Pinned Bubble Dynamics in Locally Fluidized Granular Media

Recent studies of soft robot movement in sand have revealed that local high Reynolds number airflows can aid in intrusion, but the multiphase flow phenomena that emerge beneath the surface remain unexplored. We experimentally study the simplest version of such dynamics, the grain flow and structure formation created by a downward impinging buried air jet in a granular medium at different depths and flow rates. By bringing the probe to a clear sidewall, we observe rich phenomenology and a diversity of states including pinned granular bubbles and breaching cavities. For certain parameters of depth and airflow, bubbles can display a regular oscillation in their surfaces; we refer to these as "bubblators" and their dynamics are due to a creeping boundary flow that creates damped travelling vertical dunes along the bubblators length. A dimensional model based on self-similar turbulent jets captures the frequency scaling of these oscillations. We also find that pinned bubbles and bubblators can also be stably transported through space simply by moving the inlet source.