The Effect of Air on Granular Size Separation in a Vibrated Granular Bed

TL;DR

This study investigates how air influences the vertical movement of a large sphere in a vibrated granular bed, revealing air drag as the key factor in the non-monotonic density dependence of the sphere's rise and sink times.

Contribution

It demonstrates that air drag solely accounts for the density dependence of the sphere's motion and provides a detailed analysis supported by experiments, modeling, and simulations.

Findings

Air drag causes non-monotonic density dependence of sphere motion.

Motion influenced by particle size, initial position, pressure, and convection.

Model and simulations agree quantitatively with experimental results.

Abstract

Using high-speed video and magnetic resonance imaging (MRI) we study the motion of a large sphere in a vertically vibrated bed of smaller grains. As previously reported we find a non-monotonic density dependence of the rise and sink time of the large sphere. We find that this density dependence is solely due to air drag. We investigate in detail how the motion of the intruder sphere is influenced by size of the background particles, initial vertical position in the bed, ambient pressure and convection. We explain our results in the framework of a simple model and find quantitative agreement in key aspects with numerical simulations to the model equations.