Anomalous sinking of spheres in apparently fixed powder beds discovered

Will improve technologies for recycling waste

Feb 1, 2016

A group of researchers led by OSHITANI Jun (Associate Professor, Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University) and TSUJI Takuya (Associate Professor, Department of Mechanical Engineering, Graduate School of Engineering, Osaka University) examined the state of the surface of apparently fixed powder beds in which air weak enough not to move the powder is injected, and observed the following anomalous sinking phenomena, a world first:

  1. 1. Unlike the case of fixed powder beds without air injection, anomalous sinking of spheres due to local fluidization of powder beds was observed.
  2. 2. The final sunken depth of a sphere varied with the sphere density and air strength.

  1. 3. When the sphere density is close to the powder bed density, spheres with smaller densities sank deeper than ones with bigger densities.

Sinking of objects in fluidization, in which powder is fluidized due to air injection, is used as a dry-type gravity separation technology for recycling of wastes, segregating waste plastics and non-ferrous metals. However, with this technology, only two kinds of objects with different densities, floating objects and sinking objects, can be separated at one time.
If this unique sinking phenomenon discovered by this group is used, a dry-type gravity separation technology for separating three objects with different densities can be developed, increasing the efficiency of recycling wastes.


The sinking of an intruder sphere into a powder bed in the apparently fixed bed regime exhibits complex behavior in the sinking rate and the final depth when the sphere density is close to the powder bed density. Evidence is adduced that the intruder sphere locally fluidizes the apparently fixed powder bed, allowing the formation of voids and percolation bubbles that facilitates spheres to sink slower but deeper than expected. By adjusting the air injection rate and the sphere-to-powder bed density ratio, this phenomenon provides the basis of a sensitive large particle separation mechanism.

To learn more about this research, please view the full research report entitled " Anomalous sinking of spheres due to local fluidization of apparently fixed powder beds " at this page of the Physical Review Letters website.

Related links

Technical Glossary