The flow dynamics of three-phase magmas: the coupling of rheology and permeability via bubble-particle interactions

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Volcanic activity is naturally linked to the flow of molten rock, e.g. the rise of magma from a deep reservoir to the volcano, or the flow of lava down the volcano's flanks. For the assessment of hazards associated with a volcanic eruption, it is of pivotal importance to predict and model flow processes as accurately as possible. A central parameter governing the flow properties of a magma is the gas content of he melt, the amount of crystals and bubbles suspended in the viscous silicate melt, and the degree to which these two phases interact. Accounting for, or neglecting the effect of bubbles and solid particles in eruption models can change their results by the orders of magnitudes, and could make them fatally flawed. Two key issues in volcanology are affected by crystals and bubbles, and their interactions: the bulk flow behaviour of the magma or lava, i.e. its rheology, and the ability of bubbles to connect to a continuous degassing network, i.e. its permeability. The objective of this project is to perform laboratory experiments on three-phase magma-analogue samples, and to quantify the influence of bubble and crystal concentrations on rheology and permeability on the basis of these experiments. This will lead to the formulating of a 'rheological constitutive equation', i.e. an equation that relates suspension properties (e.g. bubble & particle concentration) and external forces (stress) to the resulting sample deformation. This will enable the results to be incorporated into more complex eruption models and to enhance their degree of sophistication and thus their accuracy. The experiments comprise shearing of a liquid-bubble-particle mixture between two concentric cylinders, on of which rotates. A vital part of the study will be the visualization of the interactive processes that occur when such a sample is subject to shear. During the measurement, these processes will be observable through a transparent outer cylinder. After the experiment, flow-induced bubble and particle textures of frozen samples will be accurately characterized under the microscope. The textural data obtained will then be used to reconstruct a 'virtual porous medium' with the same textural properties. For this medium, parameters such as permeability can be calculated numerically. Furthermore, the textures will compared to microtomographic 3D images of natural volcanic rocks from Stromboli volcano. This will allow inferences on shear history and permeability development processes in natural magma. Despite its central importance for many processes in natural sciences, as well as in industrial and commercial settings, the fundamental flow properties of three-phase systems remain largely unexplored. With the project proposed here, I intend to develop the first validated set of constitutive equations for suspensions containing both solid particles and bubbles, and by that hope to contribute a substantial portion towards a refined understanding of these systems.