  The settling efficiency, and stability with respect to settling, of a dilute
suspension of infinite circular cylinders in a quiescent viscoplastic fluid is
examined by means of direct numerical simulations with varying solid volume
fraction, $\phi$, and yield number, $Y$ . For $Y$ sufficiently large we find
higher settling efficiency for increasing $\phi$, similar to what is found in
shear-thinning fluids and opposite to what is found in Newtonian fluids. The
critical yield number at which the suspension is held stationary in the carrier
fluid is found to increase monotonically with $\phi$, while the transition to
settling is found to be diffuse: in the same suspension, particle clusters may
settle while more isolated particles remain arrested. In this regime, complex
flow features are observed in the sedimenting suspension, including the
mobilization of lone particles by nearby sedimentation clusters. Understanding
this regime, and the transition to a fully arrested state, is relevant to many
industrial and natural problems involving the sedimentation of viscoplastic
suspensions under quiescent flow conditions.
