INVESTIGATING THE RHEOLOGY OF PARTICLE- AND BUBBLE-BEARING LAVA USING ANALOGUE FLOWS AND NUMERICAL SIMULATIONS

Lava flows show complex evolution throughout the duration of an eruption that is controlled by the rheology of the crystal- and bubble-bearing suspension. The presence of crystals and bubbles can change the effective viscosity of lavas by orders of magnitude, which has profound implications for predictions of lava flow runout and morphology. In concentrated suspensions, interactions between phases can result in non-Newtonian behavior. Bubbly suspensions are additionally complicated by the ability of bubbles to deform in response to shear. Existing models for lava flow prediction do not capture the complexity of lava rheology, especially with respect to the influence of polydisperse bubble populations that vary in space and time.

We pair numerical simulations of confined channel flows of a Herschel-Bulkley fluid with analogue flows of particle- and bubble-bearing viscous syrup suspensions. This combination allows us to determine the consistency, power-law exponent, and yield stress of the aggregate fluid. The production of bubbles through a chemical reaction allows us to generate suspensions with high volume fractions of gas (>70%). We measure the bubble number density and size distribution of our flows by analyzing digital images of post-emplacement cross-sections through the flow. The presence of polydisperse bubble populations make our experiments a better analogue to natural flows than flows with monodisperse, spherical, rigid particles. Our experimental flows have a similar Reynolds number and range of capillary numbers to natural basaltic lava flows. We particularly focus our conditions to match those likely experienced by the Fissure 8 channelized flow of the 2018 Kilauea eruption.

Our results allow us to develop parameterizations for the effect of bubbles and combined particle-bubble populations on the rheology of multiphase polydisperse suspensions, including magma and lava. The parameterization can then be used in flow emplacement forecasting models which track the evolution of crystal and bubble size distributions and total volume fraction to highlight the role of bubble nucleation, growth, and coalescence in modifying flow behavior.