resolved diffusing wave spectroscopy experiments. In the transmission geometry, we record the speckle pattern in the far field using a CCD-camera and process the images using the Time Resolved Correlation scheme [1]. In the backscattering geometry, we record the speckle pattern at the exit plane of our scattering cell and process the images by calculating the degree of correlation for different areas of the CCD-image. This latter technique allows us to construct spatially resolved maps of the instantaneous dynamical activity of our foam and to follow its temporal evolution. Our results suggest that the local intermittent rearrangements of bubbles typically observed in microscopy are not the only cause for dephasing the scattered light. When the internal stress imbalances, that gradually build up as the foam coarsens, become large enough, an event occurs that leads to a displacement of bubbles in a large portion of the sample. This major event appears to effectively release the internal stress imbalances.

[1] L. Cipelletti, H. Bissig, V. Trappe, P. Ballesta, S. Mazoyer, J. Phys.: Condens. Matter 15, 257 (2003).