  We study the outflow of soft particles through quasi-two-dimensional hoppers
with both experiments and simulations. The experiments utilize spheres made
with soft hydrogel, silicon rubber and glass. The hopper chamber has an
adjustable exit width and tilt angle (the latter to control the magnitude of
gravitational forcing). Our simulation mimics the experiments using purely
two-dimensional soft particles with viscous interactions but no friction.
Results from both simulations and experiments demonstrate that clogging is
easier for reduced gravitational force or stiffer particles. For particles with
low or no friction, the average number of particles in a clogging arch depends
only on the ratio between hopper exit width and particle's diameter. In
contrast for the silicon rubber particles with larger frictional interactions,
arches are larger than the low friction case. Additionally, an analysis of the
number of particles left in the hopper when clogging occurs provides evidence
for a hydrostatic pressure effect that is relevant for the clogging of soft
particles, but not so for the harder (glass) or frictional (silicon rubber)
particles.
