Tony Valier-Brasier

Scaled behavior of interface waves at an imperfect solid-solid interface

Laser ultrasonic techniques allow the remote analysis of adhesion mechanisms at imperfect interfaces up to GHz frequencies. However, the sensitivity of interface waves to the properties of the contact is not very well known. In the present work, the mechanical boundary conditions are described considering that the contacting solid half-spaces are connected by tangential and normal springs. Such a modeling implies a discontinuity of the displacement field across the interface. To identify the relative amplitudes of the different types of interface waves—skimming, leaky Rayleigh (LR) and Stoneley (St) waves—a semi-analytical time domain model describing the thermoelastic laser generation is derived. The results illustrate the influence of the boundary conditions on the attenuation of the LR wave and on the existence of the St wave. In addition, a single compact and elegant dispersion equation is presented to investigate the behaviour of the interface waves propagating along a generalized imperfect boundary....

On the modeling of modes coupling in dissipative fluid-filled waveguide with corrugated surfaces

This paper aims at providing an alternative analytical model, which would be more suitable than a previous one [C. Potel and M. Bruneau, J. Sound Vib. 313, 738 (2008)], to describe the mode coupling due to scattering on small one-dimensional irregularities (parallel ridges) of the surfaces of a fluid-filled waveguide. Both models rely on standard integral formulation and modal analysis, the acoustic field being expressed as a coupling between eigenmodes of a regularly shaped waveguide, which bounds outwardly the corrugated waveguide considered. But the model presented here departs from the previous one essentially because it starts from the integral formulation for the acoustic pressure field, the solution relying on a modal expansion, whereas the previous one starts from the inner product of the set of differential equations (which govern the acoustic pressure field) and the appropriate eigenfunctions, the solution being obtained from using a one-dimensional integral formulation. Substituting this altern...

Modes coupling of shear acoustic waves polarized along a one-dimensional corrugation on the surfaces of an isotropic solid plate

This paper aims at providing an analytical model, suitable to highlight the mode coupling due to scattering on small one-dimensional irregularities (parallel ridges) of the surfaces of isotropic solid plates, when shear horizontal waves polarized along the ridges propagate perpendicularly to them. An impedancelike boundary condition at the interface between the teeth (the ridges) and the inner plate (bounded outwardly by the ridges) accounts for the inertia of the teeth to describe the roughness. Using the integral formulation, the displacement field is expressed as a coupling between eigenmodes of the inner regular-shaped plate.

Influence of shell compressibility on the ultrasonic properties of polydispersed suspensions of nanometric encapsulated droplets

Liquid droplets of nanometric size encapsulated by a polymer shell are envisioned for targeted drug delivery in therapeutic applications. Unlike standard micrometric gas-filled contrast agents used for medical imaging, these particles present a thick shell and a weakly compressible core. Hence, their dynamical behavior may be out of the range of validity of the models available for the description of encapsulated bubbles. In the present paper, a model for the ultrasound dispersion and absorption in a suspension of nanodroplets is proposed, accounting for both dilatational and translational motions of the particle. The radial motion is modeled by a generalized Rayleigh-Plesset-like equation which takes into account the compressibility of the viscoelastic shell, as well as the one of the core. The effect of the polydispersity of particles in size and shell thickness is introduced in the coupled balance equations which govern the motion of the particles in the surrounding fluid. Both effects of shell compressibility and polydispersity are quantified through the dispersion and absorption curves obtained on a wide ultrasonic frequency range. Finally, some results for larger gas-filled particles are also provided, revealing the limit of the role of the shell compressibility.

Beam distortion detection and deflectometry measurements of gigahertz surface acoustic waves

Gigahertz acoustic waves propagating on the surface of a metal halfspace are detected using different all-optical detection schemes, namely, deflectometry and beam distortion detection techniques. Both techniques are implemented by slightly modifying a conventional reflectometric setup. They are then based on the measurement of the reflectivity change but unlike reflectometric measurements, they give access to the sample surface displacement. A semi-analytical model, taking into account optical, thermal, and mechanical processes responsible for acoustic waves generation, allows analyzing the physical content of the detected waveforms.

Shear acoustic waves polarized along the ridged surface of an isotropic solid plate: Mode coupling effects due to the shape profile

The aim of the paper is to describe the modes coupling due to scattering on small one-dimensional irregularities (parallel ridges) of the surface of isotropic solid plates, when shear horizontal waves (SH-waves) polarized along the ridges propagate perpendicularly to them. In a previous paper [Valier-Brasier et al., Appl. Phys. Lett. 93, 164101 (2008)], an analytical model was presented for describing the roughness by inertia of “teeth” which bound the ridges, through an impedancelike boundary condition, whatever shape of the roughness is. In the present paper, this shape is accounted for through a more sophisticated model, used previously for describing the effects of the roughness of walls on acoustic pressure fields in fluid-filled waveguides [Valier-Brasier et al., J. Appl. Phys. 106, 034913 (2009)], and adapted here in order to describe the modes coupling due to the scattering of these SH-waves. Moreover, the effect of a spatial periodicity of the ridges on the modes coupling is discussed, emphasizin...

Sound propagation in dilute suspensions of spheres: Analytical comparison between coupled phase model and multiple scattering theory.

Sound propagation in dilute suspensions of small spheres is studied using two models: a hydrodynamic model based on the coupled phase equations and an acoustic model based on the ECAH (ECAH: Epstein-Carhart-Allegra-Hawley) multiple scattering theory. The aim is to compare both models through the study of three fundamental kinds of particles: rigid particles, elastic spheres, and viscous droplets. The hydrodynamic model is based on a Rayleigh-Plesset-like equation generalized to elastic spheres and viscous droplets. The hydrodynamic forces for elastic spheres are introduced by analogy with those of droplets. The ECAH theory is also modified in order to take into account the velocity of rigid particles. Analytical calculations performed for long wavelength, low dilution, and weak absorption in the ambient fluid show that both models are strictly equivalent for the three kinds of particles studied. The analytical calculations show that dilatational and translational mechanisms are modeled in the same way by both models. The effective parameters of dilute suspensions are also calculated.

Random acoustic metamaterial with a subwavelength dipolar resonance.

The effective velocity and attenuation of longitudinal waves through random dispersions of rigid, tungsten-carbide beads in an elastic matrix made of epoxy resin in the range of beads volume fraction 2%-10% are determined experimentally. The multiple scattering model proposed by Luppé, Conoir, and Norris [J. Acoust. Soc. Am. 131(2), 1113-1120 (2012)], which fully takes into account the elastic nature of the matrix and the associated mode conversions, accurately describes the measurements. Theoretical calculations show that the rigid particles display a local, dipolar resonance which shares several features with Minnaert resonance of bubbly liquids and with the dipolar resonance of core-shell particles. Moreover, for the samples under study, the main cause of smoothing of the dipolar resonance of the scatterers and the associated variations of the effective mass density of the dispersions is elastic relaxation, i.e., the finite time required for the shear stresses associated to the translational motion of the scatterers to propagate through the matrix. It is shown that its influence is governed solely by the value of the particle to matrix mass density contrast.

Analytical approach of Lamb waves coupling in rough isotropic plates

The present work aims at contributing to the investigation of an analytical method to describe Lamb waves which undergoes Lamb waves coupling when propagating along the rough surfaces of finite extent of isotropic, solid, and infinite plates. The motions considered are assumed to be independent of one of the coordinates for which the component of the displacement is equal to zero (two-dimensional problem). In some respect, the analytical approach is an extension of an analytical model describing the coupling of SH waves due to ridges (on the surface of a plate) parallel to the polarization of the waves, but it is treated in a somewhat different manner because the acoustic field involves here both the longitudinal and the transversal displacements of the Lamb waves. The formalism relies on an integral formulation, using Green’s functions which permit to express Lamb waves perturbations, to describe the coupling process between the longitudinal and the transversal components of the forward and the backward ...

A simple acoustofluidic chip for microscale manipulation using evanescent Scholte waves

Acoustofluidics is acknowledged as a powerful tool offering a contactless and label-free manipulation of fluids, micro-beads, and living cells. To date, most techniques rely on the use of propagating acoustic waves and take advantage of the associated acoustic radiation force in standing or progressive fields. Here, we present a new approach based on the generation of an evanescent acoustic field above a substrate. This field is obtained by means of subsonic interfacial waves giving rise to a well-defined standing wave pattern. By both imaging and probing the evanescent acoustic field, we show that these interfacial waves are guided waves known as quasi-Scholte acoustic waves. Scholte waves present very interesting features for applications in acoustofluidics. Namely, they confine the acoustic energy to the vicinity of the surface, they are nearly lossless and thus can propagate over long distances along the substrate, and finally they do not require any particular material for the substrate. With a very simple and low-cost device we show several examples of applications including patterning lines or arrays of cells, triggering spinning of living cells, and separating plasma from RBC in a whole blood microdroplet.