Particle-pressure-induced self-filtration in concentrated suspensions.

Gravity-driven flow of concentrated suspensions (solid volume fraction ϕ>0.50) of non-brownian spherical particles through a channel contraction at low Reynolds number is studied experimentally. The abrupt change in the flow area at the contraction forms distinct shear-rate regions having different fluid pressures, which are related to the concept of particle pressure. A model involving particle pressure variation coupled to a Darcy-like behavior for the fluid captures the phenomenon of "self-filtration," in which the effluent material has lower solid fraction than the input suspension. For ϕ≐ϕ crit ≈ 0.58, under added load from a weighted piston, the flow periodically alternates between fast and slow motions.