High-speed X-ray imaging of a ball impacting on loose sand

TL;DR

This study uses high-speed X-ray tomography to visualize and quantify the internal dynamics of a sand bed during a ball impact, revealing cavity collapse, bubble rise, and packing fraction variations.

Contribution

It provides the first direct, quantitative insights into the internal processes of a granular bed during impact using advanced imaging techniques.

Findings

Jet originates from cavity pinch-off point.

Entrapped air bubbles rise consistent with fluidized beds.

Significant packing fraction variations observed in different regions.

Abstract

When a ball is dropped in fine, very loose sand, a splash and subsequently a jet are ob- served above the bed, followed by a granular eruption. To directly and quantitatively determine what happens inside the sand bed, high-speed X-ray tomography measurements are carried out in a custom-made setup that allows for imaging of a large sand bed at atmospheric pressures. Herewith we show that the jet originates from the pinch-off point created by the collapse of the air cavity formed behind the penetrating ball.Subsequently we measure how the entrapped air bubble rises through the sand and show that this is consistent with bubbles rising in continuously fluidized beds. Finally, we measure the packing fraction variation throughout the bed. From this we show that there is (i) a compressed area of sand in front of and next to the ball while the ball is moving down, (ii) a strongly compacted region at the pinch-off height after the cavity collapse; and (iii) a relatively loosely packed center in the wake of the rising bubble.