Perfect absorption of water waves by linear or nonlinear critical coupling

We report on experiments of perfect absorption for surface gravity waves impinging a wall structured by a subwavelength resonator. By tuning the geometry of the resonator, a balance is achieved between the radiation damping and the intrinsic viscous damping, resulting in perfect absorption by critical coupling. Besides, it is shown that the resistance of the resonator, hence the intrinsic damping, can be controlled by the wave amplitude, which provides a way for perfect absorption tuned by nonlinear mechanisms. The perfect absorber that we propose, without moving parts or added material, is simple, robust and it presents a deeply subwavelength ratio wavelength/size $\simeq 18$

Elastic instability in stratified core annular flow

We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -- it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.Comment: 4 pages, 4 figure

Equilibrium and nonequilibrium fluctuations at the interface between two fluid phases

We have performed small-angle light-scattering measurements of the static structure factor of a critical binary mixture undergoing diffusive partial remixing. An uncommon scattering geometry integrates the structure factor over the sample thickness, allowing different regions of the concentration profile to be probed simultaneously. Our experiment shows the existence of interface capillary waves throughout the macroscopic evolution to an equilibrium interface, and allows to derive the time evolution of surface tension. Interfacial properties are shown to attain their equilibrium values quickly compared to the system's macroscopic equilibration time.Comment: 10 pages, 5 figures, submitted to PR

Linear stability analysis of a horizontal phase boundary separating two miscible liquids

The evolution of small disturbances to a horizontal interface separating two miscible liquids is examined. The aim is to investigate how the interfacial mass transfer affects development of the Rayleigh-Taylor instability and propagation and damping of the gravity-capillary waves. The phase-field approach is employed to model the evolution of a miscible multiphase system. Within this approach, the interface is represented as a transitional layer of small but nonzero thickness. The thermodynamics is defined by the Landau free energy function. Initially, the liquid-liquid binary system is assumed to be out of its thermodynamic equilibrium, and hence, the system undergoes a slow transition to its thermodynamic equilibrium. The linear stability of such a slowly diffusing interface with respect to normal hydro- and thermodynamic perturbations is numerically studied. As a result, we show that the eigenvalue spectra for a sharp immiscible interface can be successfully reproduced for long-wave disturbances, with wavelengths exceeding the interface thickness. We also find that thin interfaces are thermodynamically stable, while thicker interfaces, with the thicknesses exceeding an equilibrium value, are thermodynamically unstable. The thermodynamic instability can make the configuration with a heavier liquid lying underneath unstable.We also find that the interfacial mass transfer introduces additional dissipation, reducing the growth rate of the Rayleigh Taylor instability and increasing the dissipation of the gravity waves. Moreover, mutual action of diffusive and viscous effects completely suppresses development of the modes with shorter wavelengths

Visualisation d'écoulements courbes

Nous décrivons une méthode de visualisation d'écoulements courbes par nappes laser, particulièrement adaptée à l'étude des instabilités centrifuges, telles que l'instabilité de Taylor-Couette, de Dean, de Taylor-Görtler. Cette technique permet de visualiser de façon fine ce qui se passe très près de la paroi courbe, et la manière dont croît l'instabilité le long de l'écoulement. Le principe est simple et permet d'étendre le concept de nappe laser plane à celui de nappe laser conique ou cylindrique. Il consiste à placer dans un faisceau laser une lentille cylindrique inclinée par rapport à la direction du faisceau incident, et une optique de convergence selon que l'on veuille une nappe laser conique ou cylindrique