Time reversal of ultrasound in granular media

Time reversal (TR) focusing of ultrasound in granular packings is experimentally investigated. Pulsed elastic waves transmitted from a compressional or shear transducer source are measured by a TR mirror, reversed in time and back-propagated. We find that TR of ballistic coherent waves onto the source position is very robust regardless driving amplitude but provides poor spatial resolution. By contrast, the multiply scattered coda waves offer a finer TR focusing at small amplitude by a lens effect. However, at large amplitude, these TR focusing signals decrease significantly due to the vibration-induced rearrangement of the contact networks, leading to the breakdown of TR invariance. Our observations reveal that granular acoustics is in between particle motion and wave propagation in terms of sensitivity to perturbations. These laboratory experiments are supported by numerical simulations of elastic wave propagation in disordered 2D percolation networks of masses and springs, and should be helpful for source location problems in natural processes.Comment: 15 pages, 6 figure

Acoustique des bulles cubiques : six oscillateurs couplés

5 pages with 4 figures, supplementary material with two figures. To appear in Physical Review Letters, with Editors' suggestionInternational audienceIn this manuscript we introduce cubic bubbles that are pinned to 3D printed millimetric frames immersed in water. Cubic bubbles are more stable over time and space than standard spherical bubbles, while still allowing large oscillations of their faces. We found that each face can be described as a harmonic oscillator coupled to the other ones. These resonators are coupled by the gas inside the cube but also by acoustic interactions in the liquid. We provide an analytical model and 3D numerical simulations predicting the resonance with a very good agreement. Acoustically, cubic bubbles prove to be good monopole sub-wavelength emitters, with non-emissive secondary surfaces modes

Multiple scattering and time reversal of ultrasound in dry and immersed granular media

Le retournement temporel (RT) est une méthode qui permet de faire revivre à une onde sa vie passée et de la faire ainsi reconverger sur la source qui lui a donné naissance. Au cours de cette thèse, nous avons étudié – expérimentalement et numériquement – le RT des ondes ultrasonores dans des milieux granulaires. En se propageant de grains en grains, les ondes ultrasonores fournissent une sonde unique du réseau hétérogène 3D des contacts. Pour des ondes se propageant en régime de diffusion multiple, nous montrons que la focalisation est globalement robuste mais toutefois sensible à des mouvements des grains à des échelles spatiales bien plus fines que la longueur d’onde. À cet égard, la propagation d’une onde ultrasonore à travers le réseau discret et métastable des contacts entre grains apparaît comme une situation intermédiaire entre l’instabilité du mouvement d’une particule dans un gaz de Lorentz et la propagation d’une onde ultrasonore dans une matrice homogène remplie d’obstacles diffusants. Lorsque l’amplitude de la source augmente, nous entrons dans un régime nonlinéaire où l’onde elle-même provoque des réarrangements du milieu, ce qui conduit à la dégradation de la focalisation obtenue par retournement temporel de ladite onde. Celle-ci n’agit alors plus seulement comme une sonde, mais aussi comme une « pompe ». Enfin, nous montrons que le RT d’une onde de faible amplitude, mais allongée dans le temps par la diffusion multiple, peut être utilisé pour focaliser une onde de grande amplitude en un point du milieu et y déclencher ainsi de façon contrôlée des réarrangements irréversibles du réseau des contacts. L’ensemble de ces résultats est supporté par un modèle numérique vectoriel fondé sur un système masses-ressorts percolé bidimensionnel.: Time reversal (TR) is a technique which gives the possibility to make a wave relive its life in reverse chronology, and to focus back to its source. In this thesis, TR of ultrasound in granular media has been investigated experimentally and numerically. By propagating from grain to grain, ultrasounds provide a unique probe of the heterogeneous 3D contact network. We show that for multiply scattered waves, the focusing is essentially robust but sensitive to displacements of grains on a scale much smaller than the wavelength. In this respect, the ultrasound propagation through the discrete and metastable contact network between the grains appears to represent an intermediary situation between the instability in the propagation of a particle in a Lorentz gas and the propagation of ultrasounds in an homogeneous medium filled with scatterers. When the source amplitude is increased, a non-linear regime is reached where the wave itself triggers rearrangements in the medium, thus degrading the quality of the TR focusing. In this regime, the wave acts not only as a probe, but also as a « pump ». Finally, we show that the TR of a small-amplitude multiply-scattered wave can be used to focus a high-amplitude wave in the medium and trigger in a controlled way irreversible rearrangements of the contact network. These results are supported by a vectorial numerical model based on a 2D percolated masses-springs network

La France toujours bien placée dans l’International Physicists’ Tournament !

Du 8 au 13 avril dernier s'est tenue à Göteborg (deuxième ville de Suède) la neuvième édition de l'International Physicists' Tournament (IPT, http://iptnet.inf

Diffusion multiple et retournement temporel des ondes ultrasonores dans les milieux granulaires secs et immergés

: Time reversal (TR) is a technique which gives the possibility to make a wave relive its life in reverse chronology, and to focus back to its source. In this thesis, TR of ultrasound in granular media has been investigated experimentally and numerically. By propagating from grain to grain, ultrasounds provide a unique probe of the heterogeneous 3D contact network. We show that for multiply scattered waves, the focusing is essentially robust but sensitive to displacements of grains on a scale much smaller than the wavelength. In this respect, the ultrasound propagation through the discrete and metastable contact network between the grains appears to represent an intermediary situation between the instability in the propagation of a particle in a Lorentz gas and the propagation of ultrasounds in an homogeneous medium filled with scatterers. When the source amplitude is increased, a non-linear regime is reached where the wave itself triggers rearrangements in the medium, thus degrading the quality of the TR focusing. In this regime, the wave acts not only as a probe, but also as a « pump ». Finally, we show that the TR of a small-amplitude multiply-scattered wave can be used to focus a high-amplitude wave in the medium and trigger in a controlled way irreversible rearrangements of the contact network. These results are supported by a vectorial numerical model based on a 2D percolated masses-springs network.Le retournement temporel (RT) est une méthode qui permet de faire revivre à une onde sa vie passée et de la faire ainsi reconverger sur la source qui lui a donné naissance. Au cours de cette thèse, nous avons étudié – expérimentalement et numériquement – le RT des ondes ultrasonores dans des milieux granulaires. En se propageant de grains en grains, les ondes ultrasonores fournissent une sonde unique du réseau hétérogène 3D des contacts. Pour des ondes se propageant en régime de diffusion multiple, nous montrons que la focalisation est globalement robuste mais toutefois sensible à des mouvements des grains à des échelles spatiales bien plus fines que la longueur d’onde. À cet égard, la propagation d’une onde ultrasonore à travers le réseau discret et métastable des contacts entre grains apparaît comme une situation intermédiaire entre l’instabilité du mouvement d’une particule dans un gaz de Lorentz et la propagation d’une onde ultrasonore dans une matrice homogène remplie d’obstacles diffusants. Lorsque l’amplitude de la source augmente, nous entrons dans un régime nonlinéaire où l’onde elle-même provoque des réarrangements du milieu, ce qui conduit à la dégradation de la focalisation obtenue par retournement temporel de ladite onde. Celle-ci n’agit alors plus seulement comme une sonde, mais aussi comme une « pompe ». Enfin, nous montrons que le RT d’une onde de faible amplitude, mais allongée dans le temps par la diffusion multiple, peut être utilisé pour focaliser une onde de grande amplitude en un point du milieu et y déclencher ainsi de façon contrôlée des réarrangements irréversibles du réseau des contacts. L’ensemble de ces résultats est supporté par un modèle numérique vectoriel fondé sur un système masses-ressorts percolé bidimensionnel

Time-Reversal Focusing as a Method for Measuring the Diffusion Coefficient of Ultrasound in Dense Granular Suspensions

Time reversal is an efficient way for focusing waves deeply inside heterogeneous media [1]. In a typical experiment, the wavefield radiated by a broadband source is recorded with a set of detectors named a Time Reversal Mirror (TRM). The detectors then become sources sending back the time-reversed sequence of the recorded field. The result is a time-reversed field that focuses at the source location as if time were going backwards. In this talk, we will show how TR focusing can be used as a method for studying ultrasound transport in a dense granular suspension consisting of a random packing of glass beads immersed in water. The idea is to perform a time reversal experiment in a peculiar configuration where the TRM, consisting of one single transducer, is embedded inside the granular suspension at a depth much larger than the scattering mean free path. An array of transducers is then placed (in water) in the far field of the suspension. A single element of the array emits a short pulse and the TRM records the resulting time-dependent signal. In a first experiment, the whole signal is flipped in time and sent back from the TRM. The time-reversed wave focuses back at the source with a focal spot size scaling with the inverse depth of the TRM, which is characteristic of a diffusive regime [2]. We also study the evolution of the focal spot size against wave travel time by sending back time- reversed versions of short windows selected at different times in the scattered signal. By comparing the predictions of a microscopic theory to our experimental data, we infer the diffusion coefficient of ultrasound in the dense granular suspension. [1] A. Derode, A. Tourin, M. Fink, Phys. Rev. E 64, 036606 (2001) [2] B.A. Van Tiggelen, Phys. Rev. Lett. 91, 243904 (2003

Dissipation driven time reversal for waves

Dissipation is usually associated with irreversibility. Here we present a counter-intuitive concept to perform wave time reversal using a dissipation impulse. A sudden and strong modification of the damping, localized in time, in the propagating medium generates a counter-propagating time- reversed version of the initial wave. In the limit of a high dissipation shock, it amounts to a ‘freezing’ of the wave, where the initial wave field is retained while its time derivative is set to zero at the time of the impulse. The initial wave then splits into two waves with identical profiles, but with opposite time evolution. In contrast with other time reversal methods, the present technique produces an exact time reversal of the initial wave field, compatible with broad-band time reversal. Experiments performed with interacting magnets placed on a tunable air cushion give a proof of concept. Simulations show the ability to perform time reversal in 2D complex media