Triggering granular avalanches with ultrasound.

Granular flows triggered by vibration below the avalanche angle are ubiquitous in nature. However, the mechanism of triggering and the nature of the resulting flow are not fully understood. Here we investigate the triggering of the shear instability of granular layers by nanometer-amplitude ultrasound close to the static threshold. We find that such small-amplitude and high-frequency sound waves provoke unjamming, resulting in a self-accelerated inertial flow or a creeplike regime which stops flowing after the removal of ultrasound. We show that these effects are due to the reduction of interparticle friction at grain contacts by the shear acoustic lubrication. Our observations are consistent with the bistability inherent to velocity-weakening friction models [e.g., Jaeger et al., Europhys. Lett. 11, 619 (1990)10.1209/0295-5075/11/7/007]. This work should help to understand the local and remote triggering of landslides and earthquakes by seismic waves.

Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension.

Observing and understanding the motion of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to monitor the sinking dynamics of a steel ball in a dense glass bead packing (3D) saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology, we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to micro-slips induced at the grain contacts but without visible plastic deformation due to macroscopic rearrangements, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.

Time reversal of ultrasound through a phononic crystal.

In this Letter, we experimentally investigate time reversal focusing through a phononic crystal consisting of a periodic square arrangement of steel rods in water. An acoustic pulse is transmitted through the medium, received at a transducer array, time reversed and backpropagated. Both spatial focusing and time compression are studied and compared with those obtained through an equivalent disordered medium. With the phononic crystal, we do not observe the "hyperfocusing effect" that is typical of time reversal through disordered samples.

Time reversal of electromagnetic waves.

We report the first experimental demonstration of time-reversal focusing with electromagnetic waves. An antenna transmits a 1-micros electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time-reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity.

Observation of a coherent backscattering effect with a dipolar source for elastic waves: highlight of the role played by the source.

We report an experimental evidence of the role played by the source on the coherent backscattering effect (CBE) for elastic waves. The experiment is carried out in a chaotic cavity consisting of a silicon plate whose shape is a quarter stadium. With a monopolar source, it has already been shown that the time-integrated squared amplitude at the point source is twice as large as at the other points around the source. Here, by using a dipolar source, we show that we instead obtain two peaks with the same axis as the dipole one's. The shape of this "bicone" is well explained with a modal theory assuming that the source may be modeled by two sources with opposite phases. Then, the theory is generalized to any multipolar emitter and/or receiver. Particularly, we study CBE when emitter and receiver are reciprocal.

Random multiple scattering of ultrasound. I. Coherent and ballistic waves.

This is the first article in a series of two dealing with the statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering. Only the first-order moment is considered here. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through two-dimensional samples with various thicknesses. The samples consist of random collections of parallel steel rods immersed in water. Experimental results show that the ensemble-averaged transmitted wave forms ("coherent wave") exhibit a time-dependent frequency spectrum. Within the independent scattering approximation, this is well explained by individual resonances of the scatterers. The coherent wave only appears after ensemble averaging and has to be distinguished from the "ballistic wave," i.e., the first well-defined pulse that crosses the sample, which can be measured on every realization of disorder. A physical interpretation is given, which is based on the separation of the coherent wave between a rigid and a resonant contribution.

Random multiple scattering of ultrasound. II. Is time reversal a self-averaging process?

This is the second article in a series of two dealing with the statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering. Second-order moments in time and space are considered here. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through two-dimensional samples with various thicknesses. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed, and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation on a single realization of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by the array aperture. As the sample thickness is increased, the quality of focusing saturates, which we believe is linked to the Thouless factor g. In the thickest sample, g approximately 30, which is still well above the localization threshold.

Sensitivity to perturbations of a time-reversed acoustic wave in a multiple scattering medium.

We present experimental results on the robustness of acoustic time-reversal focusing in a multiple scattering medium undergoing perturbations. Time reversal in such a medium can be viewed as a correlation technique, analogous to diffusive wave spectroscopy. Moreover, the recent introduction of telecommunication techniques based on time reversal in changeable media naturally raises the question of sensitivity to a perturbation of the medium. We consider the situation where the reversibility of an acoustic wave generated by a linelike source is gradually destroyed when a perturbation is added, either locally (by removing scatterers) or globally (by changing the temperature) to the multiple scattering medium.

Coherent backscattering of an elastic wave in a chaotic cavity

We report the first experimental evidence of coherent backscattering enhancement for transient elastic waves propagating in a two-dimensional chaotic cavity. The time-integrated squared amplitude at the point source is twice as large as at the other points around the source. Contrary to analogous optical experiments, this effect is already clearly observable on a single realization. Especially, the spatial shape of the coherent backscattering enhancement is well predicted by a generalization of the existing theory.

Transport parameters for an ultrasonic pulsed wave propagating in a multiple scattering medium

A set of ultrasonic experimental methods was developed to characterize a multiple scattering medium in terms of l(s), l*, l(a), respectively, the elastic, transport, and absorption mean free paths and D the diffusion constant. Actually, these quantities are the key parameters for a wave propagating in a disordered medium. Although they are widely used in optics, they are less common in acoustics. The underlying model is based on the expansion of the average solution for the heterogeneous Green's function equation. To validate this theoretical approach, a sample made of randomly located steel rods was used as a prototype. Through time-resolved measurements of the transmitted amplitude, the difference between the ballistic and the coherent wave is highlighted. In varying the sample thickness, l(s) is determined, the coherent and diffusive regime are distinguished, and the transition from one to the other is followed. Furthermore, as a limit to a description of the average intensity based on the diffusion approximation, the existence of a coherent backscattering effect is shown. This latter gives a method to estimate D and l*. These quantities being determined, it becomes possible to infer l(a) using average time-resolved intensity measurements. Finally, some applications to coarse-grain stainless steels are discussed.

Limits of time-reversal focusing through multiple scattering: long-range correlation

Experimental results of time-reversal focusing in a high-order multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The time-reversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side-lobes appear. We interpret these side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the array's aperture. With a 16-element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of time-reversal mirrors in a random medium.

Acoustical imaging through a multiple scattering medium using a time-reversal mirror.

Acoustical imaging is based on the ability to focus an acoustic beam inside the zone of interest. This remains an issue through a high-order multiple scattering medium because the electronic delay lines that enable one to focus through a multiple scattering medium are a priori unknown. Using time-reversal principles, we show that images can be obtained through a very disordered medium. Surprisingly, the images are better than those obtained in a homogeneous medium with a classical imaging device.

Coherent backscattering of an elastic wave in a chaotic cavity.

We report the first experimental evidence of coherent backscattering enhancement for transient elastic waves propagating in a two-dimensional chaotic cavity. The time-integrated squared amplitude at the point source is twice as large as at the other points around the source. Contrary to analogous optical experiments, this effect is already clearly observable on a single realization. Especially, the spatial shape of the coherent backscattering enhancement is well predicted by a generalization of the existing theory.