Tuesday, May 24th, 11am at MEB10
Institut de Physique de Rennes, CNRS/Université Rennes 1
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 acoustic resonance 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 . 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 . We show that subharmonic oscillations appear also already at low applied pressure amplitude . We report another nonlinearity, termed “compression-only”: an oscillating bubble compresses more than it expands . We rationalise all these findings using a model for the bubble shell by Marmottant et al. , on which we propose a weakly nonlinear analysis which theoretically explains the condition of appereance of all these nonlinearities.
Fast and/or confined film and foam flows are of great practical interest, because of the use of foams in porous media for enhanced oil recovery, and of the development of microfluidics with bubble assemblies. We first present an elementary experiment at the scale of single soap films pushed through tubes. Varying their velocity, we observe that SDS films get curved downstream, and we show that film rupture occurs as the film curvature exceeds that of the tube . With another solution (SLES/CAPB/myristic acid) giving strongly viscoelastic interfaces, the behaviour is totally different; soap films are deformed at much lower velocity, with a strong dependence on the amount of liquid. We identify an intermittent flow regime, reminiscent of stick-slip in solid friction. Moreover, we show by tracer-tracking that the moving film has a long-range influence on the wetting film ahead of it.
 S. M. van der Meer et al., J. Acoust. Soc. Am. 121, 648 (2007).
 M. Overvelde et al., Ultrasound Med. Biol. 36, 2080 (2010).
 J. Sijl et al., J. Acoust. Soc. Am. 128, 3239 (2010).
 J. Sijl et al., J. Acoust. Soc. Am. 129, 1729 (2011).
 P. Marmottant et al., J. Acoust. Soc. Am. 118, 3499 (2005).
 B. Dollet & I. Cantat, J. Fluid Mech. 652, 529 (2010).