Mechanism of inverse and sandwich density segregations in a vibrated particle bed

The mechanism of density segregation of fine particles under vertical vibration was investigated. The role of permeation air flow on inverse segregation (low-density particles accumulate at the bottom and high-density particles at the top) and the sandwich segregation (low-high-low density profiles) was examined quantitatively. The air pressure in the vibrated particle bed was measured directly under various vibration conditions. The numerical analysis on the coupling motion of particle bed and permeation flow was also performed. It was found that air pressure inside the particle bed changed periodically, which was due to relative motion of the particle bed to the vessel caused by vertical vibration. This periodic pressure change has a significant effect on the mechanism of density segregation. The pressure fluctuation at the bottom of the bed due to the "piston effect," and the upward propagation of pressure variance play major roles in segregations. The timescales of these two pressure changes determine the segregation patterns, i.e., inverse or sandwich segregations. When the former timescale is large, the pressure in the bed changes monotonically in the vertical direction and, as a result, inverse segregation develops. When the latter timescale is large, an inflection point appears in vertical pressure profiles, resulting in sandwich segregation where high-density particles accumulate at the center.