Propagation of elastic waves in correlated dispersions of resonant scatterers.

The propagation of coherent longitudinal and transverse waves in random distributions of spherical scatterers embedded in an elastic matrix is studied. The investigated frequency range is the vicinity of the resonance frequencies of the translational and rotational motion of the spheres forced by the waves, where strong dispersion and attenuation are predicted. A technique for making samples made of layers of carbide tungsten beads embedded in epoxy resin is presented, which allows control of the scatterers distribution, induce short-range positional correlations, and minimize the anisotropy of samples.

Thorough ultrasonic rheology of soft, visco-elastic materials: Example of crosslinked Polyurethane elastomer.

We present a thorough procedure for measuring the rheological properties of soft, highly attenuating, visco-elastic materials at ultrasonic frequencies. The material chosen for this illustration is a crosslinked Polyurethane (PU) elastomer (Sika UR3440 type), which is widely used in the field of underwater acoustics. We determine its complex longitudinal modulus M and shear modulus G as function of frequency in the range 1-5 MHz and of temperature in the range 5-40 °C.

Numerical simulation of universal morphogenesis of fluid interface deformations driven by radiation pressure.

We report on numerical simulation of fluid interface deformations induced by either acoustic or optical radiation pressure. This is done by solving simultaneously the scalar wave propagation equation and the two-phase flow equations using the boundary element method. Using dimensional analysis, we show that interface deformation morphogenesis is universal, i.

Propagation of coherent shear waves in scattering elastic media.

We measure the reflection and transmission of shear waves by slabs of random dispersions of hard, dense spheres in a viscoelastic matrix. By modeling the slab as a Fabry-Pérot interferometer, we determine the effective wave number of coherent shear waves in this scattering medium and its effective mass density. We evidence the effect of the resonant rigid-body translation and rotation of the spheres on the propagation.

Contactless thin-film rheology unveiled by laser-induced nanoscale interface dynamics.

One of the classical limitations for the investigation of the local rheology of small scale soft objects and/or confined fluids is related to the difficulty to control mechanical contact and its consequences. In order to overcome these issues, we implement a new local, active, fast and contactless optical strategy, called optorheology, which is based on both the optical radiation pressure of a laser wave to dynamically deform a fluid interface and interferometry to probe this deformation with nanometric resolution. This optical approach is first validated by measuring the surface tension and the viscosity of transparent Newtonian liquids.

Acoustic Sensing of Forces Driving Fast Capillary Flows.

The popping sound of a bursting soap bubble is acquired using microphone arrays and analyzed using spherical harmonics decomposition. Using the theoretical framework of aeroacoustics, we demonstrate that this acoustic emission originates mainly from the capillary stresses exerted by the liquid soap film on the air and that it quantitatively reflects the out-of-equilibrium evolution of the flowing liquid film. This constitutes the proof of concept that the acoustic signature of violent events of physical or biological origin could be used to measure the forces at play during these events.

Viscoelastic shear modulus measurement of thin materials by interferometry at ultrasonic frequencies.

This paper presents a technique for measuring the complex shear modulus of thin slabs of viscoelastic solids based on the measurement of the reflection and transmission of plane shear waves through a sample inserted between two delays lines. Reproducible shear wave transmission through the sample is achieved by inserting bond layers with controlled thickness between the delay lines and the sample and by characterizing beforehand the bond rheology. The frequency dependent complex shear modulus is quantitatively evaluated from the transmission and reflection coefficients using an exact model of interferences within the delay line-bond-sample-bond-delay line sandwich and by selecting the solution among the muliple solutions of the inverse problem from considerations on time of flight and sample thickness.

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.

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.

Acoustic rotational manipulation using orbital angular momentum transfer.

We report on the first quantitative test of acoustic orbital angular momentum transfer to a sound absorbing object immersed in a viscous liquid. This is done by realizing an original experiment that is to spin a millimeter-size target disk using an ultrasonic vortex beam. We demonstrate the balance between the acoustic radiation torque calculated from the Brillouin stress tensor and the viscous torque evaluated from the steady state spinning frequency.

Liquid-column sustainment driven by acoustic wave guiding.

We report on the formation and sustainment of liquid columns with aspect ratios much larger than the value at the onset of the Rayleigh-Plateau instability. This is achieved by using the passive feedback of the radiation pressure applied on the column surface by an acoustic beam injected at the upper end of the column and guided along it. We develop an analytical model that describes the coupling between the acoustic wave guiding and the balance between acoustic and capillary surface forces exerted on the column surface and find a satisfactory agreement with the experiment.

Liquid optical fibers with a multistable core actuated by light radiation pressure.

We report on spatiotemporal behavior of self-adapted dielectric liquid columns generated and sustained by light radiation pressure. We show that single- or multivalued liquid column diameter depends on the excitation light beam. When the beam diameter is sufficiently small, we observe a well-defined stationary column diameter.

Stretching and squeezing of sessile dielectric drops by the optical radiation pressure.

We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids.

Liquid transport due to light scattering.

Using experiments and theory, we show that light scattering by inhomogeneities in the index of refraction of a fluid can drive a large-scale flow. The experiment uses a near-critical, phase-separated liquid, which experiences large fluctuations in its index of refraction. A laser beam traversing the liquid produces a interface deformation on the scale of the experimental setup and can cause a liquid jet to form.

Thermocapillary valve for droplet production and sorting.

Droplets are natural candidates for use as microfluidic reactors, if active control of their formation and transport can be achieved. We show here that localized heating from a laser can block the motion of a water-oil interface, acting as a microfluidic valve for two-phase flows. A theoretical model is developed to explain the forces acting on a drop due to thermocapillary flow, predicting a scaling law that favors miniaturization.

Bistability of a compliant cavity induced by acoustic radiation pressure.

We report on the first observation of multiple-order bistability due to acoustic radiation pressure in a compliant acoustic cavity formed between a spherical ultrasonic transducer immersed in water and the free liquid surface located at its focus. The hysteretic behavior of the cavity length, observed both with amplitude ramps and frequency sweeps, is accurately described using a one-dimensional model of a compliant Fabry-Pérot resonator assuming the acoustic radiation pressure to be the only coupling between the cavity and the acoustic field.

Light-induced deformation and instability of a liquid interface. II. Dynamics.

We study the dynamics of the deformation of a soft liquid-liquid interface by the optical radiation pressure of a focused cw Gaussian laser beam. We measured the temporal evolution of both the hump height and the hump curvature by direct observation and by detecting the focusing effect of the hump acting as a lens. Extending the results of Yoshitake [J.

Light-induced deformation and instability of a liquid interface. I. Statics.

We study in detail the deformations of a liquid-liquid interface induced by the electromagnetic radiation pressure of a focused cw laser beam. Using a simple linear model of static equilibrium of the interface under the effect of radiation pressure, buoyancy, and Laplace pressure, we explain the observed hump height variations for any value of the optical Bond number Bo=(omega0/lc)2 (lc is the capillary length and omega0 is the waist of the beam) in the regime of weak deformations and show that the deformations are independent of the direction of propagation of the laser. By increasing the beam power, we observe an instability of the interface leading to the formation of a long jet when the laser propagates from the more refringent phase to the less refringent one.

Experimental study of the Doppler shift generated by a vibrating scatterer.

We report an experimental study of the backscattering of a sound wave of frequency f by a surface vibrating harmonically at frequency F (F << f) and amplitude A in the regime where the Doppler effect overcomes bulk nonlinear effects. When the duration to of the analyzed time series of the scattered wave is small compared to the vibration period, the power spectrum of the backscattered wave is proportional to the probability density function of the scatterer velocity, which presents two peaks shifted from f by roughly 2fAomega/c (omega = 2piF). On the contrary, when t0 >> F(-1), sidebands at frequencies f +/- nF (n integer) appear in the power spectrum, which are due to the phase modulation of the backscattered wave induced by its reflection on a moving boundary.