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J.-C. Géminard

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Degassing regimes and associated acoustics

    A host of broad-interest phenomena involve bursting bubbles at fluid surfaces. In daily life, jam or purée cooking produces sonic bubbles that can project fragments at bursting. In the geophysical context, giant bubbles bursting at the top of a volcano vent, or at the surface of a lava lake, are examples whose understanding might be crucial for predicting volcanic activity. Although less considered, bubble bursting also occurs at the surface of aqueous foams typically produced by wash or beauty products or even by poured beer. The analysis of acoustics emission is then a natural way of investigating bursting systems, revealing the collapse or bursting mechanism and properties of the fluid gas mixture.

    We report experimental measurements of the acoustic emission associated with the bursting of a gas bubble at the free surface of a non-Newtonian fluid [Divoux, 2008]. On account of the viscoelastic properties of the fluid, the bubble is generally elongated. The associated frequency and duration of the acoustic signal are discussed with regard to the shape of the bubble and successfully accounted for by a simple linear model. The acoustic energy exhibits a high sensitivity to the dynamics of the thin film bursting, which demonstrates that, in practice, it is barely possible to deduce from the acoustic measurements the total amount of energy released by the event. Our experimental findings provide clues for the understanding of the signals from either volcanoes or foams, where one observes respectively, the bursting of giant bubbles at the free surface of lava and bubble bursting avalanches.

Images of the bubble right before bursting and associated acoustic signals. In the images (scale bar, 1 cm) one notices that the bubbles are more elongated when the gel concentration c is larger. In addition, we report the signal from the microphone outside. We observe that the typical frequency decreases and the characteristic duration of the acoustic emission increases when the bubble length is increased.

    In addition, we report an experimental study of the intermittent dynamics of a gas flowing through a column of a non-Newtonian fluid [Divoux, 2009]. In a given range of the imposed constant flow rate, the system spontaneously alternates between two regimes: bubbles emitted at the bottom either rise independently one from the other or merge to create a winding flue which then connects the bottom air entrance to the free surface. The observations, similar to observations of the degassing regime through immersed granular matter [see more here], are reminiscent of the spontaneous changes in the degassing regime observed on volcanoes and suggest that, in the nature, such a phenomenon is likely to be governed by the non-Newtonian properties of the magma. We focus on the statistical distribution of the lifespans of the bubbling and flue regimes in the intermittent steady state. The bubbling regime exhibits a characteristic time whereas, interestingly, the flue lifespan displays a decaying power-law distribution. The associated exponent, which is significantly smaller than the value 1.5 often reported experimentally and predicted in some standard intermittency scenarios, depends on the fluid properties and can be interpreted as the ratio of two characteristic times of the system.

Left : Sketch of the experimental setup. Air, from a mass-flow controller F, is injected, at a constant flow rate Q, in a chamber (volume V connected to the bottom of the fluid column, height h). By means of a pressure sensor P we measure the overpressure P inside the chamber as a function of time. In the picture on the right-hand side, one can notice that the density of bubbles trapped into the gel strongly depends on the altitude. The scale is given by the inner diameter of the plexiglass tube, 74 mm. Right : Typical overpressure variation P(t) vs time t. At a constant flow rate, the typical signal from the overpressure sensor exhibits a spontaneous alternation between rest and activity periods. Rapid drops of the overpressure mark the emission of successive bubbles at the bottom of the column whereas an almost constant overpressure corresponds to a flue connecting the injection hole to the free surface. The analysis consists of measuring the lifespan of the bubble and flue regimes. The figures from top to bottom display the signal at different time scales.

    Finally, these laboratory experiments have been performed in order to assess the physical mechanisms at stake when giant gas bubbles burst at the top of a magma conduit [Vidal, 2010]. An overpressurized gas cavity is initially closed by a thin liquid film, which suddenly bursts. The acoustic signal produced by the bursting is investigated. The key result is that the amplitude and energy of the acoustic signal strongly depend on the film rupture time. As the rupture time is uncontrolled in the experiments and in the field, the measurement of the acoustic excess pressure in the atmosphere, alone, cannot provide any information on the overpressure inside the bubble before explosion. This could explain the low energy partitioning between infrasound, seismic and explosive dynamics often observed on volcanoes. This simple experiment provides an insight into the physical mechanisms involved in the bursting of a slug of well-controlled geometry and overpressure, in static conditions. Even in a fully controlled laboratory experiment, the amplitude and energy of the pressure wave propagating into the atmosphere after bursting cannot be predicted from the initial slug overpressure and vice versa. We demonstrated that two processes are responsible for this unpredictability: (1) the rupture time of the bubble film, which cannot be controlled in the experiments; and (2) the energy loss due to the film curvature at bursting, which excites more or less efficiently the cavity. When the rupture time tburst is larger than the characteristic propagation time tprop inside the cavity, the acoustic signal amplitude (and, thus, the energy) drops. The energy fraction (Ea/Ep) transferred into the acoustic signal radiated outside decreases drastically when the rupture time tburst increases.

Acoustic emission associated with the bursting of a gas bubble at the free surface of a non-Newtonian fluid
T. Divoux, V. Vidal, F. Melo and J.-C. Géminard, Phys. Rev. E 77 (2008) 056310.

Intermittent outgassing through a non-Newtonian fluid
T. Divoux, E. Bertin E, V. Vidal and J.-C. Géminard, Phys. Rev. E 79 (2009) 056204.

Dynamics of soap bubble bursting and its implications to volcano acoustics
V. Vidal, M. Ripepe, T. Divoux, D. Legrand, J.-C. Géminard and F. Melo, Geophys. Res. Lett. 37 (2010) L07302.

Degassing and dynamics of crater formation

    We report the formation of a crater at the free surface of an immersed granular bed, locally crossed by an ascending gas flow. In two dimensions, the crater consists of two piles which develop around the location of the gas emission. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission dynamics. We describe the related granular flows and give an account of the influence of the experimental parameters, especially of the grain size and of the gas flow.

The granular flows - Initially, a thin layer of colored grains is deposited at the free surface of the initially flat and horizontal bed. Then, a series of ascending rolls pushes the grains away from the vertical central axis. Subsequently, the grains gently deposit back onto the free surface of the bed. Along the piles flanks, provided that the local angle exceeds the angle of avalanche, we observe continuous avalanches. Inside the crater, the flowing granular-material partly replaces the grains advected upwards at the center. Along the outer flanks, the deposited granular material either flows or sits at the free surface. One can clearly observe, in the inset, that new material, deposited far away from the center forms a thin layer of material which remains at rest.

Dynamics of crater formations in immersed granular materials
G. Varas, V. Vidal and J.-C.
Géminard, Phys. Rev. E 79 (2009) 021301.


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Last update : 2012-06-26.