The Effect of Mechanical Shaking on the Rising Velocity of Bubbles in High-Viscosity Shear-Thinning Fluids

The rising velocity of an air bubble in a non-Newtonian shear-thinning fluid at low Reynolds numbers is generally similar to the Newtonian case given by Stokes' law. However, when the shear-thinning fluid is subject to mechanical oscillations, the rising velocity could significantly increase. Here, we present a series of experiments quantifying the rising velocity of single bubbles during shaking in very high-viscosity (2,000-30,000 Pa.s) shear-thinning silicone oils. Air bubbles (18-30 mm diameter) were injected in a tank mounted on a shaking table. The tank was horizontally oscillated, at accelerations between 0.4 and 2 g. We observed a small increase in the rising velocity of the shaking cases at our experimental conditions. The increase was larger when bubbles were large and accelerations were high. Larger accelerations experienced the largest observational errors and we emphasize the exploratory nature of our results. We also measured the change in bubble diameter during the oscillations and computed the shear rate at the bubble surface. Maximum shear rates were in the range of 0.04-0.08 s(-1). At these shear rates, our analysis indicates that shear thinning behavior of our analog fluids is expected to be small and compete with elastic behavior. This transitional viscous/elastic regime helps explain the small and variable results of our experiments. Our results are relevant to the study of earthquake-volcano interactions. Most crystal-free silicate melts would exhibit a purely viscous, shear-thinning behavior in a natural scenario. Seismically enhanced bubble rise could offer an explanation for the observed increased degassing and unrest following large earthquakes. Plain Language Summary The speed at which air bubbles rise in a liquid is generally well-known. However, in non-Newtonian fluids (liquids with non-constant viscosity), previous experiments have shown that this speed could be significantly increased by shaking the fluid container. In this study, we present experiments where we measure the rising speed of an air bubble in a viscous non-Newtonian fluid while shaking it. We found that bubbles rose slightly faster during shaking, especially for strong shaking and/or large bubbles. However, the shaking of the table reduced the precision of our measurements, and our results need to be further validated. We were also able to measure how quickly the bubbles deformed during shaking. When the bubbles deform quickly, our fluids may behave like solids rather than liquids. This helps explain why our results are less striking than previous experiments. Our results can be applied to the study of volcanoes. Magma inside a volcano contains vapor bubbles. When an earthquake occurs close to a volcano, these bubbles may accelerate toward the surface and lead to more gas coming out of the volcano.