Ultrasound transmission through monodisperse 2D microfoams*

While the acoustic properties of solid foams have been abundantly characterized, sound propagation in liquid foams remains poorly understood. Recent studies have investigated the transmission of ultrasound through 3D polydisperse liquid foams (Pierre et al., 2013, 2014, 2017). However, further progress requires to characterize the acoustic response of better-controlled foam structures. In this work, we introduce a new experimental setup designed to study the transmission of ultrasound (70 – 1000 kHz) through model liquid foam samples generated by microfluidics. We present measurements of the ultrasonic transmission through monodisperse 2D microfoams (i.e. a single layer of monodisperse bubbles) with various liquid fractions and bubble sizes. In such a material, we show that the acoustic wave is sensitive to the spatial distribution of the liquid network, but that the sound velocity within the monolayer is only sensitive to the gas phase. Finally, we find that the attenuation in the monolayer cannot be explained by thermal dissipation alone, as already observed in 3D polydisperse foams. On the long term, this work could contribute to the design of optimized acoustic metamaterials created by solidification of liquid foams.