The role of interstitial gas in determining the impact response of granular beds

TL;DR

This study investigates how interstitial gas influences the impact response of granular beds, revealing that gas can either facilitate or hinder penetration depending on the initial packing density, due to its effect on bed compaction resistance.

Contribution

It demonstrates the dual role of interstitial gas in granular impact dynamics, showing its contrasting effects based on initial packing density through high-speed imaging.

Findings

Gas facilitates penetration in loose beds.

Gas impedes penetration in dense beds.

Interstitial air affects bed packing resistance.

Abstract

We examine the impact of a solid sphere into a fine-grained granular bed. Using high-speed X-ray radiography we track both the motion of the sphere and local changes in the bed packing fraction. Varying the initial packing density as well as the ambient gas pressure, we find a complete reversal in the effect of interstitial gas on the impact response of the bed: The dynamic coupling between gas and grains allows for easier penetration in initially loose beds but impedes penetration in more densely packed beds. High-speed imaging of the local packing density shows that these seemingly incongruous effects have a common origin in the resistance to bed packing changes caused by interstitial air.