Why Care about Air (when the particles are large)?
Movies showing how the density dependence of the Brazil Nut Effect is related to interactions with the interstitial fluid
An overview of the phenomenon, including magnetic resonance imaging (MRI) and high-speed video clips:
(be patient.....this is a 20Mb file and will take some time to load; if you want to get a higher resolution, 66Mb file click here)
A series of more detailed high-speed video clips, showing how the interstitial fluid (= air) interacts differently with intruders of different density:
Side-by-side comparison of a nylon and a steel intruder (spheres, 1 inch diameter each) in a bed of glass particles (0.5mm diameter). The left video clip show the situation under ambient pressure conditions, the one on the right was taken at reduced pressure (200mTorr). In each movie, the light (nylon) intruder is the one on the left and the heavy steel sphere is on the right. Notice how, in the left movie clip, the nylon intruder is sucked into the bed by the vertical pressure gradient across the bed. This gradient is set up during the first part of the vibration cycle, when air streams into the bed to fill the gap that has opened up at the bottom once the bed lifts off. At the very end of the cycle, the heavy intruder buries itself back into the bed while the lighter cannot penetrate much at all.
Side-by-side comparison of a steel and a nylon sphere (1 inch diameter each) under ambient conditions but without the bed of small particles. Notice that both spheres move in unison, showing that simple Stokes drag does not affect the behavior seen in the videos above.