Wed February 12th 2014
Seminar Acoustics of liquid foams
Benjamin Dollet


Aqueous foams are concentrated dispersions of bubbles within a continuous liquid phase, containing surfactants to stabilize the liquid films between bubbles. We investigate their linear acoustic properties: speed of sound and attenuation, as they age due to drainage and coarsening, leading to an increase of the average bubble size with time. We show that at given frequency, they display a maximum of attenuation for a certain bubble radius [1], and that the speed of sound is non-monotonous, with a value compatible with Wood's model at small bubble size, but close to the speed of sound in air at large bubble size. These results are quantitatively captured by a model based on the acoustical response of the elementary building blocks of aqueous foams: thin soap films supported by thick liquid channels (termed Plateau borders). This approach fully reconciles the scattered, and seemingly contradictory, existing data in the literature. In particular, we show that foams are natural acoustic metamaterials, displaying effective negative density over extended ranges of frequency and size. In the large-amplitude regime, we also present our recent results on shock wave mitigation by aqueous foams.

[1] I. Ben Salem et al., Soft Matter 9, 1194 (2013).
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The 10th Complex Motion in Fluids 2021
Max Planck Gesellschaft
Centre for Scientific Computing