Acoustic emission associated with the bursting of a gas bubble at the free surface of a non-Newtonian fluid

An experimental device for characterizing degassing processes and related elastic fingerprints: Analog volcano seismo-acoustic observations

A challenging objective of modern volcanology is to quantitatively characterize eruptive/degassing regimes from geophysical signals (in particular seismic and infrasonic), for both research and monitoring purposes. However, the outcomes of the attempts made so far are still considered very uncertain because volcanoes remain inaccessible when deriving quantitative information on crucial parameters such as plumbing system geometry and magma viscosity. In order to improve our knowledge of volcanic systems, a novel experimental device, which is capable of mimicking volcanic degassing processes with different regimes and gas flow rates, and allowing for the investigation of the related seismo-acoustic emissions, was designed and developed. The benefits of integrating observations on real volcanoes with seismo-acoustic signals generated in laboratory are many and include (i) the possibility to fix the controlling parameters such as the geometry of the structure where the gas flows, the gas flow rate, and the fluid viscosity; (ii) the possibility of performing acoustic measurements at different azimuthal and zenithal angles around the opening of the analog conduit, hence constraining the radiation pattern of different acoustic sources; (iii) the possibility to measure micro-seismic signals in distinct points of the analog conduit; (iv) finally, thanks to the transparent structure, it is possible to directly observe the degassing pattern through the optically clear analog magma and define the degassing regime producing the seismo-acoustic radiations. The above-described device represents a step forward in the analog volcano seismo-acoustic measurements.

Determining the bubble cap film thickness of bursting bubbles from their acoustic emissions

A study of the sound generated by 2.5 mm radius bubbles bursting on the surface of fresh water is presented. The sound pulses are found to be sensitive to the time interval between the bubble reaching the water surface and bursting. Bubbles that burst within a few 10's of milliseconds behave like a Helmholtz resonator and radiate a swept chirp pulse. Bubbles that persist for 100's of milliseconds or more exhibit more complex acoustic behavior. An analysis of the resonator behavior provides an estimate of the film thickness in reasonable agreement with a fluid drainage model.

Degassing cascades in a shear-thinning viscoelastic fluid

We report the experimental study of the degassing dynamics through a thin layer of shear-thinning viscoelastic fluid when a constant air flow is imposed at its bottom. The fluid is an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Over a large range of parameters, the air is periodically released through a series of successive bubbles, hereafter named cascades. Each cascade is followed by a continuous degassing, lasting for several seconds, corresponding to an open channel crossing the fluid layer. The periodicity between two cascades does not depend on the injected flow rate. Inside one cascade, the properties of the overpressure signal associated with the successive bubbles vary continuously. The pressure threshold above which the fluid starts flowing, fluid deformation and pressure drop due to degassing through the thin fluid layer can be simply described by a Maxwell model.

Acoustic waveform of continuous bubbling in a non-Newtonian fluid

We study experimentally the acoustic signal associated with a continuous bubble bursting at the free surface of a non-Newtonian fluid. Due to the fluid rheological properties, the bubble shape is elongated, and, when bursting at the free surface, acts as a resonator. For a given fluid concentration, at constant flow rate, repetitive bubble bursting occurs at the surface. We report a modulation pattern of the acoustic waveform through time. Moreover, we point out the existence of a precursor acoustic signal, recorded on the microphone array, previous to each bursting. The time delay between this precursor and the bursting signal is well correlated with the bursting signal frequency content. Their joint modulation through time is driven by the fluid rheology, which strongly depends on the presence of small satellite bubbles trapped in the fluid due to the yield stress.

Intermittent outgassing through a non-Newtonian fluid

We report an experimental study of the intermittent dynamics of a gas flowing through a column of a non-Newtonian fluid. 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 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.