INSTABILITY OF SEDIMENTING BIDISPERSE SUSPENSIONS

When an initially homogeneous suspension consisting of two species of spherical particles (with differing size and/or density) suspended in a fluid (i.e. a bidisperse suspension) is allowed to sediment it is known that variations in the concentrations of the particles spontaneously occur due to some instability. Vertical columns containing one or the other species of particle are formed, with the resulting induced motion causing augmented particle sedimentation rates. A theory is developed here which explains the instability and column formation in terms of macroscopic equations derived by considering the effect of two-particle collisions which are assumed to involve some non-hydrodynamic process (such as physical contact) between the particles. Based on the derived macroscopic equations, conditions for instability of the suspension are obtained. These results are then compared with existing experiments and theory. © 1990.