Rise or sink: spherical intruder in density frequency dependence static   granular fluid
  A simple model for intruder in vibrating granular bed is constructed by introducing a term that governs frequency dependent granular bed density. Varying the vibrating frequency will drive the buoyant force acting on the intruder by the granular bed. Swapped regions of rising and sinking intruder compared to other reported result have bee found. 
Sparisoma Viridi, Seramika Ari Wahjoedi, Suparno Satira, Freddy P. Zen
cond-mat.soft