8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P6.9Acoustics in foams: new experimental resultsBenjamin Dollet, 1 Reine-Marie Guillermic, 1 Imen Ben Salem, 1 Marion Erpelding, 1Jérôme Crassous, 1 and Arnaud Saint-Jalmes 11 CNRS/Université Rennes 1, Institut de Physique de Rennes, Campus Beaulieu,Bment 11A 35042, Rennes, FranceWe report a set of new experimental results on the acoustic properties of aqueous foams. Manyissues are still pending on foam acoustics; and the local mechanisms underlying sound propagationand attenuation inside foam remain elusive. Experimentally, problems often arise from thecoupling between such acoustic measurements and the foam aging, which irreversibly modifiesboth the bubble size and the liquid fraction (also creating vertical gradients of these parameters).First, we present new measurements of sound velocity and sound attenuation (in the ultrasonicfrequency range) on various types of aging foams, coupled with measurements of the evolution ofliquid fraction and bubble size. This allows us to study accurately the dependence of the soundfeatures with the bubble and sample sizes and the liquid fraction. In particular, we observe anon-monotonic behavior for the attenuation as foam ages, strikingly reminiscent of results foundfor bubbly liquids, that we rationalize in terms of acoustic resonance. Second, we have studiedhow a sound wave (emitted by an external speaker) induces bubble displacement. We show thatit is possible to detect - by a light scattering technique (Diffusive Wave Spectroscopy) - the tinyacoustic-induced motion of the bubbles. Comparisons between theory and experiments allow usto elucidate the non-trivial acoustic displacement profile inside the foam; in particular, we find thatthe acoustic wave creates a localized shear in the vicinity of the solid walls holding the foam, inagreement with novel predictions on foam mechanical behavior at high frequency.9