ETUDE THEORIQUE ET EXPERIMENTALE DE LA RHEOLOGIE ET DU VIEILLISSEMENT DANS LES SUSPENSIONS COLLOIDALES EN PHASE PATEUSE

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Vibrating soap lm: origin of the dissipation

We investigate the complex dispersion relation of a transverse antisymmetric wave on a horizontal soap film. Experimentally, the complex wave number $k$ at a fixed forcing frequency is determined by measuring the vibrating amplitude of the soap film: the wavelength (linked to the real part of $k$) is determined by the spatial variation of the amplitude; the decay length (linked to the imaginary part of $k$) is determined by analyzing the resonance curves of the vibrating wave as a function of the frequency. Theoretically, we compute the complex dispersion relation taking into account the physical properties of the bulk liquid and gas phase, and of the gas-liquid interfaces. The comparison between the computation (developed to the leading order in our experimental conditions) and the experimental results confirms that the phase velocity is fixed by the interplay between surface tension, and liquid and air inertia, as reported in previous studies. Moreover, we show that the attenuation of the transverse antisymmetric wave originates from the viscous dissipation in the gas phase surrounding the liquid film. This result is an important step to understand the propagation of an acoustic wave in a liquid foam, in a bottom-up approach

Soap film vibration: origin of the dissipation

International audienceWhen a soap film is vibrated (antisymmetric mode), the wave attenuation is dominated by the viscous dissipation in surrounding air

Dissipation of ultrasonic and audible sound waves in liquid foams

Recent results have shown the rich dynamical properties of liquid foams when submitted to an acoustic wave. However the mechanisms responsible for the observed strong attenuation are still unclear. In this paper we collect and compare the main results concerning dissipation of two separate studies investigating the propagation of sound waves in liquid foams. The first one developed a model to describe the different behaviours observed at ultrasonic frequency (60-600 kHz); a reasonable agreement with experiments was found by adding a phenomenological term of dissipation. In the second study the measurement of a dissipative term of unknowm origin was performed in the audible range (0.2-5 kHz). The aim of the present paper is to make a link between the dissipation found in these two studies: by comparing the results, we find a similar value for both dissipation terms, suggesting a common origin, even if they emerge in completely different frequency ranges

Rapid settling of a colloidal gel

International audienc

Anomalous diffusion in microchannel under magnetic field

4 pagesInternational audienceWe have performed experiments to characterize the diffusion of an aqueous ferrofluid in water submitted to a magnetic field. Experiments were carried out in a microfluidic device to take advantage of the low Reynolds number flow conditions at the microscale. We have measured the concentration profile across the microchannel, which defines a characteristic length of the diffusion zone. This characteristic length varies as the square-root of the distance from the entrance of the channel divided by the mean velocity, which evidences a diffusive regime. However the application of a magnetic field is shown to inhibit the diffusion, with an increasing efficiency as the field intensity increases. We propose an explanation of this effect based on the anisotr opy of the diffusion coefficient due to the magnetic field. This hypothesis is corroborated by numerical simulations

Experimental investigation of the acoustic properties of liquids foams

International audienceLiquid foams are mixtures of gas and liquid, with a liquid volume fraction $\phi _{l} {\leq}0.3$, stabilized with surfactants. Because of their composition and structure, liquid foams are very complex and unstable media with particular acoustical properties. Experimental studies are not very numerous; they have shown a strong dependence of the velocity and attenuation of sound on parameters such as $\phi _{l}$ and the average bubble size [1]. The existent theories explain a part of the acoustic behaviour of liquid foams, but are not exactly in accordance with the experimental data found in the literature. For example, the experimental phase velocities [Phys. Rev. E 66 (2002) 021404] are larger than the velocities predicted by the Wood approximation [A. B. Wood, A Textbook of sound (Bell, London, 1932)]. We present an experimental investigation of liquid foams by an ultrasonic setup based on two broadband air transducers (40-200kHz). The acoustic properties are deduced from the reflected signal. We discuss experimental results with commercial (shaving foams) and custom-made controlled foams and compare them with available models. The evolution of the acoustic properties with the aging of the foams is also discussed

Investigating the origin of acoustic attenuation in liquid foams

Liquid foams are known to be highly efficient to absorb acoustic waves but the origin of the sound dissipation remains unknown. In this paper, we present low frequency (0.5-4kHz) experimental results measured with an impedance tube and we confront the recorded attenuations to a simple model that considers the foam as a concentrate bubbly liquid. In order to identify the influence of the different parameters constituting the foams we probe samples with different gases, and various liquid fractions and bubble size distributions. We demonstrate that the intrinsic acoustic attenuation in liquid foam is due to both thermal and viscous losses. The physical mechanism of the viscous term is not elucidated but the microscopic effective viscosity evidenced here can be described by a phenomenological law scaling with the bubble size and the gas density. In our experimental configuration a third dissipation term occurs. It comes from viscous friction on the wall of the impedance tube and it is well described by Kirchhoff law considering the macroscopic effective viscosity classically measured in rheology experiments