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

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Rapid settling of a colloidal gel

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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

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

The Acoustics of Liquid Foams

International audienceBeside other transport properties of liquid foams, like the optical or electrical ones, the acoustics of liquid foams reveals a great complexity and non-trivial features. Here we present a review of recent experimental and theoretical results on how a sound wave interacts with either a macroscopic foam sample or with its isolated building blocks (films and Plateau borders). The analysis of the literature allows us to determine what is now well understood , what could be measured in foams by acoustics, and what are the remaining issues and perspectives in this research field. Addresse

PROPAGATION OF ACOUSTIC WAVES IN A FOAM - PART I: EXPERIMENTS

International audienceWe present an experimental investigation of the acoustic properties of liquid foams. Velocity and attenuation of sound were measured in different foam samples, with a systematic study of the role of parameters such as the frequency, the liquid volume fraction, the bubble size, the nature of the gas, and the nature of the surfactants. Three experimental setups were used to cover a frequency range spanning three orders of magnitude (500 Hz to 600 kHz): an impedance tube [Pierre, 2013a], a pair of narrow-band 40 kHz transducers [Ben Salem, 2013], and a pair of broadband transducers [Pierre, 2013b]. Two main regimes of propagation were identified, with a limit at approximately fR=5 kHz.mm, where f is the frequency and R the average bubble size. At low frequencies (or for small bubbles), the velocity was found to follow the so called mixture law, i.e. depending only on the liquid volume fraction, with no clear influence neither on the bubble size, nor on the type of surfactant. A noticeable exception were Gillette foam samples, in which the velocity was higher than expected. The attenuation, on the other hand, was found to depend mainly on the nature of the gas and the bubble size. Three sources of attenuation were identified: thermal losses due to heat exchange during bubble oscillations, local viscous losses, and viscous losses along the wall of the tube (Kirchhoff losses). The latter term could be related to a macroscopic viscosity of the foam, whose order of magnitude and dependence on R were in good agreement with previous measurements at lower frequencies [Costa, 2013].In the second regime, at high frequencies (or for large bubbles), the propagation of acoustic waves was dispersive, with a peak of attenuation and a velocity reaching values substantially higher than in the previous regime. The frequency of the maximum of attenuation was close to the Minnaert resonance of the individual bubbles of the foams, suggesting that foams might behave acoustically like (very concentrated) bubbly liquids