The Effect of Air on Granular Size Separation in a Vibrated Granular Bed
  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. 
Matthias E. M\"obius, Xiang Cheng, Peter Eshuis, Greg S. Karczmar,   Sidney R. Nagel, Heinrich M. Jaeger