Intruders in the Dust: Air-Driven Granular Size Separation

TL;DR

This study investigates how large particles move within a shaken granular bed, revealing complex behaviors influenced by density, initial position, boundary conditions, and gas pressure, with a model explaining the observed phenomena.

Contribution

It provides new insights into the non-monotonic density dependence and phase boundary of intruder motion in shaken granular materials, supported by experimental imaging and modeling.

Findings

Light and heavy intruders move faster than medium-density ones.

Intruders can rise or sink depending on initial conditions and gas pressure.

A model explains the transition between rising and sinking regimes.

Abstract

Using MRI and high-speed video we investigate the motion of a large intruder particle inside a vertically shaken bed of smaller particles. We find a pronounced, non-monotonic density dependence, with both light and heavy intruders moving faster than those whose density is approximately that of the granular bed. For light intruders, we furthermore observe either rising or sinking behavior, depending on intruder starting height, boundary condition and interstitial gas pressure. We map out the phase boundary delineating the rising and sinking regimes. A simple model can account for much of the observed behavior and show how the two regimes are connected by considering pressure gradients across the granular bed during a shaking cycle.