  We study the outflow dynamics and clogging phenomena of mixtures of soft,
elastic low-friction spherical grains and hard frictional spheres of similar
size in a quasi-two-dimensional (2D) silo with narrow orifice at the bottom.
Previous work has demonstrated the crucial influence of elasticity and friction
on silo discharge. We show that the addition of small amounts, even as low as
5\%, of hard grains to an ensemble of soft, low-friction grains already has
significant consequences. The mixtures allow a direct comparison of the
probabilities of the different types of particles to clog the orifice. We
analyze these probabilities for the hard, frictional and the soft, slippery
grains on the basis of their participation in the blocking arches, and compare
outflow velocities and durations of non-permanent clogs for different
compositions of the mixtures. Experimental results are compared with numerical
simulations. The latter strongly suggest a significant influence of the
inter-species particle friction.
