When the powder stored in a cylindrical vessel is being tapped, the powder particles flow in a manner somewhat similar to that of convection of boiling water. The particles located around the central axis of the cylinder tend to bounce up more briskly than those located closer to the vessel wall, because of the friction between the particles and the wall. About the cylinder axis, the vacancies made by the relative movement of the powder by the tapping are immediately filled up by the centripetal flow of the powder particles. Consequently, the ascending flow of particles occur around the axis and the descending flow occur on the contrary near the vessel wall. Moreover the rotation of particles is observed which is due to the velocity gradient of the descending powder flow. The segregation of powder particles is presented as occurring in the vessel, and as the result, it is shown that only relatively larger particles are found in the top layer, and only small particles are found in the bottom payer, and it is mixture of large and small particles that is found in between.
Percolation was once believed to be the sole mechanical factor for particle segregation. The authors propose in this paper a closer approach to precise explanation that the real segregation mechanism is in the combination of percolation in the top layer and of ascending and descending flow of particles. The authors point out also an additional factor as pertinent to the segregation, in particular, that larger particles are slower than smaller particles in their descending speed.
