The motion of a large intruder particle inside a vertically shaken bed of smaller particles was investigated. Magnetic resonance imaging was used for the purpose of analysis. A nonmonotonic density dependence with intruders moving faster than those whose density is same as that of granular bed was found out. For light intruders, a rising or sinking behavior was observed.
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We thank N. Mueggenburg and E. Corwin for fruit?>ful discussions, J. Rivers for MRI support, and Peter Eshuis for help with the experiment. This work was supported by the NSF under CTS-0090490, and MRSEC, DMR-0213745, and by DOE under W-7405-ENG-82. M. E. M. acknowledges support from the Burroughs-Wellcome Foundation.