Sound propagation in liquid foams: Unraveling the balance between physical and chemical parameters.

We present experimental results on the propagation of an ultrasonic wave (40 kHz) in liquid foams, as a function of the foam physical and chemical parameters. We have first implemented an original setup, using transducers in a transmission configuration. The foam coarsening was used to vary the bubble size (remaining in the submillimeter range), and we have made foams with various chemical formulations, to investigate the role of the chemicals at the bubble interfaces or in bulk. The results are compared with recently published theoretical works, and good agreements are found. In particular, for all the foams, we have evidenced two asymptotic limits, at small and large bubble size, connected by a nontrivial resonant behavior, associated to an effective negative density. These qualitative features are robust whatever the chemical formulation; we discuss the observed differences between the samples, in relation to the interfacial and bulk viscoelasticity. These results demonstrate the rich and complex acoustic behavior of foams. While the bubble size remain here always smaller than the sound wavelength, it turns out that one must go well beyond mean-field modeling to describe the foam acoustic properties.