Effects of salinity on the flow of dense colloidal suspensions

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 other 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 similar to 5 degrees 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 similar to 5 degrees to similar to 20 degrees. 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. We studied how the flowing dynamics of dense suspensions of silica micro-particles in microfluidic rotating drums experiments is modified when the inter-particles repulsion is progressively screened by increasing the ionic force of the suspension.