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Liquid foams : acoustic properties and flows in channels

Benjamin Dollet Institut de Physique de Rennes

In spite of their beauty and numerous applications, liquid foams remain poorly understood in some fundamental aspects, like their acoustical and mechanical behaviour.

Liquid foams are known to be efficient to absorb the energy of pressure waves, and used as such to mitigate blast waves, but the underlying mechanisms are still obscure. We investigate their linear acoustic properties : speed of sound and attenuation, as functions of the frequency and bubble radius. We show that at given frequency, they display a maximum of attenuation for a certain bubble radius, and a strong acoustic dispersion. 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. In particular, we show that foams are natural acoustic metamaterials, displaying effective negative density over extended ranges of frequency and size [1]. In the large-amplitude regime, we also present our recent results on shock wave mitigation by aqueous foams.

In mechanics, liquid foams belong to the vast class of complex fluids, displaying elastic, plastic and viscous responses. Bubbles being easy to image, they are a well suited material to address fundamental issues in soft glassy rheology. Two recent experiments will be presented, which illustrate the elastoplastic flow of foams in straight [2] and convergent [3] channels, and the influence of dissipation, nonlocality, and delayed relaxation. We will also show our recent, and unprecedented, time- and space-resolved foam flow measurements in 3D, using fast X-ray tomography [4].

[1] J. Pierre, B. Dollet, V. Leroy, Phys. Rev. Lett. 111, 148307 (2014).
[2] B. Dollet, A. Scagliarini, M. Sbragaglia, J. Fluid Mech. 766, 556 (2015).
[3] B. Dollet, C. Bocher, Eur. Phys. J. E 38, 123 (2015).
[4] C. Raufaste, B. Dollet, K. Mader, S. Santucci, R. Mokso, EPL 111, 38004 (2015).