Silo outflow of soft frictionless spheres

Abstract

Outflow of granular materials from silos is a remarkably complex physical phenomenon that has been extensively studied with simple objects like monodisperse hard disks in two dimensions (2D) and hard spheres in 2D and 3D. For those materials, empirical equations were found that describe the discharge characteristics. Softness adds qualitatively new features to the dynamics and to the character of the flow. We report a study of the outflow of soft, practically frictionless hydrogel spheres from a quasi-2D bin. Prominent features are intermittent clogs, peculiar flow fields in the container and a pronounced dependence of the flow rate and clogging statistics on the container fill height. The latter is a consequence of the ineffectiveness of Janssen's law: the pressure at the bottom of a bin containing hydrogel spheres grows linearly with the fill height.

High-speed x-ray tomography of silo discharge

“…In contrast, the flow velocity of the soft HGS decreases noticeably with progressing discharge of the container. This is in accordance with earlier qualitative observations in 2D containers [4], where it was found that the pressure near the outlet is nearly hydrostatic for HGS, and lower pressure leads to a reduced outflow rate.…”

“…Silo discharge experiments in 2D containers with hydrogel spheres swollen in distilled water (9.2 mm diameter) have been reported before [4]. These experiments demonstrated qualitative differences of the discharge dynamics through narrow orifices as compared to rigid spheres.…”

“…The complete content of the flat-bottomed bins participated in the outflow. The flow profile at sufficiently large distance from the bottom had plug-flow character [4].…”

“…For a large variety of hard, frictional spherical grains, it has been shown that the diameter of the opening must be larger than approximately five times the particle diameter to avoid clogs [1,2]. When the particles are soft and slippery, they flow through much narrower outlets without clogging [3][4][5]. For non-spherical particles, shape parameters influence the discharge statistics [6,7].…”

“…with the determination of flow rates [1,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], the statistics of clogging [2,[23][24][25][26][27][28][29][30], the influences of obstacles near the orifice on the flow rates [31,32], and the role of gravity and embedding fluids [33][34][35]. The role of softness has been considered only recently in a few papers [3][4][5].…”