Probing acoustics of liquid foams by optical diffusive wave spectroscopy

We have studied the effect of an external acoustic wave on bubble displacements inside an aqueous foam. The signature of the acoustic-induced bubble displacements is found using a multiple light scattering technique, and occurs as a modulation on the photon correlation curve. Measurements for various sound frequencies and amplitudes are compared to analytical and numerical simulations. These comparisons finally allow us to elucidate the non-trivial acoustic displacement profile inside the foam; in particular, we find that the acoustic wave creates a localized shear in the vicinity of the solid walls holding the foam. This study of how bubbles "dance" inside a foam as a response to sound turns out to provide new insights on both foam rheology at high frequencies, foam acoustics and sound transmission into a foam, and analysis of light scattering data in samples undergoing non-homogeneous deformations.