A Wet Hourglass: silo discharge of a shear-thickening suspension

While for a Newtonian liquid the gravity-driven discharge of a vessel through a small orifice slows down as the vessel empties, we report a strikingly different picture for suspensions of small repulsive particles. Using model suspensions of polystyrene beads in water, which shear-thicken at high solid fractions, a constant discharge rate is observed as for a (dry) sandglass, albeit with a different dependance on the orifice size. We explain this law in terms of the vessel dimensions, the liquid viscosity, the critical shear stress above which the concentrated suspensions shear-thicken steeply, and the critical solid fraction above the latter stress scale. Discharges with a pressurized vessel, as well as the self-dilution of the extrudate and unsteadiness of the flow they may cause, will also be discussed.