Effects of salinity on flows of dense colloidal suspensions

Abstract

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 ${\theta }_c\sim 5°$ 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 $\sim 5°$ to $\sim 20°$. 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.

10 pages, 7 figures

References in corpus (15)

• Discontinuous Shear Thickening of Frictional Hard-Sphere Suspensions
• Shear thickening, frictionless and frictional rheologies in non-Brownian suspensions
• Colloidal Gels: Equilibrium and Non-Equilibrium Routes
• Discontinuous shear thickening without inertia in dense non-Brownian suspensions
• Jamming of frictional spheres and random loose packing
• Frictionless bead packs have macroscopic friction, but no dilatancy
• Onset of mechanical stability in random packings of frictional spheres
• Dynamics of Granular Stratification
• Measuring surface charge: why experimental characterization and molecular modeling should be coupled
• The Role of Friction in Compaction and Segregation of Granular Materials
• Effect of Friction on Dense Suspension Flows of Hard Particles
• Interparticle friction leads to non-monotonic flow curves and hysteresis in viscous suspensions
• Athermal analogue of sheared dense Brownian suspensions
• Brownian granular flows down heaps
• Creeping Avalanches of Brownian Granular Suspensions