Blast wave attenuation in liquid foams: role of gas and evidence of an optimal bubble size

Abstract

Liquid foams are excellent systems to mitigate pressure waves such as acoustic or blast waves. The understanding of the underlying dissipation mechanisms however still remains an active matter of debate. In this paper, we investigate the attenuation of a weak blast wave by a liquid foam. The wave is produced with a shock tube and impacts a foam, with a cylindrical geometry. We measure the wave attenuation and velocity in the foam as a function of bubble size, liquid fraction, and the nature of the gas. We show that the attenuation depends on the nature of the gas and we experimentally evidence a maximum of dissipation for a given bubble size. All features are qualitatively captured by a model based on thermal dissipation in the gas.

The Acoustics of Liquid Foams

“…Although the nature of the dissipation mechanism is still controversial [25e27, 31,36,37,42,72], a recent experimental study [38], conducted for various foams made with different gases and with various bubble sizes, using tubes of different diameters, allowed to identify three sources of dissipation, as illustrated in Fig. 4: two intrinsic ones, related to the sound attenuation in the bulk of the foam, and an extrinsic one, related to the boundary conditions (i.e.…”

“…Removing the contribution of this extrinsic dissipation source, thermal losses are a natural candidate to explain sound attenuation. Their dependence on the gas thermal diffusivity depends on whether the bubble oscillations are isothermal or adiabatic [42], which can complicate data interpretation. However, a recent experimental study [38] reports that thermal losses were not sufficient to explain the level of attenuation, which suggests that a second intrinsic source of loss must be considered.…”

“…New routes of optimization have also been investigated in the field of mitigation of shock wave, evidencing a possible optimization of absorption both by the choice of gas and bubble size [42]. Lastly, researchers are also designing new instruments using an acoustic wave as an original source of solicitation: for instance, new and unconventional rheometry can be proposed at frequencies much higher that what can be done by standard methods [44,45], as well as instruments for monitoring the velocity of a flowing foam [74].…”

Bubble entrainment by a sphere falling through a horizontal soap foam

“…At the other extreme, foam films can be used to separate individual particles based on their size [4]. In between, processes such as froth flotation and explosion suppression [2,5] rely on the extent to which particles are trapped by foam films. Once in the film, particles may rotate and, depending on parameters such as the contact angle, may cause rupture [6].…”

Bubble entrainment by a sphere falling through a horizontal soap foam

“…At the other extreme, foam films can be used to separate individual particles based on their size [4]. In between, processes such as froth flotation and explosion suppression [2,5] rely on the extent to which particles are trapped by foam films. Once in the film, particles may rotate and, depending on parameters such as the contact angle, may cause rupture [6].…”