Packing and flow profiles of soft grains in 3D silos reconstructed with X-ray computed tomography

Abstract

The outflow of hard grains from storage containers with narrow outlets has been extensively studied in the past. Most experiments focused on discharge rates and avalanche statistics. Flow fields inside such containers have been detected optically in two-dimensional (2D) or quasi-2D geometries. Soft grains behave qualitatively different in many respects, both in their static packing properties and during silo discharge. We employ X-ray computed tomography to map the particles in a 3D container and we compare the static packing characteristics and the flow profiles of soft hydrogel spheres with those of hard spheres. The local fill fraction of the soft grains depends upon the depth below the granular surface. The outflow of the soft, low frictional hydrogel spheres involves the complete container volume, stagnant zones are absent.