NONLINEAR BUBBLE DYNAMICS IN A SLOWLY DRIVEN FOAM

Sudden topological rearrangement of neighboring bubbles in a foam occu r during coarsening, and can also be induced by applied forces. Diffus ing-wave spectroscopy measurements are presented of such dynamics befo re, during, and after an imposed shear strain. The rate of rearrangeme nts is proportional to the strain rate, and the shape of the correlati on functions shows that they are spatially and temporally uncorrelated . Macroscopic deformation is thus accomplished by a nonlinear microsco pic process reminiscent of dynamics in the propagation of earthquake f aults or the flow of granular media.