Pinned Bubble Dynamics in Locally Fluidized Granular Media

Recent studies of soft robot movement in sand [Naclerio & Karsai et al., Science Robotics, 2021] have shown how airflows impinging into granular media can aid in intrusion by creating local flows that transport grains. To investigate the physics of such multiphase flows, we experimentally study emergent flow phenomena in single air-driven cavities in a bed of granular media (1 mm poppy seeds) at Reynold's numbers of up to 3000. A downward impinging buried air jet creates locally pinned bubbles as grains circulate within the bubble cavity. We observe emergent oscillatory behavior in these bubbles for both sidewall and mid-bed experiments for certain combinations of depth, airflow, and effective grain strength. These ‘bubblators’ are cavities oscillating at characteristic frequencies of approximately 1 Hz as damped travelling waves along the length of the bubble. These frequencies chiefly depend on the cavity’s overall intrusion depth. The oscillation arises from the periodic vorticity switching of granular flow along the bubble boundaries. Bubblators form when a pinned bubble overexpands and collapses into a pinned oscillating state. We also show that these pinned structures can also be transported vertically within the granular medium without significant reductions in volume.