Single Layer Of Bubbles Research Articles

Acoustic harmonic generation and phase conjugation with a single layer of bubbles

Bubbles are well known for being strong nonlinear scatterers in acoustics. This feature is actually used in medical ultrasound, and nonlinear phenomena, such as phase conjugation [1] or second-harmonic generation [2], had been reported in bubbly liquids. We carried out experiments and calculations on a particularly simple bubbly medium: a single layer of bubbles trapped in a yield-stress fluid. We show the existence of an ideal bubble concentration that maximizes the nonlinear second harmonic generation by the layer. It results from the interplay between the nonlinear response and the strong multiple scattering in the system [3]. The same nonlinear mechanism can be used to obtain phase conjugation, with a probe wave at frequency f, and pump wave at 2f. The single layer of bubbles is then a sub-wavelength phase conjugation mirror. We studied its efficiency in terms of direction and magnitude of the reflected wave. [1] D. V. Vlasov et al., Sov. Phys. Acoust. 29 (1983). [2] J. Wu, Z. Zhu, J. Acoust. Soc. Am 89 (6) (1991) [3] O. Lombard et al., EPL, 112 (2015).

Bubble-raft collapse and the nonequilibrium dynamics of two-state elastica.

We report on the collapse of bubble rafts under compression in a closed rectangular geometry. A bubble raft is a single layer of bubbles at the air-water interface. A collapse event occurs when bubbles submerge beneath the neighboring bubbles under compression, causing the structure of the bubble raft to go from single-layer to multilayer. We studied the collapse dynamics as a function of compression velocity. At higher compression velocity we observe a more uniform distribution of collapse events, whereas at lower compression velocities the collapse events accumulate at the system boundaries. We propose that this system can be understood in terms of a linear elastic sheet coupled to a local internal (Ising) degree of freedom. The two internal states, which represent one bubble layer versus two, couple to the elasticity of the sheet by locally changing the reference state of the material. By exploring the collapse dynamics of the bubble raft, one may address the basic nonlinear mechanics of a number of complex systems in which elastic stress is coupled to local internal variables.

Nonlinear multiple scattering of acoustic waves by a layer of bubbles

We present a theoretical and experimental study of the acoustic second-harmonic generation by a single layer of bubbles. This simple system allows us to investigate the subtle interplay between nonlinear effects and multiple scattering. A perturbative model is shown to give an excellent agreement with the experimental measurements, and we demonstrate the existence of an optimal concentration of bubbles, for which the harmonic generation is maximum. The potential of bubble screens as efficient subwavelength acoustic nonlinear sources is discussed.

Transmission of ultrasound through a single layer of bubbles

We investigate, both experimentally and theoretically, the effect of coupling between resonant scatterers on the transmission coefficient of a model system of isotropic scatterers. The model system consists of a monodisperse layer of bubbles, which exhibit a strong monopole scattering resonance at low ultrasonic frequencies. The layer was a true 2D structure obtained by injecting very monodisperse bubbles (with radius a approximately 100 microm) into a yield-stress polymer gel. Even for a layer with a low concentration of bubbles (areal fraction, n pi a(2), of 10-20%, where n is the number of bubbles per unit area), the ultrasonic transmission was found to be significantly reduced by the presence of bubbles (-20 to -50 dB) and showed a sharp minimum at a particular frequency. Interestingly, this frequency did not correspond to the resonance frequency of the individual, isolated bubbles, but depended markedly on the concentration. This frequency shift is an indication of strong coupling between the bubbles. We propose a simple model, based on a self-consistent relation, which takes into account the coupling between the bubbles and gives good agreement with the measured transmission coefficient.