A study of the sedimentation of noncolloidal bidisperse, concentrated suspensions by an acoustic technique

This paper uses an acoustic technique to determine the concentration profile developing during the sedimentation of noncolloidal bidisperse suspensions of glass beads in a Newtonian fluid. Various bead diameter ratios have been used and a wide range of relative concentrations is covered. From the shock front velocities and the concentrations in different zones, the sedimentation velocities of small and large particles in a homogeneous suspension of respective concentrations cs0 and cl0 have been determined. The semidilute regime (c0=ss0+cl0<20%) has many similarities with the dilute regime, where large particles provide the dominant hydrodynamic hindrance to settling. In the concentrated regime (c0≳35%), a mutual hindrance leads to a velocity reduction of large particles and to an enhancement of small ones, as compared to a monodisperse suspension. The data clearly demonstrate that size segregation in the concentrated regime disappears at a critical concentration, which for the size ratio 1.68 is equal to c0=45%.