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NONLINEARITIES IN ULTRASOUND CONTRAST AGENT MICROBUBBLES AND ACOUSTICS OF LIQUID FOAMS

"NONLINEARITIES IN ULTRASOUND CONTRAST AGENT MICROBUBBLES AND ACOUSTICS OF LIQUID FOAMS"

par
Benjamin DOLLET Institut de Physique de Rennes, CNRS-Université Rennes 1

le MArdi 17 Mai à 14h00

I will discuss the acoustic behaviour of two different objects : first ultrasound contrast agent microbubbles, then liquid foams.

Ultrasound contrast agents are microbubbles which are encapsulated by a phospholipid monolayer, both to prevent them from too fast dissolution and to carry therapeutic molecules for targeted drug delivery. In medical applications, once injected in the blood pool, they constitute very efficient ultrasound scatterers, which enables to image organ perfusion or to detect tumors. We have developed an optical spectroscopy method to investigate the resonance properties of the contrast agents. It shows that the viscoelasticity of the phospholipid monolayer modifies drastically their response compared to uncoated gas bubbles, changing the linear resonance properties [1]. Furthermore, it also greatly enhances the nonlinearities, compared to uncoated bubbles : we show that the resonance frequency is a decreasing function of the applied pressure amplitude, already at a few kPa [2]. We show that subharmonic oscillations appear also already at low applied pressure amplitude [3]. We report another nonlinearity, termed "compression-only" : an oscillating bubble compresses more than it expands [4]. We rationalise all these findings using a model for the bubble shell by Marmottant et al. [5], on which we propose a weakly nonlinear analysis which theoretically explains the condition of appereance of all these nonlinearities.

Liquid foams are known to absorb efficiently pressure waves, but the underlying mechanisms are still largely unclear. I will present our preliminary measurements of sound propagation and attenuation in a coarsening shaving foam. The foam exhibits a maximum of attenuation at a finite time, which corresponds to a rapid variation of the phase lag between the emitted and received signals. I will discuss this behaviour in the frame of bubble resonance.

[1] S. M. van der Meer et al., J. Acoust. Soc. Am. 121, 648 (2007).

[2] M. Overvelde et al., Ultrasound Med. Biol. 36, 2080 (2010).

[3] J. Sijl et al., J. Acoust. Soc. Am. 128, 3239 (2010).

[4] J. Sijl et al., J. Acoust. Soc. Am. 129, 1729 (2011).

[5] P. Marmottant et al., J. Acoust. Soc. Am. 118, 3499 (2005).

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