  We experimentally study the effects of salt concentration on the flowing
dynamics of dense suspensions of micrometer-sized silica particles in
microfluidic drums. In pure water, the particles are fully sedimented under
their own weight, but do not touch each others due to their negative surface
charges, which results in a "frictionless" dense colloidal suspension. When the
pile is inclined above a critical angle $\theta_c \sim 5{\deg}$ a fast
avalanche occurs, similar to what is expected for classical athermal granular
media. When inclined below this angle, the pile slowly creeps until it reaches
flatness. Adding ions in solution screens the repulsive forces between
particles, and the flowing properties of the suspension are modified. We
observe significant changes in the fast avalanche regime: a time delay appears
before the onset of the avalanche and increases with the salt concentration,
the whole dynamics becomes slower, and the critical angle $\theta_c$ increases
from $\sim 5{\deg}$ to $\sim 20{\deg}$. In contrast, the slow creep regime does
not seem to be heavily modified. These behaviors can be explained by
considering an increase in both the initial packing fraction of the suspension
$\Phi_0$, and the effective friction between the particles $\mu_p$. These
observations are confirmed by confocal microscopy measurements to estimate the
initial packing fraction of the suspensions, and AFM measurements to quantify
the particles surface roughness and the repulsion forces, as a function of the
ionic strength of the suspensions.
