Gas Bubbles Rising in a Complex Fluid

Collaboration with Thibaut Divoux, Valérie Vidal, Louis Gostiaux and Hervé Gayvallet

Introduction

We study experimentally the dynamics of gas bubbles rising through a column of complex fluid. The study could be relevant in some geophysical situations; volcanoes, emission of gas through a layer of sediments, ...

Abstract of our first publication

Packings of non-cohesive grains, immersed in a fluid, differ significantly from classical porous media as the grains, subjected to stresses and flows, can move within the sample, changing then the local properties of the material. We study experimentally the conditions for a gas to pass through a layer of immersed granular material. Above a threshold pressure, which depends mainly on the grains size and on the surface free energy of the liquid-gas interface, the gas creates a channel within the whole thickness of the layer.

A volume V of gas is trapped underneath an immersed granular layer. The pressure within the gas is increased by introducing a constant mass flow D of gas in the lower chamber.

The pressure P in the chamber increases almost linearly with time as long as the granular layer seals the system. When the pressure reaches a threshold value, the gas can escape the chamber, leading to a sudden pressure drop. The typical behavior of the pressure as a function of time is given in the figure above. For small mass flow, the pressure signal is almost periodic in time.

Changing the water height h shifts the pressure signal according to the hydrostatics. In the following, we give results extrapolated at h=s, the water level above the granular layer being zeroed.

The maximal pressure does not change with the thickness h of the layer as long as it remains smaller than the radius of the container.

The formation of a channel within the granular layer accounts for this result surprizing at first sight.

Both the maximum and minimum pressures reached by the signal do not depend on the mass flow D of gas. By contrast, the mean pressure within the sample decreases with increasing D.

The pressure signal loses its periodicity when the mass flow is increased. The channel can remain open for a long periods of time during which the pressure almost equals the outside pressure. Continuous flow of gas is expected above a critical mass flow D_c.

The maximum overpressure scales like 1/R where R is the typical radius of the grains. The maximum overpressure corresponds to the maximum curvature of the air-water interface when the gas passes between the grains.

At the free surface of the granular layer, advected by the liquid flow, the grains tend to form a crater around the emission hole.

Related publication

Dynamics of a gas bubble rising through a thin immersed layer of granular material :
an experimental study,
Gostiaux L., Gayvallet H. , and Géminard J.-C., Granular Matter 4 (2002) 39.