Diffusing acoustic wave spectroscopy

Latex agglutination analysis by novel ultrasound scattering techniques

The latex agglutination test is employed to visualize antigen-antibody reactions through the aggregation of antibody-coated particles in the presence of an antigen. In the present study, we developed an ultrasound scattering technique to detect latex agglutination in an optically turbid media. However, the ultrasonic technique had less sensitivity to the dilute particle suspension than the optical techniques because of its wavelength. Therefore, we applied a time-correlation approach to detect small amounts of these aggregates using a sophisticated noise correction algorithm in the frequency domain. The lowest concentration of avidin used to detect aggregations of the biotin-coated particle using the ultrasound scattering technique was found to be 0.625 μg/ml. Furthermore, since the density differences between the particle and liquid were larger for silica suspensions than for polystyrene (PS) suspensions, a larger signal was proposed to be expected from silica suspensions. Nevertheless, it was found that latex agglutinations with the PS particle were more sensitive than those with the silica particles. The dynamic ultrasound scattering analysis along the sedimentation direction also supported the presence of strongly scattered intensity components of the PS aggregates, which is proposed to be due to the resonance scattering from PS spherical particles. Therefore, this technique can be employed to enhance scattering signals from particles for application in the agglutination test using ultrasound.

Detection of defects in a 2D fluid-solid periodic cluster

The present work deals with locating a defect buried in a medium composed of a fluid matrix and small solid inhomogeneities. Classical imaging methods are based on delay and sum principle and would implicitly assume that the undamaged medium is homogeneous. The topological imaging framework however allows to take into account the heterogeneous nature of the undamaged medium and potentially to take advantage of it. In this work, it is applied to a demanding test case with different assumptions on the knowledge of the medium properties using a specifically-designed fluid-solid compatible imaging function. It leads to the definition of three imaging processes whose results are compared using respectively synthetic and experimental data. The results show the relevance of using the inhomogeneities' locations information, but not necessarily at all steps of the imaging process, leading to the definition of so called hybrid topological imaging method.

Probing intermittency and reversibility in a dense granular suspension under shear using multiply scattered ultrasound

We study the rheology of a dense granular suspension under shear strain with the simultaneous detection of multiply scattered ultrasound through the shear band. At a low shear rate, the dissipation is rate-independent and determined by the frictional contacts between grains. Under quasistatic shear, the stress-strain curve contains elastic loading parts interrupted by stress drops. Such an intermittency is concomitant with some large decorrelation events as measured by the ultrasound probe, sensitive to the position of the grains. Under cyclic shear, the correlations between the scattered ultrasonic waves show that at low shear strain, the grains exhibit reversible motion. Beyond this linear regime, some irreversible motion of the grains is detected. Moreover, the correlation between successive ultrasound signals suggests that some specific rearrangements, which add to the homogeneous flow, take place near the maximum strain.

Tracking fluids in multiple scattering and highly porous materials: Toward applications in non-destructive testing and seismic monitoring

Seismic and ultrasonic waves are sometimes used to track fluid injections, propagation, infiltrations in complex material, including geological and civil engineered ones. In most cases, one use the acoustic velocity changes as a proxy for water content evolution. Here we propose to test an alternative seismic or acoustic observable: the waveform decorrelation. We use a sample of compacted millimetric sand as a model medium of highly porous multiple scattering materials. We fill iteratively the sample with water, and track changes in ultrasonic waveforms acquired for each water level. We take advantage of the high sensitivity of diffuse coda waves (late arrivals) to track small water elevation in the material. We demonstrate that in the mesoscopic regime where the wavelength, the grain size and the porosity are in the same order of magnitude, Coda Wave Decorrelation (waveform change) is more sensitive to fluid injection than Coda Wave Interferometry (apparent velocity change). This observation is crucial to interpret fluid infiltration in concrete with ultrasonic record changes, as well as fluid injection in volcanoes or snow melt infiltration in rocky glaciers. In these applications, Coda Wave Decorrelation might be an extremely interesting tool for damage assessment and alert systems.

Locating structural changes in a multiple scattering domain with an irregular shape

Locadiff is a method for imaging local structural changes in a random, heterogeneous medium. It relies on the combination of a forward model to calculate the sensitivity kernel of the source-receiver pairs, with an inversion method to determine the position of the changes. So far, the sensitivity kernel has been evaluated based on an analytical solution of the diffusion equation, which lacks the flexibility to handle problems where the domain has boundaries with an irregular shape. Moreover, the accuracy of the previous inversion method, based on linear algebra tools, was very sensitive to the values of the inversion parameters. This paper introduces a more generic approach to solve both these issues. The first problem is tackled by the implementation of a numerical method as an alternative for solving the diffusion equation. The second problem is tackled by the introduction of enhanced optimization algorithms to improve the stability of the inversion. This improved version of Locadiff is validated via both numerical examples and experimental data from an actual civil engineering problem.

Bubbly flow velocity measurement in multiple scattering regime

We propose a technique to measure the velocity of a bubble cloud based on the coda correlation. The method is founded on successive recordings of multiple scattered waves from a bubble cloud. Our model predicts the dependence between the correlation coefficient of these coda waves and the velocity of the bubble cloud under diffusion approximation. The Acoustic experiments are validated by simultaneous optical measurements in a water tank, with a good agreement between the acoustical and the optical methods (relative difference smaller than 7%). This technique can be transposed to any particle flow velocity problems involving multiple scattering effects in acoustics.

Sensitivity of coda wave interferometry to fluid migration through rock

The sound speed of a porous medium changes with fluid substitution when the fluids have different acoustic properties. The authors demonstrate that coda wave interferometry is capable of sensing subtle local sound speed changes associated with minute fluid displacements, Δh. In fact the resolution on fluid motion is given by a simple scaling relationship, Δh_min/λ∼t^-γe^2αt, where t is the waveform time, λ is the wavelength, γ is a constant that varies based on the nature of the acoustic propagation, and α is a system specific acoustic attenuation coefficient. In contrast to the conventional notion that later arrivals (further into the coda) give greater sensitivity to fluid movement, this scaling relationship suggests that there is a temporal optimum in sensitivity to Δh. This is the case even though later arrivals exhibit signal intensities well above the noise floor. The authors elucidate the physical basis for determining the waveform time at which the sensitivity is optimal.

Multiple scattering of an ultrasonic shock wave in bubbly media

This experimental study deals with the propagation of an ultrasonic shock wave in a random heterogeneous medium, constituted of identical 75μm radius bubbles, trapped in a yield-stress fluid. The fundamental frequency of the incident wave (in the MHz range) was much larger than the resonance frequency of bubbles (38kHz). A well-expanded coda, resulting from the multiple scattering of the incident shock wave through the heterogeneous medium, was experimentally measured in transmission. Despite the significant amplitude of the shock wave (90kPa), no sign of nonlinear response of the bubbles was detected. Both the coherent and incoherent fields were successfully described by linear theories. Using a shock wave presents the advantage of characterizing the medium over a large frequency range (1.5-15MHz).

Contact laws between nanoparticles: the elasticity of a nanopowder

Studies of the mechanical contact between nanometer-scale particles provide fundamental insights into the mechanical properties of materials and the validity of contact laws at the nanoscale which are still under debate for contact surfaces approaching atomic dimensions. Using in situ Brillouin light scattering under high pressure, we show that effective medium theories successfully predict the macroscopic sound velocities in nanopowders if one takes into account the cementation of the contacts Our measurements suggest the relevance of the continuum approach and effective medium theories to describe the contact between nanoparticles of diameters as small as 4 nm, i.e. with radii of contact of a few angstroms. In particular, we demonstrate that the mechanical properties of nanopowders strongly depend on the surface state of the nanoparticles. The presence of molecular adsorbates modifies significantly the contact laws.

Dynamic sound scattering: Field fluctuation spectroscopy with singly scattered ultrasound in the near and far fields

Dynamic sound scattering (DSS) is a powerful acoustic technique for investigating the motion of particles or other inclusions inside an evolving medium. In DSS, this dynamic information is obtained by measuring the field autocorrelation function of the temporal fluctuations of singly scattered acoustic waves. The technique was initially introduced 15 years ago, but its technical aspects were not adequately discussed then. This paper addresses the need for a more complete account of the method by describing in detail two different implementations of this sound scattering technique, one of which is specifically adapted to a common experimental situation in ultrasonics. The technique is illustrated by the application of DSS to measure the mean square velocity fluctuations of particles in fluidized suspensions, as well as the dynamic velocity correlation length. By explaining the experimental and analytical methods involved in realizing the DSS technique in practice, the use of DSS will be facilitated for future studies of particulate suspension dynamics and particle properties over a wide range of particle sizes and concentrations, from millimeters down to nanometers, where the use of optical techniques is often limited by the opacity of the medium.

Diffuse ultrasound monitoring of stress and damage development on a 15-ton concrete beam

This paper describes the use of an ultrasonic imaging technique (Locadiff) for the Non-Destructive Testing & Evaluation of a concrete structure. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through the monitoring of a 15-ton reinforced concrete beam subject to a four-point bending test causing cracking. The experimental results show that Locadiff achieved to (1) detect and locate the cracking zones in the core of the concrete beam at an early stage by mapping the changes in the concrete's micro-structure; (2) monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acousto-elastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. The performances of the Locadiff technique in the detection of early stage cracking are assessed and discussed.

Dynamics of micron-sized particles in dilute and concentrated suspensions probed by dynamic ultrasound scattering techniques

A novel ultrasound technique called Frequency-Domain Dynamic ultraSound Scattering (FD-DSS) was employed to determine sedimentation velocities and the diameters of microparticles in a highly turbid suspension. The paper describes the importance of the scattering vector q for dynamic scattering experiments using broadband ultrasound pulses because q (or frequency) corresponds to the spatial length scale whereas the pulses involve inevitable uncertainty in the time domain due to the frequency distribution of broadband pulse. The results obtained from Stokes velocity of monodispersed silica and polydivinylbenzene (PDVB) particles were compared to those obtained by a Field Emission Scanning Electron Microscope (FE-SEM). A novel method to extract the particle size distribution is also demonstrated based on an ultrasound scattering theory.

Imaging multiple local changes in heterogeneous media with diffuse waves

This study focuses on imaging local changes in heterogeneous media. The method employed is demonstrated and validated using numerical experiments of acoustic wave propagation in a multiple scattering medium. Changes are simulated by adding new scatterers of different sizes at various positions in the medium, and the induced decorrelation of the diffuse (coda) waveforms is measured for different pairs of sensors. The spatial and temporal dependences of the decorrelation are modeled through a diffuse sensitivity kernel, based on the intensity transport in the medium. The inverse problem is then solved with a linear least square algorithm, which leads to a map of scattering cross section density of the changes.

Revealing the structure of a granular medium through ballistic sound propagation

We study the propagation of sound through a bidimensional granular medium consisting of photoelastic disks, which are packed into different crystalline and disordered structures. Acoustic sensors placed at the boundaries of the system capture the acoustic signal produced by a local and well-controlled mechanical excitation. By compressing the system, we find that the speed of the ballistic part of the acoustic wave behaves as a power law of the applied force with both exponent and prefactor sensitive to the internal geometry of the contact network. This information, which we are able to link to the force-deformation relation of single grains under different contact geometries, provides enough information to reveal the structure of the granular medium.

Local time in diffusive media and applications to imaging

Local time is the measure of how much time a random walk has visited a given position. In multiple scattering media, where waves are diffuse, local time measures the sensitivity of the waves to the local medium's properties. Local variations of absorption, velocity, and scattering between two measurements yield variations in the wave field. These variations are proportional to the local time of the volume where the change happened and the amplitude of variation. The wave field variations are measured using correlations and can be used as input in a inversion algorithm to produce variation maps. The present article gives the expression of the local time in dimensions one, two, and three and an expression of its fluctuations, in order to perform such inversions and estimate their accuracy.

The seismic noise wavefield is not diffuse

Passive seismology is burgeoning under the apparent theoretical support of diffuse acoustics. However, basic physical arguments suggest that this theory may not be applicable to seismic noise. A procedure is developed to establish the applicability of the diffuse field paradigm to a wavefield, based on testing the latter for azimuthal isotropy and spatial homogeneity. This procedure is then applied to the seismic noise recorded at 65 sites covering a wide variety of environmental and subsoil conditions. Considering the instantaneous oscillation vector measured at single triaxial stations, the hypothesis of azimuthal isotropy is rejected in all cases with high confidence, which makes the spatial homogeneity test unnecessary and leads directly to conclude that the seismic noise wavefield is not diffuse. However, such a conclusion has no practical effect on passive imaging, which is also possible in non-diffuse wavefields.

High-efficiency second-harmonic generation in doubly-resonant χ(²) microring resonators

By directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving high-efficiency second harmonic generation (SHG) in a structure consisting of a microscale doubly-resonant ring resonator side-coupled to two adjacent waveguides. We find that ≳ 94% conversion efficiency can be attained at telecom wavelengths, for incident powers in the milliwatts, and for reasonably large bandwidths (Q ∼ 1000s). We demonstrate that in this high efficiency regime, the system also exhibits limit-cycle or bistable behavior for light incident above a threshold power. Our numerical results agree to within a few percent with the predictions of a simple but rigorous coupled-mode theory framework.

A statistical approach to direct density of states measurements in disordered systems

A statistical method for measuring the modal density of elastic waves through direct mode counting in strongly scattering disordered systems is presented. To illustrate this approach, the results of ultrasonic experiments in a highly porous sintered glass bead network are reported. This method is shown to yield a reliable and robust measurement of the density of states, enabling mode-counting techniques to be applied to increasingly complex systems, where modal overlap and sensitivity to experimental conditions have previously hampered definitive results.

Probing slow dynamics of consolidated granular multicomposite materials by diffuse acoustic wave spectroscopy

The stiffness of a consolidated granular medium experiences a drop immediately after a moderate mechanical solicitation. Then the stiffness rises back toward its initial value, following a logarithmic time evolution called slow dynamics. In the literature, slow dynamics has been probed by macroscopic quantities averaged over the sample volume, for instance, by the resonant frequency of vibrational eigenmodes. This article presents a different approach based on diffuse acoustic wave spectroscopy, a technique that is directly sensitive to the details of the sample structure. The parameters of the dynamics are found to depend on the damage of the medium. Results confirm that slow dynamics is, at least in part, due to tiny structural rearrangements at the microscopic scale, such as inter-grain contacts.

Temporal changes in the lunar soil from correlation of diffuse vibrations

It was recently demonstrated that one can reconstruct the impulse response between passive sensors by cross-correlating diffuse waves or ambient noise. Using seismic waves recorded on the moon, we show here that not only direct waves can be retrieved, but also late arrivals that have been scattered before reaching the seismometers. As these late arrivals propagate for a longer time, they are more sensitive to weak perturbations of the medium such as velocity changes. This high sensitivity of scattered waves is used to monitor periodic velocity changes in the lunar soil by measuring small delays of the passively retrieved coda waves. The velocity changes result from temperature variations due to periodic heating of the lunar surface by the sun.

Diffusive transport of light in a two-dimensional disordered packing of disks: analytical approach to transport mean free path

We study photon diffusion in a two-dimensional random packing of monodisperse disks as a simple model of granular media and wet foams. We assume that the intensity reflectance of disks is a constant r . We present an analytic expression for the transport mean free path l;{*} in terms of the velocity of light in the disks and host medium, radius R and packing fraction of the disks, and the intensity reflectance. For glass beads immersed in air or water, we estimate transport mean free paths about half the experimental ones. For air bubbles immersed in water, l;{*}R is a linear function of 1epsilon , where epsilon is the liquid volume fraction of the model wet foam. This throws light on the empirical law of Vera [Appl. Opt. 40, 4210 (2001)] and promotes more realistic models.

Sound velocity and attenuation in bubbly gels measured by transmission experiments

Measurements of the phase velocity and attenuation of sound in concentrated samples of bubbly gels are presented. Hair gel was used as a matrix material to obtain well characterized distributions of bubbles. Ultrasonic measurements were conducted over a large range of frequencies, including the resonance frequencies of the bubbles. Surprisingly good agreement with Foldy's prediction was found, even for monodisperse samples at resonance frequencies, up to volume fraction of 1%. Beyond this concentration, the effects of high-order multiple scattering were observed. These results support the feasability of ultrasonic techniques to investigate the size distribution of bubbles in a weak gel or liquid.

Temperature-dependent diffusing acoustic wave spectroscopy with resonant scatterers

The influence of a slight temperature change on the correlation of multiply scattered acoustic waves is studied, and experimental results are discussed. The technique presented here, similar to diffusing-acoustic-wave spectroscopy, is based on the sensitivity of a multiply scattering medium to a slight change. Ultrasonic waves around 3 MHz are transmitted through a sample made of steel rods in water and recorded by an array of transducers at different temperatures. The cross correlations between highly scattered signals are computed. The main effect of the temperature change is a simple dilation of the times of arrival, due to a change of the sound velocity in water. But the scatterers also play a role in the progressive decorrelation of wave forms. An analysis resolved in both time and frequency shows that at some particular frequencies, the resonant behavior of the scatterers is responsible for a significantly larger decorrelation. Interestingly, the experimental results allow one to detect the presence of a small resonance that was not detected earlier on the same scatterers with classical measurement of the scattering mean free path. A simple model is proposed to interpret the experimental results.

Characterization of ultrasound-induced fracture of polymer-shelled ultrasonic contrast agents by correlation analysis

Beyond a characteristic value of the negative peak pressure, ultrasound fracture the shell of ultrasonic contrast agents (UCAs). Existing criteria for ascertaining this threshold value exploit the dependence of the amplitude of the UCA acoustic response on the incident pressure. However, under the common experimental conditions used in this work, these criteria appear to be unreliable when they are applied to UCAs that are stabilized by a thick polymeric shell. An alternative criterion for determining the onset of shell fracture is introduced here, which uses variations of the shape of the acoustic time-domain response of an UCA suspension. Experimental evidence is presented that links the changes of the cross-correlation coefficient between consecutive time-domain signals to the fracture of the shells, and consequent release of air microbubbles. In principle, this criterion may be used to characterize similar properties of other types of particles that cannot undergo inertial cavitation.

Ultrasonic properties of a suspension of microspheres supporting negative group velocities

The ultrasonic attenuation coefficient, phase velocity, and group velocity spectra are reported for a suspension that supports negative group velocities. The suspension consists of plastic microspheres with an average radius of 80 microm in an aqueous medium at a volume fraction of 3%. The spectra are measured using a broadband method covering a range from 2 to 20 MHz. The suspension exhibits negative group delays over a band near 4.5 MHz, with the group velocity magnitude exceeding 4.3 x 10(8) m/s at one point. The causal consistency of these results is confirmed using Kramers-Kronig relations.

Mesoscopic phase statistics of diffuse ultrasound in dynamic matter

Temporal fluctuations in the phase of waves transmitted through a dynamic, strongly scattering, mesoscopic sample are investigated using ultrasonic waves, and compared with theoretical predictions based on circular Gaussian statistics. The fundamental role of phase in diffusing acoustic wave spectroscopy is revealed, and phase statistics are also shown to provide a sensitive and accurate way to probe scatterer motions at both short and long time scales.

Diffusive wave spectroscopy of a random close packing of spheres

We are interested in the propagation of light in a random packing of dielectric spheres within the geometrical optics approximation. Numerical simulations are performed using a ray tracing algorithm. The effective refractive indexes and the transport mean free path are computed for different refractive indexes of spheres and intersticial media. The variations of the optical path length under small deformations of the spheres assembly are also computed and compared to the results of Diffusive Wave Spectroscopy experiments. Finally, we propose a measure of the transport mean free path and a Diffusive Wave Spectroscopy experiment on a packing of glass spheres. The results of those experiments agree with the predictions of this ray tracing approach.

Observation of coherent precursors in pulsed mode-stirred reverberation fields

We report on measured statistics of pulsed radio-frequency fields inside a dynamic (mode-stirred) reverberant cavity. Unlike for time-harmonic excitation, the early transient received power during the buildup of energy is quasiperiodic with respect to the stir angle and exhibits unusually long correlations. These correlations decay at the same exponential rate as the increase of energy. By contrast, the probability distribution of the power reaches its asymptotic form in a more rapid but oscillatory manner.

Time-resolved diffusing wave spectroscopy beyond 300 transport mean free paths

We presented theoretical and experimental demonstrations of the possibilities of performing time-resolved diffusing wave spectroscopy: We successfully registered field fluctuations for selected photon path lengths that can surpass 300 transport mean free paths. Such performance opens new possibilities for biomedical optics applications.

Observation of multiple scattering of kHz vibrations in a concrete structure and application to monitoring weak changes

In this paper, we present two experimental studies of mechanical wave propagation in a concrete building around 1 kHz. The first experiment is devoted to the observation of the coherent backscattering enhancement, which demonstrates the presence of multiple diffractions in the late part of the wave records. An application of multiple diffraction and reverberations is proposed in a second experiment. Thanks to their sensitivity to weak changes of the medium, the late records are used to monitor weak change in concrete wave velocity induced by thermal variations. The velocity change measurements have a precision of deltac/c=10(-4). Such a precision is difficult to obtain with direct waves. This experiment is the first step to other applications like stress, damage, aging, or crack monitoring in concrete structures.

Time-resolved-correlation measurements of temporally heterogeneous dynamics

Time resolved correlation (TRC) is a recently introduced light scattering technique that allows one to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by cI(t, tau), the degree of correlation between speckle images recorded at time t and t + tau. Heterogeneous dynamics results in significant fluctuations of cI(t,tau) with time t. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of cI are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number N of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a N--> infinity extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.

Extracting the Green's function from the correlation of coda waves: a derivation based on stationary phase

The Green's function of waves that propagate between two receivers can be found by cross-correlating multiply scattered waves recorded at these receivers. This technique obviates the need for a source at one of these locations, and is therefore called "passive imaging." This principle has been explained by assuming that the normal modes of the system are uncorrelated and that all carry the same amount of energy (equipartitioning). Here I present an alternative derivation of passive imaging of the ballistic wave that is not based on normal modes. The derivation is valid for scalar waves in three dimensions, and for elastic surface waves. Passive imaging of the ballistic wave is based on the destructive interference of waves radiated from scatterers away from the receiver line, and the constructive interference of waves radiated from secondary sources near the receiver line. The derivation presented here shows that the global requirement of the equipartitioning of normal modes can be relaxed to the local requirement that the scattered waves propagate on average isotropically near the receivers.

Coda-wave interferometry in finite solids: recovery of P-to-S conversion rates in an elastodynamic billiard

We study the temperature dependence of diffuse reverberant ultrasound in elastic bodies. Transient wave forms are found to undergo an almost pure dilation of 0.0277% per degree, related to the temperature dependence of wave speeds. The wave forms also suffer a distortion that, we argue, depends on the rate of conversion between the dilatational (P) and shear (S) waves. Distortion is found to scale in a manner consistent with theoretical arguments but also appears to be a function of the degree of ray chaos in the body, indicating that the mixing rates are slower in more regular bodies.

Field fluctuation spectroscopy in a reverberant cavity with moving scatterers

We report a study of transient ultrasonic waves inside a reverberant cavity containing moving scatterers. We show that the elastic mean free path and the dynamics of the scatterers govern the evolution of the autocorrelation of acoustic wave field. A parallel is established between these results and a closely related technique, diffusing acoustic wave spectroscopy. Excellent agreement is found between experiment and theory for a moving stainless steel ball in a water tank, thereby elucidating the underlying physics, and a potential application, fish monitoring inside aquariums, is demonstrated.