Periodic Nonlinear Bubble Motion in Aqueous Foam under Oscillating Shear Strain

The elastic response of aqueous foam to oscillating shear strain is probed by diffusing-wave spectroscopy. At low strains, the local bubble displacements are shown to be compatible with linear elastic behavior. At higher strains, a crossover to nonlinear but still fully periodic displacements is observed. To explain these results, we present a phenomenological model based on unstable bubble configurations.