  Using experiments and simulations, we study the flow of soft particles
through quasi-two-dimensional hoppers. The first experiment uses oil-in-water
emulsion droplets in a thin sample chamber. Due to surfactants coating the
droplets, they easily slide past each other, approximating soft frictionless
disks. For these droplets, clogging at the hopper exit requires a narrow hopper
opening only slightly larger than the droplet diameter. The second experiments
use soft hydrogel particles in a thin sample chamber, where we vary gravity by
changing the tilt angle of the chamber. For reduced gravity, clogging becomes
easier, and can occur for larger hopper openings. Our simulations mimic the
emulsion experiments and demonstrate that softness is a key factor controlling
clogging: with stiffer particles or a weaker gravitational force, clogging is
easier. The fractional amount a single particle is deformed under its own
weight is a useful parameter measuring particle softness. Data from the
simulation and hydrogel experiments collapse when compared using this
parameter. Our results suggest that prior studies using hard particles were in
a limit where the role of softness is negligible which causes clogging to occur
with significantly larger openings.
