Mechanism of anomalous sinking of an intruder in a granular packing close to incipient fluidization

Objects released into a granular packing close to incipient fluidization may float or sink depending on their density. Contrary intuition, earlier work reported that under certain conditions, lighter sphere can further and slower than heavier one. In this study, we reveal the mechanism of intriguing phenomenon using both magnetic resonance imaging discrete particle simulation. Our findings suggest local around sinking formation detachment gas bubbles play critical role in driving anomaly.